oEPA
United States
Environmental Protection
Agency
Robert S Kerr Environmental Research
Laboratory
Ada OK 74820
EPA-600. 2-79-062
March 1979
Research and Development
Environmental
Planning Manual for
Salinity
Management in
Irrigated
Agriculture
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1 Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7 Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution-sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-79-062
March 1979
ENVIRONMENTAL PLANNING MANUAL
FOR
SALINITY MANAGEMENT IN IRRIGATED AGRICULTURE
by
Gaylord V. Skogerboe
Wynn R. Walker
Robert G. Evans
Agricultural and Chemical Engineering Department
Colorado State University
Fort Collins, Colorado 80523
Grant No. R-804672
Project Officer
James P. Law, Jr.
Source Management Branch
Robert S. Kerr Environmental Research Laboratory
Ada, Oklahoma 74820
ROBERT S. KERR ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
ADA, OKLAHOMA 74820
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DISCLAIMER
This report has been reviewed by the Robert S. Kert Environmental
Research Laboratory, U.S. Environmental Protection Agency, and approved for
publication. Approval does not signify that the contents necessarily reflect
the views and policies of the U.S. Environmental Protection Agency, nor does
mention of trade names or commercial products constitute endorsement or
recommendation for use.
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FOREWORD
The Environmental Protection Agency was established to coordinate
administration of the major Federal programs designed to protect the quality
of our environment.
An important part of the Agency's effort involves the search for
information about environmental problems, management techniques, and new
technologies through which optimum use of the Nation's land and water resources
can be assured and the threat pollution poses to the welfare of the American
people can be minimized.
EPA's Office of Research and Development conducts this search through a
nationwide network of research facilities.
As one of these facilities, the Robert S. Kerr Environmental Research
Laboratory is responsible for the management of programs to: (a) investigate
the nature, transport, fate and management of pollutants in groundwater; (b)
develop and demonstrate methods for treating wastewaters with soil and other
natural systems; (c) develop and demonstrate pollution control technologies
for irrigation return flows; (d) develop and demonstrate pollution control
technologies for animal production wastes; (e) develop and demonstrate tech-
nologies to prevent, control or abate pollution from the petroleum refining
and petrochemical industries; and (f) develop and demonstrate technologies
to manage pollution resulting from combinations of industrial wastewaters or
industrial/municipal wastewaters.
This report contributes to the knowledge essential if the EPA is to meet
the requirements of environmental laws that it establish and enforce pollution
control standards which are reasonable, cost effective and provide adequate
protection for the American public.
<*
Q
William C, Galegar
Director
Robert S . Kerr Environmental
Research Laboratory
111
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PREFACE
No other natural resource in the United States may be subjected to as
many legal, socio-economic and institutional arrangements as water. There
are federal, state, and local laws, policies, and administrative regulations.
In addition, there is an abundance of small water districts, natural resource
districts, groundwater districts, and many other special districts each with
its own priorities, programs, regulations, responsibilities, and areas of
jurisdiction. These institutions are often in direct conflict with one
another; moreover, if one represents agricultural uses and others represent
industrial and municipal uses, conflicts are frequently solved via the legal
system which may take several years.
A governmental restriction that ties these diverse institutions together
is the national water quality program, PL 92-500, Section 208. Administration
and implementation of the Section 208 programs is a difficult task since
plans must be formulated for regional water quality goals that are compatible
with all users. The most difficult area of water use to accurately assess
and formulate workable pollution control programs is agricultural water use.
The agricultural assessment is complicated by its diffuse nature and the
pollution potential from pesticides, fertilizers, sediment, and salinity. To
compound the problem, each type of pollution or combination of pollutants has
a unique "site-specific" set of solutions for each agricultural area. The
pollution potential of salinity in irrigated agriculture is an area of major
national and international concern.
The EPA research and development program regarding irrigation return
flow quality control has made substantial progress in the past several years.
Considerable effort has been given to salinity problems and their control in
irrigation return flows with more and more attention being devoted to identi-
fying processes for implementing both technological (hardware) and institu-
tional (software) measures for reducing salt loads from irrigated agriculture.
This manual is an attempt to compile these research results in such a manner
that they can be used by action organizations responsible for implementing,
managing, and controlling water pollution from irrigated agriculture.
Gaylord V. Skogerboe
Wynn R. Walker
Robert G. Evans
IV
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ABSTRACT
An environmental planning manual for salinity management in irrigated
agriculture has been prepared. The primary focus of this manual is a
delineation of the combinations of technological and institutional solutions,
the various levels of planning effort, use of existing data and necessary
field investigations which are required for the different planning levels,
methods of data analysis, technological and socio-economic considerations
in implementing a salinity control program, and finally, recommendations for
formulating an action program.
It is intended tHat the primary audience for this manual would be
environmental planners such as EPA Regional Offices, state water pollution
control agencies, regional councils of governments, and 208 (Section 208 of
PL 92-500) planning groups. In addition, it is intended to serve as a guide
to be used and tailored at the discretion and guidance of the supervisory
personnel to persons without prior training or experience in assessing the
nonpoint source pollution problems of irrigation return flows due to salinity.
This report was submitted in fulfillment of Grant No. R804672 to
Colorado State University, under sponsorship of the U.S. Environmental
Protection Agency. This report covers the period of August 22, 1976 to
August 31, 1978, and work was completed as of August 31, 1978.
v
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CONTENTS
Foreword
Preface iv
Abstract v
Figures ix
Tables xiii
Abbreviations and Symbols xiv
Acknowledgments xvi
1. Planning Framework 1
Purpose of the manual 1
Scope 2
Meeting national goals 2
Identification of salinity problems 4
Development of best management practices 5
Implementation of best management practices 7
2. Irrigation in the United States 12
Present water and land use 12
Trends in agricultural water use 21
3. Irrigated Agriculture and Salinity Problems 28
Soil-water-plant relationships 32
Irrigation water management 39
Water law 44
4. Potential Technological Solutions 48
Improving the water delivery subsystem 49
Improving the on-farm subsystem 55
Improving the water removal subsystem 67
Collection, treatment, and disposal of irrigation return flows. 68
Nontechnical alternatives 70
5. Inflow-Outflow Analyses 72
Procedure 72
Data collection 76
6. Hydro-Salinity Analyses 79
Developing water budgets 80
Delivery subsystem 84
Farm subsystem 85
Water removal subsystem 87
vxi
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7. Soil Moisture—Chemistry Simulation 98
Modeling considerations 99
8. Field Investigations 106
Procedure 107
Delivery subsystem investigations 107
On-farm subsystem investigations 124
Water removal subsystem investigations 143
9. Developing Best Management Practices 154
Alternative management practices 154
Analysis of field data 155
Cost-effectiveness analyses 156
10. Implementation of Best Management Practices 161
Introduction 161
Legal considerations 161
Economic considerations 162
Organizing for implementation 165
Training 167
Farmer activities 168
Monitoring, evaluation, and refinement 170
References 171
Appendices 192
A. Description of selected irrigation return flow models 192
B. References required for irrigation return flow studies 225
C. Conversion factors 230
Vlll
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FIGURES
Number Page
1 Planning framework for developing best management
practices in a subbasin ..... 8
2 Planning framework for developing best management practices
for salinity management in a river basin 9
3 Planning framework for developing best management practices
for only one irrigated area . „ 10
4 Map of the United States showing the 17 western states and
the commonly used water resource regions 13
5 Irrigation development in the 17 western states, 1889-1974. . . 14
6 Water withdrawals and consumption in the United States for
1975. . 18
7 Estimated consumption of irrigation water and irrigation
return flows by water resource regions in the United
States for 1975 19
8 Trends in use of irrigation water supplies 22
9 Schematic representative of the potential for nonpoint source
pollution from irrigated agriculture. .... 33
10 Typical water-holding capacities of different textured soils. . 35
11 Water delivery, farm, and removal irrigation subsystems .... 40
12 Schematic representation of intake opportunity time and
typical advance and recession curves for surface ir-
rigation 59
13 Definition sketch of surface irrigation application uniformity
for (a) the case where part of the field is underirrigated,
(b) the case of zero underirrigation, and (c) conditions of
significant overirrigation. . 60
IX
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Number
Page
14 Elevation drawing of cutback furrow irrigation system with
spiles in the ditch sidewall 61
15 Schematic illustration of a tailwater recovery and reuse
system for surface irrigation 62
16 An illustration of the concept of trickle irrigation in
which only a small part of the field, the wetted profile
where crop roots are growing, is irrigated by emitters
bringing water to each individual plant or group of
plants 65
17 A schematic diagram of a typical trickle irrigation system
control head 66
18 Schematic diagram of a typical desalination system 69
19 An illustration of a watershed which includes an irrigated
component 73
20 Schematic example of inflow-outflow analysis 75
21 Delineation of a study area and a possible monitoring
network for a hydro-salinity investigation 81
22 Schematic representation of the investigative procedure
for evaluation of specific irrigation events and seasonal
irrigations 86
23 Detailed schematic for individual irrigation event on-farm
subsystem analyses 88
24 Detailed schematic for seasonal on-farm subsystem analyses... 89
25 Schematic representation of the many parameters that must
be considered in on-farm hydro-salinity investigations. ... 90
26 Schematic of a generalized hydro-salinity model 92
27 Illustrative flow chart of the root-zone budgeting procedure. . 94
28 Flow chart of the groundwater modeling procedure 96
29 Typical relationships defining the hydraulic properties
of a soil 100
30 Generalized block diagram of the model 101
31 Generalized block diagram of moisture flow program 103
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Number Page
32 Generalized block diagram of a Biological-c'hemical
program 105
33 Schematic representation of the Collins flow gage . 110
34 Typical rating curves for current meter measurements Ill
35 Trajectory method and impeller meters for measuring
discharge 112
36 Portable installation of a propeller meter used to measure
discharge from a pump for farm or pump efficiency studies . 113
37 Schematic representation of various weirs used in
agricultural water management 115
38 Schematic representation of some common types of flumes used
in agricultural water measurement 116
39 Two closed-to-open conduit methods for measuring headgate
diversions 119
40 Schematic representation of methodology for chemical dilution
techniques of flow measurement 120
41 Schematic representation of the ponding test method for
seepage measurement 122
42 Typical relationship of water depth and area which need to
be determined for seepage tests 124
43 Schematic of instrumentation required for on-farm hydrology
investigations 127
44 ,Schematic of the hydrologic variables to be considered in an
on-farm subsystem investigation 128
45 Schematic of constant water-table lysimeter 131
46 Schematic of the construction of a hydraulic weighing
lysimeter 133
47 Typical calibration curve for hydraulic weighing lysimeter. . 134
48 Typical areal photograph used for land use mapping showing
the land use mapping index used in Table 11 137
49 Finished map corresponding to the areal photograph shown in
Figure 48 138
XI
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Number
50 Typical hydrograph of field tailwater for surface irrigation. . 140
51 Schematic of vacuum extractors installation with detail .... 145
52 Schematic view of pressure-vacuum lysimeters for soil water
quality sample collection 146
53 Sketch of typical observation well construction and the
associated driller's log « • • 148
54 Representative geologic, electric, and gamma-ray logs that
are used for the identification of the hydrologic properties
of the geologic substrata 149
55 Schematic representation of observation well clusters for
stratified aquifer situations 150
56 Typical piezometer installations that can be used for
vertical water gradients and water quality information. . . . 152
57 Conceptual decomposition model of a regional or basin salinity
control strategy 157
58 Dimensionless level 1 cost-effectiveness curves for the
Grand Valley 160
Xll
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Number
1
2
3
4
5
6
7
8
TABLES
Irrigated Acreage in the United States for Selected Years . . .
Water Diverted and Consumed for Irrigation by States, 1975. . .
Estimated Water Use and Projected Requirements
Regional Estimations of Water Use and Projected Requirements. .
Regional Projections of Irrigated Land in the Conterminous
Unites States
Regional Projections of Irrigation Withdrawals. .
Regional Projections of Irrigation Consumption
Crop Yield Response at Various Levels of Electrical Con-
ductivity of the Saturation Extract of Soils
Page
15
16
20
23
24
25
26
37
9 Classification of Soils by the Soluble Salt Content and
Exchangeable Sodium Percentage 38
10 Table for Computing Seepage Losses 125
11 Suggested Land Use Mapping Index 135
Xlll
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
ac
AF
BTU
cal/gm
cfd
cfs
cmd
CMI
degrees C or °C
degrees F or °F
ft
gm
gpm
ha
ha-m
hr
hp
in
km
Ib
1/s
m
-acre, (43,560 ft ) one acre equals 0.4046
hectare
-acre-foot, volume of water to cover one acre
a depth of one foot, one Acre-foot equals
0.1233 hectare-meters
-British Thermal Unit
-calories per gram
-cubic feet per day
-cubic feet per second, volume flow rate of
water, one cfs equals 0.0283 cubic meter per
second
-cubic meter per day
-Colorado Miner's Inch, one Colorado Miner's
Inch equals 0.74 liters per second
-Centigrade temperature (also called Celsius)
scale
-Fahrenheit temperature scale
-feet, unit of length, one foot equals 0.3048
meters
-gram, 454 grams equal one pound
-gallons per minute, volume flow rate of
water, one gallon per minute equals 0.0631
liters per second
-hectare, metric unit of area, one hectare
equals 2.471 acres
-hectare-meter, volume of water to cover one
hectare to a depth of one meter, one ha-m
equals 8.108 AF
-hour, 60 minutes
-horsepower, one horsepower equals 7.460 x
10 erg/sec
-inch, one inch equals 2.54 centimeters
-kilometer, metric unit of length, one kilo-
meter equals 0.621 mile
-pound (mass)
-liters per second, volume flow rate of water
-meter
-cubic meters per second, volume flow rate of
water
xiv
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me/1 —milliequivalents per liter
rag/1 —milligrams per liter, equal to one ppm
mi —mile, one mile equals 1.609 kilometers
xv
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ACKNOWLE DGMENTS
The authors are indebted to several individuals who significantly
contributed to the completion of this manual. The cooperation of the
Drainage District, irrigation companies, landowners, irrigators and numerous
others in the Grand Valley, where many of the concepts and procedures pre-
sented in this manual were developed and tested, is especially appreciated.
A special thanks goes to Dr. Harold R. Duke of the USDA, Science and
Education Administration for reading and making constructive criticisms on
early drafts of this manual. A special acknowledgment goes to Ms. Annette
Ward for her productive and diligent efforts in editing and making this report
much more readable.
A very special thank you goes to Ms. Sue Eastman, Ms. Mary Lindburg, and
Ms. Melanee Lowdermilk for their steadfast and cheerful typing of the many
drafts of this report.
Finally, the efforts and advice given by the EPA Project Officer, Dr.
James P. Law, Jr., have been extremely helpful in the successful pursuit of
this project. He has generously given of his time to cooperatively achieve
the goals of the project.
xvi
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SECTION 1
PLANNING FRAMEWORK
The increasing complexity of assessing the physical, economic, legal,
sociological, and institutional aspects of irrigated agriculture has necessi-
tated the development of extensive resource materials. The need for current
and detailed information on salinity management in irrigated agriculture
along with specially trained personnel has been acknowledged by staffs
involved in regional water quality management. Recently, numerous research
projects regarding irrigation return flow quality control have been com-
pleted. It is highly desirable that these research results be disseminated
for use by action organizations responsible for implementing, managing,
and controlling water pollution from irrigated agriculture.
This manual has been developed to provide a basis for the determination
of the significance of salinity problems, delineation of the combinations of
technological and institutional solutions, determination of the various
levels of planning, evaluation of field investigations required for the
different planning levels, and selection of methods for data analysis.
Ultimately, these areas lead to a planning framework for the implementation
of action programs.
PURPOSE OF THE MANUAL
This manual is designed to assist areawide administrators responsible
for nonpoint source agricultural salinity control programs under the juris-
diction of the Federal Water Pollution Control Act of 1972 (Public Law
92-500) and the Clean Water Act of 1977 (PL 95-217). Congressional mandates
have specified that regional investigations, commonly referred to as
Section 208 studies, be undertaken to identify significant nonpoint sources
of pollution, to develop plans for the control of these pollutants, and
to design programs for the implementation of these pollution abatement plans.
The legal basis for requiring these studies is detailed in Section 201(c)
and Sections 208(b)(2)(C)(F) through (K) of P.L. 92-500.
This manual is to provide administrators working under the regulations
of Section 208 a basis to judge the adequacy of technical studies contracted
under their supervision. Environmental planners, state water pollution
control agencies, regional councils of governments, agriculturalists,
environmentalists, consulting engineers, and action agency personnel can
also be assisted by this manual. Generally, information presented in the
manual serves as a guide and reference source for any person actively engaged
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in the analyses and development of best management practices for irrigated
agriculture. This report provides an interdisciplinary framework for (a)
identifying salinity pollution problems in agriculture; (b) developing the
best management practices for salinity control; and (c) implementing best
management practices.
SCOPE
It should be noted this manual does not specifically recommend or
present descriptions of the advantages and disadvantages of various investi-
gative techniques for salinity control since the volume of material would be
overwhelming. It does, however, stress the need for an interdisciplinary
approach for studying salinity which is necessary when viewing the physical
complexity of the soil-water-plant regimes, the socio-economic and political
implications of water, and the rapid expansion of irrigated agriculture
throughout the United States. Throughout the text it is emphasized that
there are no set values for the various parameters and specific solutions
that universally apply to the salinity control problems of irrigated agri-
culture. Instead, this report presents a rational framework by which any
irrigated area suspected of contributing significant amounts of nonpoint
source pollution can be evaluated.
Generally, this report is concerned with gravity diversion irrigation
delivery systems which are common in the western United States. On the
other hand, due to power costs and other economic considerations which
reflect the true cost of water, pumped groundwater systems are usually
operating at higher irrigation water use efficiencies and alternatives for
improved water management are often limited. Rawlins (1976) discusses some
alternatives which can be considered for extensive groundwater use areas.
Procedures and methods for evaluating surface return flow problems with
sediment and biocides and groundwater nitrate pollution are beyond the scope
of this manual, but these subjects are adequately addressed by Evans and
Duseja (1973), Dornbush et al., (1974), McNeal and Carlile (1976), Pratt
(1972), Carter and Bondurant (1976), and Wendt et al., (1976). It should
be noted that many of the investigative procedures and some potential
solutions discussed in this manual for salinity control apply also to the
control of other types of nonpoint source pollution. Additional information
on water quality management problems due to irrigated agriculture can be
found in Law and Witherow (1970), and King and Hanks (1975) .
MEETING NATIONAL GOALS
There are several national goals that impinge on the subject of salinity
management in irrigated agriculture. Besides reducing water pollution in
order to have cleaner water, it is also necessary to continually increase
crop production, to provide water to meet new water demands by agriculture,
municipalities and industries, and to reduce energy consumption.
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The forecasted increase in irrigated acreage will result in higher
pollution potentials from agriculture throughout the United States. Therefore
it should be expected that control of pollution from irrigation return flows
and other agricultural sources in all areas of the country will receive more
and more emphasis in the future.
Improved irrigation water management practices will not only reduce
pollution from irrigated agriculture, but will also increase crop production,
reduce energy consumption and increase water supplies available for new
demands of other beneficial uses. The necessity for pollution control pro-
grams and the concomitant benefits to other national goals will continue to
direct more and more attention to the control of irrigation return flows.
Consequently, more agricultural areas will need to be evaluated and questions
regarding the quantity and quality of the irrigation return flows will need
to be answered.
Because many of the nonpoint pollution parameters tend to be site-
specific, every area should be evaluated individually. The same conditions
encountered in one area may result in an entirely different set of conse-
quences in another area. The conditions in areas such as the Grand Valley
of Colorado are not necessarily the same in the Imperial Valley of
California, Wellton-Mohawk Project of Arizona, Platte River of Colorado,
Wyoming and Nebraska, Snake River of Idaho, Oregon and Washington, or the
Red River of North Dakota. To assume nonsite-specific "representative"
value implies that decisions can be made without technical information with
a reasonable probability of success. This is clearly not the case, and very
often the components of a nonpoint source pollution control program are not
intuitive.
Mineral pollution is the most serious water quality problem in many areas
of the United States, most notably in the Colorado River Basin. The pro-
blem is serious because the Basin is approaching conditions of full develop-
ment and utilization of the available water resources. While the salinity
problem may seem unique to basins of the arid West, it will ultimately be
faced hy nonarid areas as water use approaches the available supply.
The United States Environmental Protection Agency (1971) reported that
existing damages due to salinity to Lower Colorado River Basin users would
increase from $16 million annually in 1970 to $51 million annually by the
turn of the century, if planned developments do not include appropriate
salinity control measures. More recent estimates by the United States
Bureau of Reclamation (Bessler and Maletic, 1975) show present damages at
$53 million annually, which is projected to be $124 million annually by the
year 2000. Irrigated agriculture accounts only for 37 percent of the total
salt load of the Colorado River at Imperial Dam.
The Upper Colorado River Basin water users are particularly affected
by these conditions because most of the future developments involve trans-
basins diversions, in-basin oil shale development, and possible hydroelectric
and thermoelectric production. None of these water uses add significantly
to the salt loading aspect, but each diminishes the quantity of water avail-
able for diluting the salt loads already being carried. Future development
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of water resources in the Upper Colorado River Basin must be associated with
more rigid salinity controls on the existing salt sources, many of which
are related to agricultural water uses.
IDENTIFICATION OF SALINITY PROBLEMS
Salinity includes inorganic chemical salts of sodium, magnesium,
potassium, and calcium that form bicarbonates, carbonates, chlorides, sul-
fates and nitrates which are dissolved in water or are present in soil, It
is a naturally occurring pollutant that is always carried by the rivers or
streams in varying concentrations. Salinity control is a controversial
subject that illustrates only one of the many aspects of the quality-quantity
conflicts that occur in water short areas. It is necessary to derive a
salinity control program that balances water quality and water quantity with
in-stream and off-stream beneficial uses such as fisheries and recreation
with irrigation and industrial uses. It is hoped that this manual will
provide insight to developing realistic as well as acceptable nonpoint
source pollution control programs that can be implemented. It is necessary
that the solutions be acceptable to the irrigators, environmentalists, and
regulating agencies.
In studying salinity problems of an irrigated area, the first step is
to determine the impact of the resulting pollution. This may be accom--
plished by an inflow-outflow analysis which essentially analyzes the
quantities of water and salt entering and leaving an irrigated area. If the
subsurface irrigation return flows pass through the groundwater reservoir
and then return to the river, an inflow-outflow analysis can be performed
using gaging stations upstream and downstream from the irrigated area.
However, if the groundwater level is deep, which usually is the result of
heavy pumping, the outflow must be measured in the groundwater reservoir,
where increasing salinity concentrations of the presence of nitrates alone
could indicate pollution of the groundwater reservoir,
The important question is whether certain uses of the water downstream
are impaired as a result of the increased salinity concentrations. If the
groundwater reservoir is being polluted, then it would be important to
evaluate the continued use of groundwater supplies. For instance, if
nitrate concentrations in the groundwater were increasing with time, it
would be important to know whether the water is suitable for present and
future domestic use.
The existence of salinity problems in the irrigated area is also of
environmental concern. Irrigation water supplies having high salinity con-
centrations, or more likely, high groundwater levels resulting from ex-
cessive water use, poor natural drainage, or a combination of factors, are
likely to create significant environmental changes such as permitting
development of different plant and animal species.
Once a salinity problem is recognized in an irrigated area, a more
difficult problem becomes the identification of the sources of the problem.
The first question to be resolved is how much of the salt pollution results
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from irrigated agriculture, municipalities and industries, or natural
sources such as runoff from precipitation and snowmelt. Typically, these
questions are difficult to answer and require considerable analysis. The
required approach is to prepare water and salt budgets for the area by
developing a hydro-salinity model as described in Section 6. The sources of
salinity from irrigated agriculture are the result of seepage losses from
canals and laterals, as well as deep percolation losses from overirrigation
of croplands. Therefore, it becomes necessary to measure seepage and deep
percolation losses throughout the irrigated area. Field procedures for
measuring these losses are discussed in Section 8.
For most areas, a reduction in seepage and deep percolation losses will
not result in a corresponding decrease in the volume of salts leaving the
area. For such situations, it is necessary to evaluate the chemical changes
in the subsurface return flows as they move through the soil profile and are
transported through the groundwater reservoir. Such an analysis involves
soil moisture-chemistry simulation, which is described in Section 7.
Basic Considerations
Irrigated agriculture is not always the major contributor to the salinity
problem in an area. For example, in the Upper Colorado River Basin, natural
runoff and natural point sources contribute more than 60 percent of the total
salt load, and irrigated agriculture contributes about 37 percent. The
salinity concentrations from irrigated agriculture are usually much higher,
the areal extent is much smaller, and efficient and cost-effective control
is easier to attain.
Irrigation has two primary objectives: (a) to supply the essential
moisture for plant growth; and (b) to leach or dilute the chemical salts
in the applied water and in the soil. The first objective of supplying
the necessary moisture can be accomplished in several ways. Regardless
of the method used, the purpose of irrigation is to periodically replenish
the soil moisture depleted by the consumptive demands of the plant.
The second objective of irrigation is very important and will often
occur naturally during an irrigation as deep percolation. In fact, if
these salts are not periodically flushed or leached from the crop root zone,
the land will become nonproductive in a relatively short time. It is also
this second objective of irrigation that is responsible for the salinity
problems in irrigation return flows. Sustained agriculture must have a
certain amount of salinity in its return flows.
DEVELOPMENT OF BEST MANAGEMENT PRACTICES
Once the magnitude of the subsurface irrigation return flows are
quantified, it becomes possible to identify appropriate technologies for
alleviating water quality degradation. The first step is to move from the
potential solutions described in Section 4 to solutions that are appro-
priate to the particular area under study.
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Those solutions identified as appropriate should be tested on farmers'
fields in order to develop alternatives that are acceptable to farmers. At
the same time, the soil moisture-chemistry simulation should be undertaken
so quality and quantity impacts of reducing subsurface irrigation return
flows being transported ultimately to the stream can be evaluated. This
process of shifting from potential solutions to appropriate solutions to
developing acceptable solutions is very important if a salinity control
program is to be developed which can be implemented successfully.
Appropriate solutions must also consider the institutional constraints,
such as the existing water law and water rights systems. For example, a
tailwater reuse system may not be a best management practice to be implemented
in some areas if the reduced irrigation return flows interfere with downstream
water rights. Also, evaporation and disposal of saline waters may deprive
another appropriator of his water right, regardless of how saline that
water may be.
The cost-effectiveness of a particular technology is defined herein as
the cost of implementing the technology versus its effectiveness in reducing
the salt load in the groundwater reservoir or river, as well as the concern
for reclaiming unproductive salinized soils. -The cost-effectiveness of
each appropriate technology must be developed for the "site-specific" case
under study to arrive at an optimal mix of technologies that will alleviate
the salinity problems. The best management practices will be those tech-
nologies that will also be acceptable to the farmers. The solutions must
be acceptable to the irrigator, since it is his management of the system
that will maximize the potential of the improved irrigation practices.
Mandated efficiencies are very difficult to enforce without a commitment
from the irrigator-farm manager. This is discussed in more detail in Sections
9 and 10.
River Basin Salinity Planning
The majority of Section 208 planning efforts are directed towards
identifying measures required, to control salinity in individually irrigated
areas or subbasins. However, plans must also be developed for the entire
river basin in order to determine the level of the programs to be imple-
mented. It is important to understand that salinity planning involves three
steps. First, evaluation of alternatives for each area leads to management
practices that can effectively control salinity from use of irrigation
water. These plans should indicate optimal policies for reducing salinity
by amounts ranging from the maximum potential control achievable to no
cu.iit.rol as prescribed by each specific area.
There are several ways that the salinity control policies, program costs,
and the resulting effectiveness of a program can be evaluated. One method is'
a cost-effectiveness technique which relates the implementation cost to the
corresponding net unit reduction in salt loading for the salinity control
measures. The cost-effectiveness method is used in this report. A total
benefit approach can also be used which considers local benefits such as ~MI-
creased crop production, aesthetics, and reclamation of saline loads, as '..-?!!
-------
as the reduction in downstream salinity damages, In some cases, only the po-
tential downstream benefits of a salinity control project are used for pro-
gram comparisons. The subarea analysis is illustrated in Figure 1,
In each subarea, the magnitude of the problem must be determined and
the sources delineated. This should include the magnitude of subsurface
irrigation return flows resulting from seepage losses and deep percolation
losses, along with changes in chemical composition of these flows enroute to
the groundwater reservoir and, subsequently, the river. When this is
accomplished, the best management practices must be developed to alleviate
the salinity problems. Appropriate solutions should be demonstrated and
evaluated on farmers' fields to develop alternatives that are acceptable to
both the farmers and various regulatory agencies. In addition, cost-
effectiveness analysis must be applied to the acceptable solutions to arrive
at best management practices. Finally, the basis for implementation of the
best management practices must be established.
The second 'step in establishing the most cost-effective salinity control
program is to define the best management practices by optimizing cost-
effectiveness relationships from individual areas into a single strategy.
This planning function delineates the level of salinity control required in
each individual subbasin to achieve the overall goal for water quality
improvement with the least cost. Relative levels of implementation among
the areas are also determined.
The third step assigns the required level of salinity control that
should be implemented in each subbasin or irrigated area. The basin planning
process for salinity control involves integration of planning studies at the
local level into the total basin framework to define the best management
practices on a basin-wide scale. By integrating the results outlined in
the first step, the best management practices for salinity control in each
area are identified. A summary of this process is shown schematically in
Figure 2.
If it is not possible or feasible to determine a basin-wide salinity
control program, the processes involved in alleviating salt pollution
from an individual area must be determined as shown in Figure 3. This
analysis is identical to the process outlined in the first step, but the
scale is much smaller and the best management practices are tailored to
the subarea.
Finally, best management practices should include educational programs,
technical assistance programs, improved water delivery systems, and im-
proved irrigation practices. Financial assistance programs should also be
considered in nonpoint source control programs.
IMPLEMENTATION OF BEST MANAGEMENT PRACTICES
Once the best practices for salinity management in the irrigated area
are determined, there is a need to develop methods for implementing these
practices. Section 10 of this manual discusses many of the issues involved
-------
2 o
CO
o
o
O 4-
if
o>
o
a>
CO
£
Natural Sources
Water Delivery
Subsystem
Potential Solutions
Soil Moisture
Chemistry Simulation
Institutional
Considerations
Inflow-Outflow
Analysis
Hydro-Salinity Model
Magnitude and
Sources of Salinity
Irrigated Agriculture
On- Farm Water Use
Subsystem
I
Appropriate Solutions
Field Demonstrations
and Evaluations
Acceptable Solutions
Cost-Effectiveness Analysis
Best Management Practices
Evaluate Next
Subarea
t
Other — Municipal,
Industrie 1 , Turf, Lawns
Water Removal
Subsystem
Jsys
Figure 1, Planning framework, for developing best management
practices in a subbasin.
-------
Identify Extent and
Magnitude of Salinity
Detriments in the Basin
.
E
4) «
Establish the Goals
of a Basin-Wide
Salinity Control Program
Inflow-Outflow
Analyses to
Identify Areas of
Possible Programs
Yes
(A
0>
-
ID ^
— 00 O
O
•ft" -
O ID ^
Optimize Individual
Plans Into Basin Plan
OQ
Identify the
Salinity Control
Level Optimally Selected
For Each Subarea
Individual Suboreo
Salinity Control
Planning
Express Subareas Best
Management Practices
as Functions of Salinity
Control Levels
Subarea Analysis
.J
Determine the Level of
Best Management Practices
to be Implemented
in Each Subarea
Legal Issues
.
to *-
cm o
o o
'£.
-------
Inflow-Outflow
Analysis
o
o 62
cvo
g "
o 2<
O Q_
c • —
-------
in implementing solutions to water quality problems in irrigated agriculture
and, in particular, salinity. Besides legal and economic issues in imple-
menting technology, there are important questions on how to organize for
implementation.
Implementation of salinity management measures requires strong
participation by farmers in order to have a successful program. Farmers
must be aware of the salinity problems and their solutions. There is an
additional obstacle of training technical assistance personnel for this
particular work. There is a great necessity for continuing monitoring,
evaluation, and refinement of the implementation program.
11
-------
SECTION 2
IRRIGATION IN THE UNITED STATES
PRESENT WATER AND LAND USE
Agricultural water use- has grown steadily throughout the years, The
first statistical record of irrigation in the United States was obtained
through the Census of Agriculture in 1889. That census recorded 3,361,000
acres (1,360,200 hectares) under irrigation. The most recent Census of
Agriculture (1974) recorded 41,429,000 acres 016,765,965 hectares) under
irrigation or an increase of more than 1,200 percent in 85 years. In 1975
it was estimated that 160 million acre-feet (19,7 million hectare-meters) of
water were diverted for irrigation in the United States of which about 56
percent was consumptively used (Murray and Reeves, 1977), This does not mean
the average irrigation efficiency is 56 percent. Rather, the average
efficiency is much lower, approximately 35 to 45 percent, because the "excess"
water, which is diverted, returns to streams as irrigation return flow by
surface and/or subsurface means and is then rediverted for reuse on other
lands.
The major irrigated areas in the United States are located in the 17
western states that are shown in Figure 4, The increase in irrigation for
the 17 western states in Figure 5 reflects the national increase in irrigated
acreage. An examination of Table 1 indicates that in 1977 both California
and Texas contained over 8 million acres (3.24 million hectares) of irrigated
land; Nebraska had more than 7 million; Idaho, 4 million; and Colorado,
Kansas, and Montana contained over 3 million acres (2,8, 1.6, and 1,2 million
hectares, respectively) of irrigated land. Arizona, Nevada, New Mexico,
Oregon, Utah, Washington, and Wyoming contained between 1 and 2 million
acres (0.8 and 0.4 million hectares); while Oklahoma, North Dakota, and
South Dakota contained less than 1 million acres (0.4 million hectares) of
irrigated land. In addition, Florida had almost 3 million acres. Arkansas
had 1.5 million acres, and Louisiana about 0,6 million acres of irrigated
land (1.2, 0.6, and 0.24 million hectares, respectively). The estimated
irrigation water use by state for 1975 is presented in Table 2 . Figure 6
illustrates the relative magnitude of fresh water withdrawals and consumption
in the United States for 1975. The estimated quantity of water used for
irrigation and irrigation return flows (IRF) for the same year are presented
by water resource region in Figure 7.
Irrigation results in large increases in the productivity of croplands.
Irrigated lands in the United States amount to approximately 10 percent of
the total cropland, and yet produces about 25 percent of the total
12
-------
Figure 4. Map of the United States showing the 17 western states and the commonly used water
resource regions (Water Resources Council, 1968).
-------
1889
1899 1909
1919
1929 1939 1949
1959
1969
1979
Figure 5. Irrigation development in the 17 western states, 1889-1974
(U.S. Department of Commerce, Bureau of the Census and
Irrigation Journal).
14
-------
4/
TABLE 1. IRRIGATED ACREAGE IN THE UNITED STATES FOR SELECTED YEARS—
"J
STATE
ALABAMA
ALASKA
ARIZONA I/
ARKANSAS
CALIFORNIA I/
COLORADO I/
CONNECTICUT
DELAWARE
FLORIDA
GEORGIA
HAWAII
IDAHO I/
ILLINOIS
INDIANA
IOWA
KANSAS I/
KENTUCKY
LOUISIANA
MAINE
MARYLAND
MASSACHUSETTS
MICHIGAN
MINNESOTA
MISSISSIPPI
MISSOURI
MONTANA I/
NEBRASKA I/
NEVADA I/
NEW HAMPSHIRE
NEW JERSEY
NEW MEXICO I/
NEW YORK
NORTH CAROLINA
NORTH DAKOTA I/
OHIO
OKLAHOMA I/
OREGON I/
PENNSYLVANIA
RHODE ISLAND
SOUTH CAROLINA
SOUTH DAKOTA I/
TENNESSEE
TEXAS I/
UTAH 17
VERMONT
VIRGINIA
WASHINGTON I/
WEST VIRGINIA
WISCONSIN
WYOMING I/
U.S. TOTAL
17 WESTERN STATES
OTHER STATES
IRRIGATED ACREAGE
1956-2/
25,000
3/
1,150,000
892,930
7,750,000
2, 382,000
20,000
11,000
521,200
75,000
I/
2,405,089
16,500
35,000
32,000
722,575
18,225
711,000
6,900
12,000
25,270
24,360
28,000
157,000
60,000
1,890,000
2,012,320
700,000
1,100
72,150
800,000
59,024
38,500
48,000
25,000
285,175
1,575,000
21,500
1,000
48,994
120,000
29,000
6,962,234
1,200,000
1,470
32,000
947,000
1,006
25,000
1,300,000
35,278,602
32,249,393
3,029,209
19652/
25,580
86
1,160,000
1,275,000
8,500,000
3,003,000
15,000
18,770
1,184,593
156,000
139,810
3,250,000
26,000
17,000
85,000
1,190,000
15,000
484,475
6,500
18,800
27,585
114,000
23,000
146,000
75,872
2,901,078
3,491,000
846,000
4,500
65,842
850,000
66,000
94,797
80,105
26,000
418,373
1,690,000
25,000
600
26,525
138,000
10,000
7,800,000
1,436,295
2,200
55,000
1,236,900
2,300
72,600
1,590,000
43,886,286
39,580,851
4,305,435
1977i/
58,600
1,890
1,150,000
1,698,500
8,189,176
3,060,000
9,000
34,000
2,918,244
592,100
155,128
3,934,000
68,000
58,250
165,000
3,157,500
27,500
662,695
5,555
32,570
31,300
135,000
397,500
402,400
266,558
3,114,150
7,165,100
1,304,700
6,000
174 ,000
1,239,600
53,840
120,640
134,310
43,000
951,260
1,885,000
19,000
3,000
36,295
371,000
19,200
8,900,000
2,033,769
2 ,200
61,320
1,630,800
2 ,262
253,000
1,658,960
58,393,000
49,879,325
8, 513,047
-' 17 Western States.
— From Irrigation Journal Surveys.
& Not available.
4/ 1 acre equals 0.4046 hectares.
15
-------
TABLE 2. WATER DIVERTED AND CONSUMED FOR IRRIGATION BY STATES, 1975. (MURRAY & REEVES ,
cn
State
Alabama
Alaska
Arizona
Arkansas
California
Colorado
Connecticut
Delaware
Florida
Georgia
Hawa i i
Idaho
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Missouri
Montana
Nebraska
Nevada
New Hampshire
New Jersey
Acres
irrigated
( 1 ,000!
acres)
32
0
1,400
1,400
9,000
3,100
T5
22
2,000
120
140
3,800
68
43
57
3,000
10
780
21
22
39
no
140
390
260
2,400
5,600
860
6.0
130
Total water withdrawn
(liOOO acre-feet per year) / Freshwater
/ cons umed
Ground
water
7.2
0
4,700
2,300
18,000
2,800
.4
14
1,400
26
480
3,900
32
26
21
5,200
.1
900
0
4.6
12
27
26
620
100
120
5,900
590
0
120
Surface
water
17
0
3,100
390
20,000
7,500
4.4
2.1
1,800
44
580
13,000
14
11
2.6
370
2.9
1,300
9.5
5.9
25
44
26
140
6.0
12,000
2,300
2,900
6.1
40
Re-
claimed
sewage
0
0
60
0
180
90
0
0
0
0
0
6.2
0
0
0
0
0
0
0
.2
0
0
0
0
0
0
0
3.7
0
0
(1 ,000
All ac-ft/yr)
water
24
0
7,900
2,700
39,000
10,000
4.8
16
3,200
71
1,100
17,000
46
37
23
5,600
3.0
2,200
9.5
11
37
72
52
750
110
12,000
8,200
3,500
6.1
160
24
0
6,000
2,000
23,000
5,700
4.8
16
1,400
71
560
5,300
46
37
23
4,300
2.9
2,200
9.5
10
37
72
52
380
85
3,000
6,400
1,700
6.0
120
Convey-
ance loss
( 1 ,000
ac-ft/yr)
0
0
280
190
5,900
1,200
0
0
240
0
500
4,800
0
0
0
120
0
690
0
0
0
0
0
75
2.5
2,800
1 ,700
800
0
0
Total water withdrawn r
(million gallons per day) /
Ground
water
6.6
0
4,200
2,100
17,000
2,500
.4
12
1,200
24
430
3,500
29
24
18
4,600
.1
810
0
4.1
11
24
24
550
91
110
5,200
530
0
no
Surface
water
15
0
2,800
350
18,000
6,700
3.9
1.8
1,600
40
520
1 2 ,000
12
10
2.2
330
2.6
1,100
8.5
5.2
22
40
23
120
5.5
1 1 ,000
2,100
2,600
5.4
36
Re-
claimed
sewage
0
0
54
0
160
80
0
0
0
0
0
5.6
0
0
0
0
0
0
0
.2
0
0
0
0
0
0
0
3.3
0
0
All
water
22
0
7,000
2,400
35,000
9,300
4.3
14
2,900
63
950
15,000
41
34
21
5,000
2.7
1,900
8.5
9.5
33
M
47
670
96
11,000
7,300
3,100
5.4
140
Freshwater
consumed
(mgd)
22
0
5,400
1 ,800
21,000
5,100
4.3
14
1 ,300
63
500
4,700
41
33
21
3,800
2.6
1,900
8.5
9.4
33
64
47
340
76
2,700
5,800
1 ,500
5.3
no
Convey-
ance
loss
(mgd)
0
0
250
170
5,300
1,000
0
0
220
0
450
4,300
0
0
0
110
0
610
0
0
0
0
0
67
2.3
2,500
1,600
720
0
0
(Continued)
-------
TABLE 2. (Continued)
State
New Mexico
New York
North Carolina
North Dakota
Ohio
Okl ahoma
Oregon
Pennsylvania
Rhode Island
South Carolina
South Dakota
Tennessee
Texas
Utah
Vermont
Virginia
Washington
West Virginia
Wisconsin
Wyomi ng
District of
Columbia
Puerto Rico-
Virgin Islands
United States
Acres
irrigated
Total water withdrawn /
(1,000 acre-feet ner year)-i/
(1,000 3/
acres)
1,100
83
500
130
41
1,000
2,100
29
3.8
42
200
19
8,600
1,700
2.3
44
1,600
2.4
130
1,700
0
66
54,000
Ground
water
1,500
21
59
54
6.2
1,100
1,000
6.9
.5
10
55
3.6
10,000
540
.4
4.2
260
0
57
300
0
100
63,000
Surface
water
1,800
15
38
130
14
180
5,700
32
4.7
22
320
6.1
2,600
3,300
2.0
18
5,900
1.4
22
7,300
0
160
94,000
Re-
claimed
sewage
0
0
0
0
0
0
4.0
0
0
0
0
0
60
1.0
0
0
0
0
0
0
0
0
410
All
water
3,200
36
97
180
20
1,300
6,700
39
5.
32
370
9.
13,000
3,900
2.
22
6,200
1.
79
7,600
0
260
160,000
Freshwater
consumed
(1,000
ac-ft/yr)
1,600
35
97
170
18
910
3,400
39
2 5.2
32
200
7 9.0
12,000
2,400
4 2.4
13
2,500
4 1.4
62
2,200
0
160
89,000
Convey-
ance loss
(1,000
ac-ft/yr)
24
0
0
18
0
16
1,900
0
0
0
160
.7
480
430
0
3.4
1,200
0
0
1,800
0
60
25,000
Total water withdrawn /
(million gallons per day) j>/
Convey-
Freshwater ance
Ground
water
1,300
19
53
48
5.5
1,000
920
6.1
.4
8.9
49
3.3
9,400
480
.4
3.7
230
0
51
270
0
89
57,000
Surface
water
1,600
13
34
120
13
160
5,100
28
4.2
20
280
5.3
2,300
3,000
1.8
16
5,300
1.2
20
6,500
0
140
84,000
Re-
claimed
sewage
0
0
0
0
0
0
3.6
0
0
0
0
0
53
.9
0
0
0
0
0
0
0
0
360
All
water
2,900
32
87
160
18
1,200
6,000
34
4.6
29
330
8.6
12,000
3,500
2.2
20
5,500
1.2
71
6,800
0
230
140,000
consumed
(mgd)
1,400
32
87
150
16
820
3,000
34
4.6
29
180
8.1
1 1 ,000
2,200
2.2
12
2,200
1.2
56
2,000
0
140
80,000
loss
(mgd)
21
0
0
16
0
14
1,700
0
0
0
150
.7
430
390
0
3.0
1,000
0
0
1,600
0
54
23,000
— Including Puerto Rico and Virgin Islands.
2/
— Partial figures may not add to totals due to rounding.
— One acre equals 0.4046 hectares.
4/
— One acre foot equals 0.1233 hectare-meters.
— 1 mgd (million gallons per day) equals 0.0183 m3/sec, also equals 3.07 acre-feet per day.
-------
WATER
WITHDRAWN
FRESHWATER
CONSUMED
RURAL
( I percent)
PUBLIC SUPPLIES RURAL
(7percent) (4 percent)
INDUSTRY
(6 percent)
INDUSTRY^IRftlGATiON
(58 percent)M34 percent)
PUBLIC SUPPLIES
(7 percent)
420,000 million gallons
(1,590 million ms)
per day withdrawn
IRRIGATION
(83 percent)
96,000 million gallons
(363 million m5)
per day consumed
Figure 6. Water withdrawals and consumption in the United States for
1975 (Murray and Reeves, 1977).
agricultural output (FWPCA, 1968). Other factors have also contributed to
spectacular gains in productivity. The use of more fertilizer, increased
plant populations per acre, improved crop varieties, advanced methods and
means of pest control and improved water management have all contributed to
high farm productivity. In addition to the increased productivity of
irrigated agriculture, several other factors including technological
changes have increased the irrigated acreage. These include improved water
storage, water conveyance, and irrigation water application methods such as
sprinkler irrigation systems and trickle irrigation. Evaporation suppression,
weather modification, and phreatophyte control or eradication also have made
more water available for agricultural use. Population growth and population
shifts have necessitated increased crop productivity due to larger local
demands for fresh fruits and vegetables as well as dairy and poultry products.
The Water Resources Council (1968) estimated all water requirements to
the year 2020 (Table 3). Their calculations indicate that use of water in
irrigated areas will increase by more than 50 percent during the 1965-2020
period. Most of this expanded water use will be in the midwest and southeast
portions of the United States.
Large increases in irrigated acreage are due to the development of
sprinkler and trickle irrigation systems« These irrigation methods, with
18
-------
•Port consumed
100 million gollons
per day (or less)
Note Areo of circle indicotes water use.
HAWAI 1
)
1>
CARIBBEAN
Figure 7. Estimated consumption of irrigation water and irrigation
return flows by water resource regions in the United States for
1975 (Murray and Reeves, 1977).
Note: 1 million gallons per day equals 0.0183 m3/sec and equals
3.07 acre-feet per day.
19
-------
TABLE 3. ESTIMATED WATER USE ANp PROJECTED REQUIREMENTS, (U.S. WATER RESOURCES COUNCIL, 1968)
MILLION GALLONS DAILY)-'
. , ••— " "" "" — — • — • •- - — '•' • ' • •
TYPE OF USE
Rural domestic
Municipal (public-supplied)
tvj Industrial (self-supplied)
0 Steam-electric power: . .
Fresh .
Saline
Agriculture:
Irrigation
Livestock
Total
Used
1965
2,
23,
46,
62,
21,
110,
1,
269,
351
745
405
738
800
852
726
617
Projected Requirements
1980
2000
Withdrawals
2,474 2,852
33,596 50,724
75,026 127,365
133,963 259,208
59,340 211,240
135,852 149,824
2,375 3,397
442,626
804
,610
2020
3
74
210
410
503
160
4
1,368
,334
,256
,767
,553
,540
,978
,660
,088
Used
1965
1,636
5,244
3,764
659
157
64,696
1,626
77,782
Projected Requirements
1980
2000
Consumptive Use
1,792 2,102
10,581 16,478
6,126 10,011
1,685 4,552
498 2,022
81,559 89,964
2,177 3,077
104,418
128,206
2020
2,481
24,643
15,619
8,002
5,183
96,919
4,238
157,085
— I million gallons per day = 0.0183 m3/sec.
-------
increased flexibility and efficient water control, have enabled irrigation
of more kinds of soils than surface water application methods allow. There-
fore, more land can be classified as irrigable. As a direct result, many
thousands of hectares, which were previously considered suitable only for
dryland farming or as wasteland, are being irrigated today and producing
high crop yields. This phenomenon is particularly evident in eastern
Colorado, Nebraska, and Kansas. Also, the higher application efficiencies
in established irrigated areas, due to these methods plus seepage control
by lining conveyance systems, have permitted an expansion of the irrigated
area with the "saved" water, sometimes at the expense of other irrigated
areas depending on return flows.
Development of new water sources such as groundwater in California,
Arizona, New Mexico, Texas, Oklahoma, Kansas, Nebraska, and Idaho has been
very important for the irrigation development of these areas. Figure 8 in-
dicates the United States trends in use of irrigation water supplies, where
the increased use of groundwater is clearly evident. This rapid development
of groundwater has presented numerous problems with existing surface water
rights, land subsidence, and groundwater pollution. Many areas that utilize
groundwater supplies for irrigation are severely affected by increased costs
for energy. Also, in an effort to reduce energy costs, many growers in
Nebraska and Kansas use only off-peak electric power for pumping. Off-peak
power is power which is available at times other than at peak demand intervals
and has a lower cost per kilowatt-hour.
Areas in the United States, specifically the Midwest and Southeast,
that annually receive sufficient precipitation to satisfy crop requirements
have installed supplemental irrigation systems. This is necessary since
rainfall does not often fall at exactly the right time in the right quantities.
Timely irrigations at a critical crop growth stage, applying only 0.5 to 1
foot (150 to 300 mm) of water per year, can more than double yields.
Sprinklers form a large number of these supplemental irrigation systems due
to their adaptability to large topographic differences and soil conditions,
In 1950, there were about 26 million acres of irrigated land.in the
United States of which more than 92 percent was in the 17 western states.
In 1974, this dropped to 89 percent (U.S. Department of Commerce, 1976) and
in 1977 it dropped to 85 percent (Irrigation Journal, 1978). The percentage
of irrigated agricultural land in other areas of the United States is ex-
pected to increase even more in the future.
TRENDS IN AGRICULTURAL WATER USE
The amount and distribution of water in the future will depend on
several factors; primarily climate, cropping patterns, irrigated acreage and
level of efficiency attained by the irrigators. The projected water consump-
tion for all uses is presented by regions in Table 4. Regional projections
of irrigated land, irrigation withdrawal and irrigation consumption are
presented in Tables 5, 6, and 7.
21
-------
180
160
•g 140
a>
a.
120
c
o
o 100
o>
I 80
CD
60
40
20
0
Surface water
Ground water
1
I
1
I
o
CD
-------
NJ
U)
TABLE 4. REGIONAL ESTIMATIONS OF WATER USE AND PROJECTED REQUIREMENTS (U.S.
COUNCIL, 1968) (MILLION GALLONS DAILY).!/
WATER RESOURCES
REGION
North Atlantic
South Atlantic-Gulf . . .
Great Lakes
Ohio
Tennessee
Upper Mississippi ....
Lower Mississippi ....
Souris— Red— Rainy
Missouri
Arkansas-White-Red ....
Texas-Gulf
Rio Grande
Upper Colorado
Lower Colorado
Great Basin
Columbia-North Pacific . .
California
Total
Used
1965
37,467
20,560
25,119
28,255
5,767
8,179
5,571
391
19,344
9,410
16,410
7,289
4,017
6,913
5,115
29,631
37,300
162
1,597
1,120
269,617
Projected Requirements
1980
54
53
47
41
12
14
12
23
17
29
8
5
8
7
41
56
2
4
442
2000
Withdrawals
,920 113,860
,180 87,440
,893 96,594
,749 65,109
,252 13,877
,800 30,587
,816 27,967
936 2,002
,264 27,876
,279 25,336
,080 57,330
,330 9,510
,675 6,575
,497 8,428
,055 7,550
,407 90,135
,290 120,510
535 901
,658 4,698
,010 8,325
,626
804
,610
2020
236
130
190
90
18
41
39
2
31
31
92
11
6
8
7
156
244
4
8
13
1,368
,290
,190
,960
,163
,106
,266
,442
,758
,572
,589
,640
,680
,725
,889
,800
,735
,760
,206
,587
,730
,088
Used
1965
2,023
2,695
1,199
1,134
331
770
1,470
77
10,554
5,874
7,289
4,403
1,982
3,448
2,253
10,521
20,944
12
533
270
77,782
Projected Requirements
1980
2000
2020
Consumptive Use
2,870 4,960 8,490
3,395 5,655 8,265
1,881 3,183 5,484
1,619 2,539 3,623
572 834 1,132
1,103 1,778 2,624
3,012 4,453 6,251
215 494 544
13,160 14,979 16,378
8,482 10,587 12,329
9,435 10,890 12,300
4,676 4,991 5,466
2,700 3,100 3,140
4,075 4,645 5,310
3,299 3,562 3,776
13,581 17,325 21,616
29,205 32,660 38,190
50 96 184
728 1,000 1,368
360 475 615
104,418
128,206
157,085
— 1 million gallons per day = 0.0183 mVsec = 3.07 acre-feet per day.
-------
TABLE 5. REGIONAL PROJECTIONS OF IRRIGATED LAND IN THE CONTERMINOUS UNITED STATES (U.S. WATER
RESOURCES COUNCIL, 1968) (THOUSANDS OF ACRES)!/
REGION
1960
1965
1980
2000
2020
North Atlantic 240 310 380 550 700
South Atlantic-Gulf 850 1,500 1,800 2,750 3,750
Great Lakes 100 140 230 350 470
Ohio 35 55 90 180 260
Tennessee 15 20 30 40 50
Upper Mississippi 80 140 210 390 550
Lower Mississippi 700 900 2,100 3,050 4,150
Souris-Red-Rainy 10 15 90 240 250
Missouri 6,600 7,400 8,050 9,000 9,600
Arkansas-White-Red 3,100 3,800 5,600 6,400 6,850
Texas-Gulf 5,100 5,500 5,500 5,500 5,500
Rio Grande 1,950 2,000 2,050 2,050 2,050
Upper Colorado 1,370 1,440 1,800 2,000 2,000
Lower Colorado 1,520 1,660 1,750 1,800 1,800
Great Basin 1,700 1,860 1,950 2,000 2,000
Columbia-North Pacific 5,450 6,250 7,700 9,500 11,200
California 8,420 8,850 10,150 10,750 11,100
Total 37,240 41,840 49,480 56,550 62,280
— 1 acre = Q.4046 hectares,
-------
TABLE 6. REGIONAL PROJECTIONS OF IRRIGATION WITHDRAWALS (U.S. WATER RESOURCES COUNCIL, 1968)
_ (MILLION GALLONS DAILY)^-7 _
REGION
1965
1980
2000
2020
North Atlantic 151
South Atlantic-Gulf 3,270
Great Lakes 75
Ohio 24
Tennessee 8
Upper Mississippi 95
Lower Mississippi 1,320
Souris-Red-Rainy 24
Missouri 16,039
Arkansas-White-Red 6,960
Texas-Gulf 7,450
Rio Grande 6,671
Upper Colorado 3,880
Lower Colorado 6,400
Great Basin 4,575
Columbia-North Pacific 26,400
California 26,200
Subtotal 109,542
Alaska ~U~
Hawaii 1,060
Puerto Rico 250
Total 110,852
230
3,900
110
40
18
110
3,030
200
19,300
9,400
9,400
6,840
5,300
7,700
6,200
31,400
30,950
134,128
4
1,420
300
330
6,000
170
80
23
200
4,400
562
21,600
10,700
9,000
6,840
5,350
7,000
6,100
37,500
31,700
147,555
9
1,910
350
420
8,200
230
115
29
280
6,000
576
23,000
11,500
8,500
6,840
4,900
6,500
5,800
42,500
32,600
157,990
2,570
400
135,852
149,824
160,978
— 1 million gallons per day
— Insignificant.
= 0.0183 m3/sec = 3.07 acre-feet per acre.
-------
TABLE 7. REGIONAL PROJECTIONS OF IRRIGATION CONSUMPTION (U.S. WATER RESOURCES COUNCIL, 1968)
(MILLION GALLONS DAILY)—
I/
REGION
1965
1980
2000
2020
to
CTi
North Atlantic 150
South Atlantic-Gulf 1,400
Great Lakes 68
Ohio 24
Tennessee 8
Upper Mississippi . . 83
Lower Mississippi 890
Souris-Red-Rainy 24
Missouri 9,798
Arkansas-White-Red 5,030
Texas-Gulf 5,810
Rio Grande 4,165
Upper Colorado 1,934
Lower Colorado 3,170
Great Basin 2,100
Columbia-North Pacific 10,050
California 19,290
Subtotal 63,994
Alaska 'z7~
Hawaii 477
Puerto Rico 225
Total 64,696
230
1,600
95
40
16
95
2,180
150
12,100
6,800
7,100
4,270
2,600
3,630
3,040
12,900
23,800
80,646
3
640
270
330
2,450
140
80
21
170
3,170
402
13,500
7,800
7,100
4,270
2,880
3,760
3,110
15,900
23,700
88,783
860
315
420
3,350
190
115
26
240
4,320
416
14,400
8,300
7,100
4,270
2,880
3,760
3,110
18,700
23,800
95,397
12
1,150
360
81,559
89,964
96,919
~~ 1 million gallons per day
— Insignificant.
= 0.0183 m /sec.
-------
in that climate. It is expected that the increasing trend towards fewer
but larger farms will persist.
Future expansion of irrigation in the West will face critical scrutiny
because of environmental concerns and cheaper production of many food
materials elsewhere. New water demands will create policy emphasis towards
water resource development via water markets (or water banking), and manda-
tory increases in irrigation efficiency, improved water management practices
and other water conservation measures. This would not necessarily result
in decreased acreage of irrigated cropland, but would require much higher
application efficiencies with an expected shift towards better irrigation
methods. The total amount of water use for irrigation would not decline,
but the areal use patterns will shift. More emphasis will be placed on new
food production technologies and plant genetic research. Future increases
in crop production will be contingent upon more effective management
practices in the utilization of fertilizers and pesticides with concern for
their effects on the environment.
There will be more emphasis towards reuse and desalination or other
treatment of existing water supplies, There will be expanded use of
irrigated agriculture as tertiary treatment from municipal sewage or indus-
trial effluent, and for disposal of thermal pollution (waste heat) from
steam power generator facilities. In these instances, there will be an
even greater concern for controlling the irrigation return flow from these
areas, and managing the quality and quantity of these flows.
27
-------
SECTION 3
IRRIGATED AGRICULTURE AND SALINITY PROBLEMS
Irrigation is one of the most important agricultural practices developed
by man. Irrigation, practiced in some form since the earliest recorded
history of agriculture, was the economic base for many ancient civilizations.
Today, as then, irrigated farming not only increased productivity, but
provides flexibility by allowing shifting from the relatively few dryland
crops to other higher value crops such as corn, cotton, and sugar beets that
are in greater demand. Irrigation strengthens other facets of a region's
economy since it usually creates more employment opportunities than rain-fed
agriculture through its intensive and diversified cultivation, stimulation
of agri-businesses, the provision of products for export, and creation of a
healthy domestic market (Skogerboe and Law, 1971).
Irrigation, however, is not without problems. To maintain productivity
in irrigated agriculture, salts applied onto the croplands through irrigation
water must be moved below the root zone in order not to impair plant growth.
Therefore, it is mandatory that water diverted to a crop exceed the actual
water requirement of the plants to include sufficient water for evapotranspi-
ration, leaching, seepage losses, and other transit or ditch losses, which
in many cases, are substantial.
The quantity of irrigation water diverted from a river usually far
exceeds the cropland water requirement. Data from many irrigation regions
indicate that seepage losses from canals and laterals throughout the irriga-
tion water delivery systems are high. Excess water in the delivery system is
usually bypassed back to the stream and surface flows. Added to this problem
is the overapplication of water on farm fields, which results in excessive
surface runoff from the lower end of the field (tailwater runoff), and/or
excessive quantities of water moving below the root zone (deep percolation).
This surface and subsurface water which returns to the stream is referred to
as irrigation return flow.
In some areas, the combination of seepage and deep percolation losses
can cause groundwater levels to rise near the surface (waterlogging), which
is undesirable. In many irrigated regions, groundwater levels are suffi-
ciently close to the root zone that water and salts are supplied to that
region by upward movement of groundwater due to capillarity. When upward
moving water reaches the soil surface and evaporates, salts are left behind
on the ground surface. This process of salinization not only results in
declining agricultural production, but has caused land to become barren.
28
-------
In some areas, such as the South Platte River Basin in Colorado, deep
percolation and seepage losses are politically and socially desirable since
many of the water rights (surface and groundwater) are dependent on the
irrigation return flow. In this area, the groundwater is not seriously
polluted with salts, except possibly nitrates, and salinity increases are
primarily due to concentrating effects.
Historically, some degree of salt concentration due to irrigation was
tolerated as the price for development (Law and Skogerboe, 1972). In some
areas, however, there has been so much laxity that water and land quality
degradation has become a serious matter. As pressures on water resources
become greater due to the necessity to increase the quantity and quality of
food production, there is a mounting concern for control of water quality
deterioration and soil salinization. The need for more precise information
as a basis for wise policy action is of critical importance (Skogerboe and
Law, 1971).
The major problems resulting from irrigation are due to the basic fact
that plants are large consumers of water resources. Growing plants extract
water and leave salts behind. This results in a concentration of the
dissolved mineral salts that are present in all natural water which is applied
to the land. Irrigation also adds to the salt load by leaching natural
salts arising from weathered minerals occurring in the soil profile, or
deposited in the geologic substrata. Irrigation return flows, surface and
subsurface, provide the vehicle for conveying the concentrated salts and
other pollutants to a receiving stream or groundwater reservoir.
Whenever water is diverted from a river for irrigation use, the quality
of the return flow becomes degraded. The degraded return flow then mixes
with the natural flows in the river system. This mixture is then diverted
by downstream users to satisfy their water demands. This process of diver-
sion and return flow may be repeated many times along the course of a river.
In the case of the original diversion, if the increase in pollutants con-
tained in the return flow is small in comparison to the total river flow,
water quality might not be degraded to an extent that it would be unfit for
use. However, if the quantity of pollutants, such as salinity, in the return
flow is large in relation to the river flow, it is likely the water will not
be suitable for the next user unless objectionable mineral constituents are
removed. In fact, the total salt burden of many of our western streams may be
as much as 40 percent man-caused (Law and Bernard, 1970) . It must be
remembered that in many areas the mass emissions of salt are not linearly
proportional to the volume of irrigation return flows, and this relationship
must be determined before formulating the best management practices for that
area.
The amount of pollution which can result from irrigated agriculture is
dependent upon the water management practices, agricultural (cultural) prac-
tices and soil chemical and physical properties. Water management practices
include the operation and maintenance of the water conveyance systems and
the water application method including the amount, timing and frequency of
the irrigations. Agricultural practices include seedbed preparations,
planting, and other tillage operations, as well as fertilizer and chemical
applications.
29
-------
Since the same water is usually diverted many times from the major
rivers, there is a continual water quality degradation in the downstream
direction. Consequently, as water resources become increasingly utilized
without controls, the quality in the lower reaches of the river will likely
be degraded to such a point that remaining flows will be unsuitable for many
uses. It is even possible that the waters arriving at the lower portion of
the river basin have become so polluted that some existing uses must be
discontinued. The Utah State University Foundation (1969) presents an
excellent discussion of the characteristics and problems of pollution
associated with irrigation return flows.
Major Water Problems
Water Quantity—
In most arid and semi-arid areas of the world, there is the problem of
proper timing of water availability. For example, in the western United
States, most of the water which is available for irrigation results from
runoff from the high mountain snowpacks. This surface runoff is usually
exhausted by late June, and without storage facilities such as reservoirs,
there would be very little water for the maximum irrigation demands which
occur in July and August. Due to the time lag or storage effects, subsurface
irrigation return flows in these areas will often increase the natural flows
in the streams, resulting in a larger quantity of water available later in
the season.
Maintaining or increasing agricultural productivity in an irrigated area
requires that a favorable salt balance be achieved in the root zone.
Furthermore, the salt in the applied water must be leached to deeper soil
horizons, groundwater or the drainage system to the extent that the mass of
salts leaving the area must equal or exceed that received in the water supply.
It is important that the appropriate amount of water is applied, otherwise,
groundwater levels will rise until the water table is near the ground surface.
In this case an expensive relief drainage facility would have to be constructed.
Thus, a balance must be reached such that a sufficient amount of water is
applied to the croplands to supply moisture for crop growth and leaching of
salts from the root zone, but not so much that groundwater levels approach
the surface. Historically, many societies applied too much water to their
lands, and this continues in many areas of the world today.
In order that water does not limit crop yields, a suitable amount of
water must be applied to the cropland at the right times. The timing and
quantity of required irrigation water is primarily a function of climate,
soils, and the growth stage of the crop.
In many regions, increasing urbanization and industrialization requires
the reallocation of agricultural water in order to meet new water demands.
To accommodate these water reallocations, improved water management practices
must be instituted by the agricultural sector, thereby reducing agricultural
water diversion requirements.
Improved irrigation practices will actually save or conserve little
water, except for reductions in consumptive use by phreatophytes and
30
-------
evaporation from high water tables areas. Improved practices do improve
water quality and affect the time distribution of the natural stream flows.
However, improved irrigation practices will generally not result in more
water being available in the river for fishery enhancement and recreational
.uses. For example, when considering the consumption of water in an irrigated
area, the only actual water losses, that is, water permanently removed from
the area, are due to evapotranspiration from crops and phreatophytes, which
is fairly constant on a yearly average, or water that percolates to "deep"
aquifers which are too deep to have returns to the stream. The remainder of
the water is either surface or subsurface return flows. If the total
diversions to the cropland were reduced, this "excess" water might be avail-
able for redistribution to other upstream users if this water, which was
previously return flows, was not part of a downstream water right.
In some cases, improved irrigation practices and reduced return flows
may actually damage a downstream water right. This would result because of
a change in the temporal distribution of stream flows. For example, the
South Platte River in Colorado was an intermittent stream before the intro-
duction of irrigated agriculture in the drainage area. Presently, the stream
flows continuously throughout the year as a result of the irrigation return
flows. In this case, the existing recreational facilities and downstream
water rights might be adversely affected by the rigid implementation of best
management practices in the area. In this example, there are also interstate
agreements which must be met. The above example illustrates the site-
specific nature of irrigation return flow problems.
Water Quality—
The quality of water coming from most upland watersheds in the West is
excellent. At the base of hills or mountain ranges, large quantities of water
are diverted to valley croplands. Much of the applied water, perhaps 50 to
70 percent, is lost to the atmosphere by evapotranspiration. The remaining
water supply is irrigation return flow. This return flow will be either
surface runoff (spillage and field tailwater), shallow horizontal subsurface
interflows, or will move vertically through the soil profile (seepage and
deep percolation) until it reaches a perched water table or the groundwater
reservoir. There it can be pumped and used again or be transported through
the groundwater reservoir until it reaches a river channel from which it is
rediverted further downstream.
Water transpired by plants and water lost by evaporation from soil and
open water surfaces is salt free. Water that percolates through the soil
profile contains the majority of the salts left by the consumed water and
contains a higher concentration of salts than the applied water. This is
referred to as the "concentrating" effect. This concentrating effect can
also result from interbasin diversions of high quality water, which reduces
the water in the river for dilution. As water moves through the soil
profile, it may dissolve additional naturally occurring salts resulting from
the weathering of the soil minerals. Some salts will react with other chemi-
cals in the soil and be precipitated, while an exchange between some salt
ions in the water and soil occurs simultaneously. Additional salts may be
picked up by deep percolation passing through salt bearing strata in its way
to the stream-drainage system (Law and Bernard, 1970). Salts picked up by
31
-------
the water, which are in addition to the salts applied to the land, are termed
salt "pickup." The total salt load is the net sum of the original salt in
the applied water resulting from the concentrating effect plus the salt
pickup.
Whether irrigation return flows come from surface runoff or return to
the system via the soil profile, water can be expected to undergo a variety
of quality changes due to varying exposure conditions. Surface return flows
consist mainly of precipitation and tailwater runoff from irrigated land.
Because of its limited contact and exposure to the soil particles, the
following changes in quality might be expected between application and runoff:
(a) an increase in sediments and other colloidal material, which results in
the slightly increased concentration of dissolved solids, the addition of
variable amounts of pesticides, and the addition of variable amounts of
fertilizer elements; (b) an increase in crop residues and other debris floating
from the soil surface; and (c) increased bacterial content (Utah State
University Foundation, 1969).
Subsurface flows that move through the soil profile will be affected
differently than the surface runoff. Because of its more intimate contact
with the soil and the dynamic soil-plant-water regime, the following changes
in quality are predictable: (a) a considerable increase in dissolved solids
concentration; (b) a change in relative distribution of various cations and
anions; (c) a variation in the total salt load depending on whether there was
deposition or leaching; (d) little or no sediment or colloidal material; (e)
increase in nitrate content; (f) little or no phosphorus content; (g) a
general reduction of oxidizable organic substances; and (h) a reduction of
pathogenic organisms and coliform bacteria. Each type of return flow will
affect the receiving water in proportion to respective discharges and the
relative quality of the receiving water (Utah State University Foundation,
1969).
The quality of irrigation water and return flow is determined largely
by the amount and nature of the dissolved and suspended materials it contains.
In natural waters, the materials consist of dissolved inorganic salts
leached from rocks and soil minerals after contact by water. Irrigation,
municipal and industrial use and reuse of water concentrates these salts
and adds additional kinds and amounts of pollutants. Many insecticides,
fungicides, bactericides, herbicides, nematocides, as well as plant hormones,
detergents, salts of heavy metals, and many organic compounds, leave water
less usable for irrigation and other uses. From this information it can be
concluded that increased salinity concentrations and salt loading result
primarily from subsurface irrigation return flows. A schematic representation
of these processes is presented in Figure 9.
SOIL-WATER-PLANT RELATIONSHIPS
Soil, water, and plant relationships of particular importance to
irrigated agriculture include: (a) evapotranspiration; (b) capacity of the
soil to hold water and still be well drained; (c) flow characteristics of
water in the soils; (d) physical properties of the soil matrix including
32
-------
IRRIGATED AGRICULTURE
WATER MANAGEMENT PRACTICES
AGRICULTURAL PRACTICES
Water Use
Land Use
Use of Agr. Chemicals
Conveyance
Diversion and
Delivery System
SOURCES OF
RETURN FLOW
Seepage Losses
(Subsurface
Return Flows)
Deep Percolation
(Subsurface Return
Flows)
i DEGRADING
CONSTITUENTS: I
Salinity
\v
Salinity
Nitrates
Degraded Irrigation Return Flows
Tailwater Runoff
(Surface Return
Flows)
1
Sediment
Phosphates
Crop Residue
Biocides
^^
THE IMPACT
Deterioration of Ground Water and Surface Water Quality
Figure 9. Schematic representative of the potential for nonpoint source
pollution from irrigated agriculture (Radosevich & Skogerboe,
1977).
-------
organic matter content, soil depth, soil texture, and soil structure; and
(e) soil chemical relationships including translocation and precipitation
of soluble salts and nutrients due to movement, use, and evaporation of the
soil water. Knowledge of all these relationships and their influence on each
other is critical to those desiring to improve irrigation practices and
obtain the best, most efficient use of water.
Evapotranspiration
The total evaporation occurring from soil and plant surfaces and the
plant transpiration (evaporation from the parenchyma cells through the
stomatal cells) is called evapotranspiration (ET). In addition to ET,
plants will use a small amount of water in tissue building. The sum of the
ET and the water use in tissue building is called consumptive use. However,
because the water removed by plant tissues is usually very small compared to
ET, the terms consumptive use and evapotranspiration are commonly used
interchangeably. Evapotranspiration (ET) can be measured by several methods
which are discussed in Section 8. These measured values are used to calibrate
the numerical calculation of ET for both crops and phreatophytes to local
conditions. Phreatophytes are water-loving vegetation such as cottonwood
trees, willows, salt cedars and cattails, which from an agricultural view-
point, nonbeneficially consume large amounts of water.
Soil Moisture
If there is either excessive water (waterlogging), or insufficient
water, crop growth will be retarded. While irrigation is an artificial
means of adding*water to the soil to prevent moisture deficiencies, poor
irrigation practices can create a waterlogging problem. As commonly defined,
the available moisture is the water which is held in soils at a negative
apparent pressure range from one-third bar (field capacity) to 15 bars (perma-
nent wilting point). However, the available moisture content within this
pressure range varies from 25 cm per meter of soil depth (3 in/ft) for some
silty loams, to less than 6 cm per meter of soil depth (0.75 in/ft) for some
sandy soils. As a consequence, soil type can greatly influence irrigation
management practices.
Soil at field capacity is a soil which is holding the moisture by capil-
larity only, which cannot be easily drained by the pull of gravity. When a
plant has pulled enough of the moisture out of a soil, it begins to wilt
because the soil cannot supply sufficient water to meet the plant needs.
This is known as the wilting point. The amount of water between field capa-
city and the wilting point is referred to as total available moisture or total
available water. The percentage of water at the permanent wilting point is
usually less than half that at field capacity, but is still much greater
than the water content of air dried soil (Figure 10).
When available soil moisture is below a 50 to 70 percent moisture
depletion and approaching the permanent wilting point, the limited water
supply plays an important part in retarding plant growth. When a plant
becomes "stressed" due to deficiencies in soil water, several changes can
occur in the physiological processes of the plant. If these stresses occur
34
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during a critical stage such as flowering or fruiting (critical stage(s)
vary according to plant species), crop yields can be severely reduced. If
the plant is stressed below the permanent wilting point, it will probably not
recover.
40
a
a>
x»
55 30
o
E
20
in
u
0)
c
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Soil Physical Properties
The soil matrix serves several valuable functions, including serving
as a foundation to hold the plants upright. It must furnish nutrients and
provide a balance between areation and available moisture content.
Soil texture and structure influence the intermolecular forces and
"suctions" of water in unsaturated soils. These forces can be quite sub-
stantial and include the capillarity and attractive forces resulting from
close contact of soil particles. Soil texture and primarily soil structure
greatly influence pore sizes, distribution of pore sizes, and the permeabil-
ity of soils to air, water, and roots which is as important to crop growth
as an adequate supply of nutrients. In fact, the entire soil-water-plant
system is so interrelated that failure or lack of one component can cancel
the combined benefits of the others.
The depth of soil is important because it establishes the amount of
water and nutrients that can be stored, as well as the physical limits of the
root zone. A uniform deep soil without confining substrata is necessary to
have a well-drained soil. Shallow top soils can inhibit the rate and depth
of root growth as well as subsurface drainage, if bedrock or hard pan condi-
tions occur beneath the soil layer.
Proper irrigation practices are influenced by the degree of root
proliferation since the water supply available to the plant is limited to the
distribution and soil volume explored by the crop's root system. Different
crops have different root growth patterns, hence, different moisture
extraction patterns. Obviously, a shallow-rooted crop requires more frequent
irrigations than a deep wide-rooted crop, providing both have the same soil
conditions.
The depth of soil available for root exploration, even for a normally
deep-rooted crop, may not always be evident from cursory observation of test
holes. Marked changes in soil texture with depth warrant close inspection.
For example, a sandy layer within the potential root zone of a clay soil
may effectively impede root development by resisting water entry until the
overlying soil is saturated. In this case, when irrigation water is applied
in amounts necessary to replenish the potential root zone, the top layer
becomes saturated and reduces soil aeration with consequent yield depression.
Barriers to infiltration may also be caused by mineral deposits, such as
calcium salts or "caliche", or by cultural practices such as plowing and soil
compaction by farming operations which can form hardpans.
Good internal drainage of a soil profile is a necessary requirement for
continued irrigated agriculture. If not naturally occurring, artificial
means must be used such as mole drains or tile. In addition to soil layers
or mineral deposits mentioned above, internal drainage might be hampered by
a high water table. The presence of an impermeable layer or a shallow water
table greatly affects the distribution of salts within the soil profile
(.Khoades, 1974) .
36
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Soil Chemical Properties
Soils are generally classified by their chemical suitability for
irrigation on the basis of their soluble salt concentration and the exchange-
able sodium content. Both of these parameters depend on the soil moisture
content. In soil analyses, the standard moisture content for these tests
is the saturation moisture content. Valuable references in this area are
Richards (1954) and Black et al. (1965).
Soluble salt concentrations are generally assessed by measuring the
electrical conductivity of the saturation extract in units of millimhos/cm
at 25 degrees C. Different crops are affected to different degrees by the
soluble salt concentrations. These responses have been quantified in terms
of electrical conductivity by the United States Department of Agriculture
Salinity Laboratory in Agriculture Handbook No. 60 (Richards, 1954). These
results are shown in Table 8.
TABLE 8. CROP YIELD RESPONSE AT VARIOUS LEVELS OF ELECTRICAL CON-
DUCTIVITY OF THE SATURATION EXTRACT OF SOILS (RICHARDS, 1954)
Electrical
conductivity
(mmhc/cm @ 25°) Crop response
0 to 2 Salinity effect on yield negligible
0 to 4 Yield of very sensitive crops reduced
0 to 8 Yield of many crops reduced
0 to 16 Only tolerant crops yield satisfactorily
> 16 Only very tolerant crops yield satisfactorily
Electrical conductivity is also measured in micromho/cm @ 25° C OjJmho')- -
which is 1/1000 of a millimho (mmho).
The standard method of assessing the effects of sodium is by determining
the exchangeable sodium percentage (ESP), where
Exchangeable sodium content
ESP ~ Cation exchange capacity
However, the direct technique for determination of the ESP is somewhat time
consuming and the easier, if slightly less reliable, sodium adsorption ratio
(SAR) is usually determined to estimate the exchangeable sodium content
(Withers and Vipond, 1974). The formula for the SAR is given by:
Soluble sodium concentration
SAR = '
v/Sol. calcium cone. + sol, magnesium cone.
2
37
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where the concentrations are expressed in millieqivalents per liter. An
empirical relationship between the ESP and the SAR has been obtained
(Richards, 1954) and is expressed by:
_ 100(-0.0126 + 0.01475 SAR)
1 + (-0.0126 + 0.01475 SAR)
If the ESP should exceed 15, the soils are classed as "sodic" and essentially
unusable for most agricultural purposes.
Chemical properties of soils can greatly influence the irrigability of
the soil by affecting the hydraulic characteristics and suitability of the
soil for crop production. Soils having an excess of soluble salts are
designated as saline soils, and, if the soil has an excess of exchangeable
sodium, it is termed a sodic soil. The basis of the classification of these
soils is given in Table 9. Sodic soils tend to have very poor soil structure
TABLE 9. CLASSIFICATION OF SOILS BY THE SOLUBLE SALT CONTENT AND
EXCHANGEABLE SODIUM PERCENTAGE (RICHARDS, 1954).
Electrical Conductivity
of saturation extract Exchangeable sodium
Soil (mmho/cm at 25° } percentage
Saline >4
Alkali (Sodic) <4
Saline Alkali >4
due to swelling or dispersion properties that tend to reduce the range of
pore sizes (Corey, 1977). This adversely affects the hydraulic properties
of the soil. For example, hydraulic conductivity of a soil can change as
much as three orders of magnitude when the sodium adsorption ratio is reduced
from a value of 20 to 1, providing all other soil properties are constant
(Dane, 1976).
Excess soil salinity will delay or prevent crop germination and can
substantially reduce the amount and rate of plant growth because of high
osmotic pressures that develop between the soil-water solution and the plant.
These pressures greatly impair the plant's ability to absorb water. The
osmotic pressure correlates well with total dissolved solids and appears to
be independent of the type of salts present because most soil solutions have
sufficiently similar distributions of mono and divalent ions. In addition,
some adverse effects due to salinity can include nutritional imbalances or
toxicities caused by specific ions such as boron which can be toxic in very
small quantities. In sufficient concentrations, even beneficial salts, such
as potassium nitrate, can become toxic to plants.
In addition to the soil chemical characteristics mentioned above, the
soil must also have an adequate supply of available plant nutrients.
38
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Many chemical elements are essential for plant growth and are necessary to
obtain large and satisfactory crop yields. These include calcium, iron,
magnesium, nitrogen, potassium, phosphorus, sulfur, and many other trace
elements depending on the type of crop. The availability of nutrients to the
plant largely depends upon the moisture content of the soil.
Bacterial activity is also an important part of the soil-water-plant
relationship since this action often converts nitrogen to a usable form.
This is called nitrogen-fixation. Bacterial action also breaks down organic
matter and converts other chemical compounds into forms usable by the plants.
Soil moisture content, structure, aeration, and organic matter (carbon source)
directly influence bacterial activity.
IRRIGATION WATER MANAGEMENT
Improved water management practices are needed to minimize diversions to
new croplands due to limited water supplies. Future diversions to irrigated
agriculture may have to be reduced to provide water supplies for new demands.
Minimization of water quality degradation in receiving streams resulting from
irrigated agriculture, and maximization of agricultural production on existing
croplands must be achieved. The solutions are identical. Whatever goal is
undertaken, better water management practices are mandatory.
Definition of an Irrigation System
An irrigation system can be subdivided into three major subsystems:
(a) the water delivery subsystem; (b) the farm subsystem; and (c) the water
removal subsystem. The water delivery subsystem can be subdivided further
into two components including the transport of water and pollutants from the
'headwaters of the watershed to the section of the river where water is
diverted to irrigated croplands, and the transport of water and pollutants
from the river diversion works to the individual farm (Skogerboe and Law,
1971) . This manual will only refer to the second portion of this delivery
system. The farm subsystem begins at the point where water is delivered to
the farm and continues to where water is removed from the farm. The farm
subsystem is defined vertically as beginning at the ground surface and termi-
nating at the bottom of the root zone. The water removal subsystem consists
of surface runoff from the lower end of the farm and water moving below the
root zone. These subsystems are illustrated in Figure 11.
Planning for Effective Water Management
The resource base for irrigated agriculture has not changed substantially
since its inception thousands of years ago. Development of surface water
irrigation in the West during the past hundred years has produced little
incentive for any major innovation to improve efficiency in the use of water,
which is a scarce resource. The provision of irrigation water in many
areas has been considered a governmental or collective responsibility,
and the direct charges made for water usually have not been high enough
to encourage proper budgeting of water or other necessary improved
irrigation methods. The custom of undercharging for water continues today;
39
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Open Drain
(Surface Removal)
r*
Of ^
Bottom
Root Zone
(b)Farm Subsystem (c)Water Removal Subsystem
IIliiiilliiMiili^
Subsurface Removal
(a)Water Delivery Subsystem
Figure 11. Water delivery, farm, and removal irrigation subsystems.
-------
with only a few regions charging the farmers for the real cost of water.
There are a few extremely water-scarce areas in the West where considerable
ingenuity was applied to effectively utilize water supplies. Pumped water
supplies do reflect the "true" cost of water and these areas are usually much
more innovative and willing to accept new practices.
Aggravating this situation of so-called "cheap water" is the fact that
development of irrigated agriculture in most places, even in the last few
decades, has focused almost entirely upon the construction of water delivery
subsystems. This preoccupation with installation of "hardware" results from
a naive civil engineering approach to water management (Wiener, 1972) . This
approach is probably the greatest deterrent to improved water management in
most irrigation systems today. This is especially true when other additional
problems require attention such as the need for improved soil-plant-water
management techniques, cultural practices, advisory services and input supply
systems, administration of institutions, water laws, water quality, and many
other factors that must fit together to form a most complex system. In
irrigated agriculture, there is a wide gap that frequently exists between
"hardware development" and the development of other requisites for increased
agricultural production.
The approach that has been applied to irrigated agricultural development
in the past is characterized by separating the development of water resources
from the management aspects of water resource utilization. The record shows
development being emphasized greatly while management is often neglected.
This orthodox approach was used almost exclusively with reasonable success in
the western United States. However, as the water resources become more fully
utilized, necessity for meeting new water demands, along with physical,
socio-economic, and political problems of water quality degradation, require
rejection of many conventional guidelines from the past.
In contrast to the mere development of a water'resources approach, the
"management" approach attempts to achieve water development objectives by
applying a variety of measures after studying the entire system, thereby
attempting to modify the system to meet new and changing demands* Instead
of constructing engineering structures to meet these new demands, focus
should be on water resource management, with construction works considered
only when necessary to meet water management objectives (Wiener, 1972).
Unfortunately, in most cases, water management and the many disciplines
required to produce efficient management are postponed until the post analysis
of an engineering project which aggravates not only the advance of technology,
but constrains or makes difficult the implementation of a variety of services
requiring strong institutional measures.
Operation and Management of an Irrigation System
The "heart" of the whole irrigation system is the farm subsystem. The
p-urpose of the irrigation system is to grow food. The connection between
the food and the irrigation system is the root zone. The water delivery and
water removal subsystems exist to support this "purpose." Proper operation
and management of an irrigation system requires, first, that the farm sub-
system be adequately designed, operated, and managed so the water delivery
41
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subsystem can be operated to provide proper quantities of water at the times
required by the plants. The most important constraint in the operation process
is the necessity to assure adequate drainage through the root zone in order
to maintain a favorable root-zone salt balance and provide good aeration to
enable continued agricultural productivity. At the same time, quantities of
water moving below the root zone can result in waterlogging and salinity
problems which can require expensive relief drainage facilities.
Farm Subsystem—
The most important irrigation variables in operating the farm subsystem
are climate, water, soils, crops, topography, method of irrigation, and the
skill of the irrigators. The interrelationships between these variables
dictate the capability of the land resource for producing food and fiber.
Frequently, in the arid West, water is the most limiting factor in agricul-
tural production. The capability of the available water supplies from pre-
cipitation, surface runoff, and groundwater for plant production is highly
dependent upon the efficiencies at which the water is used, which in turn
is a function of both economic and institutional factors. Use of proper
irrigation methods and practices are crucial for; (a) uniformly distri-
buting the necessary moisture throughout the field; and (b) minimizing deep
percolation losses so as not to aggravate problems in the water removal
subsystem such as waterlogging and salinity, and yet retain fertilizer where
it can be used by the plant.
Water Delivery Subsystem—
The crop water plus leaching requirements of each farm should dictate
the operation of the water delivery subsystem. These water requirements de-
termine the necessary quantities and timing of water deliveries at the farm
inlets. The water delivery network must be capable of handling these farm
water requirements.
One of the essential facilities for successfully operating an irrigation
conveyance network is adequate flow measurement devices. Each field has a
particular water requirement; the only means by which the proper amount of
water can be delivered is by measuring the water at the farm inlet. The
farmer cannot be expected to .use good water management practices if the
quantity of water being managed is unknown. Besides each farm inlet, a flow
measurement structure should be provided at all division points in the water
delivery subsystem to ensure equitable allocation of water among users. Water
measurement is discussed in more detail in Section 8.
A major problem in the water delivery subsystem is the institutional
framework controlling the operation of this portion of the irrigation system.
Generally, operation of the conveyance facilities was not related to the
requirements for sustaining long-term productive agriculture. In particular,
institutional factors such as water laws and the existing water cost
structures have constrained improved water management.
The primary requirement for sustaining an irrigation system is an
institutional framework that is compatible with the operational requirements
for the water delivery subsystem. The operational requirements, in turn,
must be dictated by water requirements for each farm as well as constraints
42
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imposed by the water removal subsystem. Thus, even if all three components
of the irrigation system are properly designed, lack of an adequate insti-
tutional framework for operating the system in accordance with good water
management criteria will likely lead to either failure of the system or at
least lower agricultural production levels.
The existing water rights system in the western United States is an
example of an institutional constraint to improved water management practices,
Water has been allocated to irrigated agriculture in the majority of the
17 western states under the doctrine of prior appropriation since the mid-
1800' s. The practice of granting water use on the basis of seniority has
become institutionalized and very resistant to change. Water has been di-
verted for agriculture since the inception of the doctrine and often by the
same conveyance and application practices employed when the right was
initiated. Fear of loss of the right through nonuse compels water users to
divert their full allotment so that change in irrigation water management
practices will occur only if strong incentives are provided. The objective
of the water right is satisfied once the water is applied. Water quality
degradation sustained downstream from surface (tailwater) runoff or sub-
surface (seepage and deep percolation) return flows is of little concern
to the appropriator. There are no water quality criteria associated with
most of the prior appropriation laws. The water rights system is discussed
in more detail later in this section.
Water Removal Subsystem—
The principal function of the water removal subsystem is to allow proper
drainage below the root zone so adequate aeration and the leaching of salts
from the root zone will occur. One of the most satisfactory mechanisms for
minimizing drainage needs is proper operation of the water delivery subsystem.
By appropriate operation, it is possible that a drainage problem will not
occur.
Another important consideration in the water removal subsystem is water
quality. If canal seepage and cropland deep percolation losses result in
water quality degradation of the underlying groundwater supplies, then use
of these supplies may become impaired. Also, return flows to the river may
limit the usefulness of the river water to downstream users. Numerous
examples of this situation can be cited throughout the West.
Administration of an Irrigation Subsystem—
The proper functioning of an irrigation system is dependent upon the
institutional framework. This framework must be compatible with the design
criteria used in constructing the system, and still provide flexibility to
achieve improved water management as the need arises. Satisfying on-farm
water management objectives cannot be achieved without controlling water
deliveries, and administration of the irrigation system requires that
satisfactory legal mechanisms exist to control water deliveries.
The provision of adequately trained personnel for the operation and
maintenance of irrigation systems may be understood, but is difficult to
accomplish. The focus must be on training not only the engineers and
technicians, but those working in conjunction with the farmer. Although
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agricultural experiment stations exist, there are usually limitations in
accomplishing on-farm improvements due to the few adequately trained farm-
level advisors capable of applying research results to the farms.
WATER LAW
One of the most important factors of implementing technological change
for salinity management in irrigated agriculture is the water rights held
by farmers and irrigation companies and districts. This is due to the many
organizations and mechanisms which have developed to control and administer
these rights.
Water law in the United States is a federated system with a delineation
of jurisdiction at the national and state levels. The nationwide federal
water law is uniform with some regional flexibility. Each of the 50 states
has adopted surface and groundwater laws that vary significantly. State
water quality laws, on the other hand, are more uniform and generally follow
the pattern set by the federal legislation of 1956 (P.L. 84-660), 1965
(P.L. 89-234), and 1972 (P.L. 92-500) and their respective amendments.
Federal Water Law
The federal government holds direct control of water rights by two
primary means: (a) the holding of water rights on federal reclamation proj-
ects by the United States Bureau of Reclamation until the project reimburse-
ment costs are paid; and (b) the Reservation Doctrine. The Reservation
Doctrine is alleged to give the federal government power to reserve water
on lands withdrawn from private purchase and lands designated for specific
federal purposes, such as national parks, forests, recreational areas,
wildlife refuges, oil shale reserves and hydro-power locations. The
reservation extends to all present and future uses, is not lost through
nonuse, and has a priority corresponding with the date of the legislation
that withdrew these lands from other uses. This doctrine has been interpreted
to include both surface and groundwaters (Radosevich and Daines, 1976).
However, the validity and applicability of this doctrine has been severely
curtailed by two recent Supreme Court decisions. Trelease (1971) presented
detailed information regarding federal-state water relations in a report to
the National Water Commission.
State Water Law
Surface water laws in the United States developed along two distinct
philosophies that are generally consistent with the geographical and
climatic characteristics of the individual state. In the eastern half of
the -country, and along the west coast, the Riparian Doctrine was adopted.
The western half of the country, as a result of much trial, error and
compromise, designed a water right system that is characteristic to arid
lands and is broadly described as the Prior Appropriation Doctrine. These
include: Montana, Idaho, Wyoming, Nevada, Utah, Colorado, Arizona, and
New Mexico. On the other hand, California, Oregon, Washington, North and
South Dakota, Nebraska, Kansas, Oklahoma, Texas, and Mississippi have
44
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spatially varied water availability; and consequently, adopted a mixed
riparian and prior appropriation system. States not listed above have
various forms of the riparian water law system.
Riparian Water Law—
The riparian water laws common to humid regions in the United States
were patterned after the early common law of England. Under this English
law, every landowner with property adjacent to a stream or body of water
was entitled to use as much of this water as was desired, as long as it was
not diminished in quality. The right to use the water was not a right for
fixed quantity of flow or volume, but was dependent upon the extent of water-
use development that had taken place. Generally, the water right was
attached to the land that could not be separated.
As a result of the conflicts that arose when emerging industries,
municipalities and agriculture in the United States began diverting water,
the American Rule of Reasonable Use was established. Under this rule,
riparian landowners can divert a "reasonable" amount of water with respect
to all other riparians on the stream. In addition, nonriparian lands may,
under certain conditions, make use of the available waters.
Prior Appropriation Water Law—
The Appropriation Doctrine is a water allocation system developed in
response to the early gold and silver mining interests in the West. Miners
recognized that the Riparian Doctrine was not workable under conditions they
encountered. In response, they applied the same principles to their water
as they did to their mines. That is, the first person to discover a mine
was protected against later claimants. In terms of water, this was trans-
lated into the familiar quote of "first in time is first in right," or, in
other words, the first person to use a specified amount of water acquired
the right to its future beneficial, reasonable, and .continuous use. Water
supplies above the specified amount were free to be appropriated by other
users.
One of the primary differences between the riparian and prior
appropriation doctrines is that in the latter doctrine, the priority of
right and not the equality of right is the basis for dividing water during
periods of scarcity. There is no proration when water shortages occur.
This means that all users are given a time priority by date. For example,
the most recent water allocation granted is the first to be denied the right
to divert water. From this it can be seen that the burden of shortages
always falls on those allocated most recently, but does guarantee a dependable
supply to those with senior rights. As a result, the economic value of water
rights greatly depends upon the priority date and the source of supply in
terms of dependability of flow.
In response to the concern that some users are more critical than
others, states with various appropriation doctrines have adopted statutes
that define "preferred uses." These designations allow a preferred use to
condemn water, with compensation, from a nonpreferred or a "less" preferred
use in times of shortages. This order of preference also serves as criteria
for the allocating agency when several uses are competing for the same
45
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unappropriated water. Generally, the order of ranking for water use is
domestic, agricultural, industrial, power, fish and wildlife, and
recreation.
In existing irrigated areas, many of the appropriated water rights are
dependent on return flows from the above lands. Any degradation of
quality or quantity of these return flows can cause economic and agronomic
hardship on the downstream users. Changes in irrigation practices, extensions
of the irrigated acreage or water transfers that change the quantity and
temporal distribution of the return flows can affect the value and use of
downstream water rights. Changes in irrigation practices are usually per-
mitted except for surface reuse systems where the size of the tailwater pond
is controlled so that, in effect, it does not result in a storage right.
Extending the irrigated acreage is not allowed, except where increases are
with unappropriated water. The amount allowed to be transferred is usually
limited only to the consumptive use. In any proposed change, it must be
shown that no other appropriator would be hurt by these new practices or
uses. Case studies and discussions on the legal interpretation of both
water law doctrines can be found in Trelease (1967), Sax (1968), Radosevich
and Hamburg (1971), and Radosevich and Allen (1975).
The existing water law systems in the western United States tend to act
as an institutional constraint to good irrigation water management. Water
rights are based on a rate of flow rather than a volume measurement, or
consideration of the crop water requirements. Also, if an irrigator or
irrigation district does not divert the total amount or rate of flow of the
adjudicated water right, the undiverted portion of the right will be appro-
priated by other users through abandonment procedures. The full water right
is usually diverted even though part of it is not needed, and the excess water
is either returned unused to the stream or applied to the land.
If best management practices were implemented and the diversion require-
ments were reduced, under the existing laws this "excess" water would then
go in order of priority to more junior appropriators up to the flow rate of
their allocated water right. This water would not necessarily remain in the
stream for fisheries enhancement or flow augmentation unless the junior appro-
priators were downstream, which is often not the case in the West. Since
water would be available for more time during the irrigation season to these
junior appropriators, the result may be increased pollution from their lands
and the net effect on the stream would remain the same, even with best
management practices under the existing water laws in these areas.
Groundwater Law—
In many areas of the United States, groundwater resources play an
increasingly major role in agriculture, municipal and industrial uses. Since
1950 there has been an estimated 240 percent increase in the use of fresh
groundwater supplies. In 1975, it was estimated that approximately 38 per-
cent of the water withdrawn in the country came from groundwater sources
(Murray and Reeves, 1977). Due to this rapid expansion in groundwater
utilization, many state laws controlling its extraction and use have been
enacted. These laws are at least as complex as the corresponding surface
water laws. There are several legal approaches to this problem. Most of the
46
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western states have elected to adopt groundwater statutes similar in
philosophy to the prior appropriation doctrine. This was necessary because
surface and groundwater are often hydraulically interconnected, and with-
drawals from one will affect the other. These laws generally permit the
appropriation of groundwater for a beneficial use provided that the intended
user complies with all the statutory requirements. It is the responsibility
of the administrative officials to determine if groundwater exists and what
adverse effects would occur if the application was approved. A more in-depth
discussion on groundwater law is presented by Corker (1971).
47
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SECTION 4
POTENTIAL TECHNOLOGICAL SOLUTIONS
All salinity prob.1 ems result from both salt pickup and salt concentrating
effects. No irrigated area contributes salts due to only one cause to the
total exclusion of the other. The relative magnitude of these sources,
however, does determine the emphasis of the salinity program to be implemented.
In areas where salt concentration is the primary cause of increased salinity,
the concentration of salinity in the subsurface return flows is usually
approximately inversely proportional to quantity of deep percolation, except
where chemical precipitation may be significant. Therefore, the emphasis of
the best management practices is directed toward reducing the chemical con-
centration of the deep percolation by measures such as minimum leaching (USDA,
Salinity Laboratory, 1977), phreatophyte control, growing more salt tolerant
crops, land retirement, and collection and disposal or treatment (desalina-
tion) of the irrigation return flows.
For areas which primarily contribute salinity due to salt pickup, the
emphasis of a salinity control program is to reduce the quantity of canal
and lateral seepage and deep percolation losses. Best management practices
will consist of canal and lateral lining to reduce seepage losses, along
with minimizing deep percolation by improved on-farm water management
practices such as installation of accurate flow measurement devices, irriga-
tion scheduling, and more uniform water applications. For most areas, the
salinity problems result from a combination of both salt concentration and
salt pickup, which requires an integrated site-specific combination of the
above types of strategies.
Achieving high irrigation efficiencies and other improved irrigation
management practices are goals not only of water quality planners, but of
individual irrigators and irrigation organizations as well. King (1973) and
Willardson and Hanks (1976) discuss many of the effects of irrigation manage-
ment on irrigation return flows. The technological solutions to salinity
problems are often the solutions applicable to reducing agricultural energy
consumption, achieving higher farm production and higher profits. In this
section, potential technological solutions to the problems of salinity are
discussed.
The collection and treatment or disposal of irrigation return flows are
usually straightforward engineering problems and are summarized near the con-
clusion of this section. For the most part, increasing the efficiency of
irrigation is a complex task. Improving physical aspects of the irrigation
system, including structural rehabilitation and redesign and instituting
48
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better management practices for the operation of the system by irrigation
scheduling, call periods, and limiting wastes, must be considered in any
program for improving the efficiencies of irrigation.
Irrigation is an integral part of the larger hydrologic system encom-
passed by a river basin or watershed. Salinity control measures must be
compatible with the objectives for water resource management and development
in the total system. These can be logically divided into measures aimed at
improving the operation and management of the three major subsystems within
the irrigated systems: (a) the water delivery subsystem; (b) the farm sub-
system; and (c) the water removal subsystem.
IMPROVING THE WATER DELIVERY SUBSYSTEM
The conveyance of water from the source of supply, such as a stream,
reservoir, or groundwater, to the inlet of the irrigated field boundaries
constitutes the water delivery subsystem. Salinity problems associated with
this subsystem are primarily a result of seepage from the channels into the
soil and substrata where salts are picked up and transported to the ground-
water, or back to the river or stream. Seepage water from the delivery
subsystem may be lost to nonbeneficial consumptive use by phreatophytes and
direct evaporation from high water table areas resulting in salt concentrating
effects. In open conveyances, evaporation from the water surfaces also con-
centrate salinity; however, these losses are generally insignificant.
The operation of the water delivery subsystem can also be expected to
indirectly affect water utilization in both the farm and water removal sub-
systems. Careful management and accurate measurement and allocation of water
results in higher on-farm irrigation efficiencies. Properly maintained and
operated conveyances reduce the water removal subsystem requirements since
less water is spilled or lost through seepage. Improvements in the water
delivery subsystem affecting the quantity and quality of irrigation return
flows fall into the following categories: (a) seepage control; and (b)
systems management and operation which includes flow measurement and control.
Seepage Control
Many unlined canals, ditches, laterals, and water courses traverse long
distances between the point of diversion and the farm. Where soils are well
structured and permeable, seepage losses may be considerable. Traditionally,
reaches with high seepage loss have been lined with a variety of alternative
materials such as concrete, asphalt, bentonite, compacted earth, and plastics
to prevent seepage with the economic justification based on the value of the
water saved. Converting to a closed conduit of concrete, asbestos-cement
(A-C) or plastic is an effective alternative that offers advantages of better
trafficability, reduced evaporation, maintenance of pressure due to gravity,
and aesthetics. The salt pickup contribution from conveyance seepage can
often exceed that leached from the irrigated land which is necessary to
maintain a salt balance in the root zone. The time required to transport
these residual salts to receiving waters depends upon the distance traveled
49
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and the hydraulic properties of the aquifer. The time is, however, usually
sufficient for the seepage flows to reach chemical equilibrium with the
soils and/or geologic substrata. In addition to the quantity of water saved,
conveyance linings ordinarily eliminate the salt loading impacts. Improved
water quality may be another benefit claimed in the economic justification
for seepage controls if soils along the routes traversed by the irrigation
channels are high in residual salts.
Costs of conveyance channel lining vary with the square root of the
channel capacity, so unit costs diminish with increased scale of construction.
Seepage rates per unit of channel area, on the other hand, tend to be higher
with smaller-sized channels because of less maintenance, greater depths to
a water table, and larger ratios of wetted perimeter to discharge capacity.
In that event, the cost effectiveness of lining each portion of the water
delivery subsystem is not readily apparent and should be evaluated further.
Flow Measurement and Control
The purposes of flow measurement and control in irrigation systems are
to ensure an adequate application of water to the croplands and to prevent
unnecessary and wasteful diversions, thereby ensuring an equitable water
allocation. Unfortunately, irrigators often understand less about the mea-
surement of their water than any other aspect of irrigation they handle.
Water control in the absence of flow measurement is very seldom adequate
to achieve irrigation efficiencies that can combat water pollution problems,
including salinity, resulting from irrigation return flows. In many instances,
managers and water masters of irrigation companies and districts tend to de-
liver excess water to individual irrigated subunits in order to maintain or
improve personal relations with the irrigators. Irrigators utilize these
excess flows to ensure that no portion of their fields are underirrigated,
ostensibly to prevent crop yield reductions. At the same time, most of these
fields are overirrigated, which not only aggravates the salinity problem,
but recent research shows that overirrigation will also reduce crop yields by
decreased nutrient availability and, in extreme cases, by poor aeration.
Inevitably, poor water management leads to inefficiencies, reduced yields,
and subsequently, to salinity problems, both locally and downstream. The
problems, however, are amenable to implementation of effective flow measuring
and water control devices combined with improved irrigation practices on the
croplands.
Water measurement structures are commercially and readily available to
farmers, water masters, or any interested individual for almost any field
condition. However, without correct installation and proper maintenance,
most water measurement structures will indicate incorrect discharge mea-
surements. Taking into account the conditions, the following criteria for
selecting a flow measuring device should be considered;
1. Discharge should be read directly rather than requiring use of
conversion tables or calculations.
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2. Structures should be standardized, inexpensive, and easy to fabri-
cate and install.
3. Structures should be hydraulically efficient (low hydraulic energy
losses), self-cleaning, and easily maintained.
It is also desirable that they be volumetrically cumulative. Flow measure-
ment structures or devices fall into three general classifications; (a)
flumes, such as Parshall flumes, trapezoidal flumes, Cutthroat flumes, and
H-Type flumes; (b) weirs; and (.c) orifices. These are discussed in more der-
tail in Section 8,
Flumes involve width constrictions for use in open channels where insuf-
ficient head is available to operate higher head loss structures such as
weirs. While most of the commonly used flumes noted above are usually oper-
ated as critical depth or free flow devices, recently many have been cali-
brated for the subcritical or submerged flow regime.
In order to control the flow of water in canals or ditches, structures
referred to as checks and drops are used. These structures specifically
control the slope and elevation of water surfaces and are critical in dividing
the water, as well as distributing water to each field. Other control
structures include culverts and field inlet devices.
A check structure is used to maintain or increase water surface eleva-
tions in open channels and should be designed to allow the flow needed down-
stream to pass over or through the structure while maintaining a constant
upstream depth. In that case, the check structure acts similar to an over-
flow weir, or an orifice, or a combination of both. Among the basic criteria
for designing a check is the necessary height to maintain sufficient water
surface elevation for distribution of the water, and a means of energy
dissipation to prevent erosion downstream.
Drop structures are usually provided in gully control and for changing
steep slopes to flatter ones. This change is usually facilitated by vertical
or inclined drops and an energy dissipating structure. The purpose of drops
in irrigation systems is to prevent erosion. In many, if not most instal-
lations, drops are used in conjunction with checks and dividers to control
flows to other points in the system.
Numerous culverts are found in irrigation conveyance and distribution
systems, as well as in farm head ditches and at points of tailwater runoff
from croplands. They are commonly placed through canal banks to divert water
into laterals. Rather than constructing small bridges, culverts are fre-
quently placed in the channel to allow vehicles such as farm machinery to
cross the channel. Because culverts may constrict flow, they also may
function as flow measuring devices. Culverts often constrain operation of
the system because of flow capacity restrictions.
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Canal Management and Operation
The degradation of water quality associated with the irrigation of
agricultural lands is often most economically controlled on croplands where
the water is applied. This requires reevaluation of the presently employed
irrigation practices. Extensive research in the related fields of agronomy
and irrigation has made available information that serves as a technical
basis for modifying existing irrigation methods. While the primary function
of an irrigation system is to apply water on the farm, the primary responsi-
bility for control lies with the operation of the conveyance and distribution
system. An important part of water management practices needing improvement
is methods used to operate the canals.
When the water quality aspects of irrigation are considered in planning
improved canal systems, quite often radically different alternative canal
management and operation practices are compared with existing ones, The
essential function of the conveyances system is to provide water deliveries
in sufficient amounts when requested. While these criteria are important
when considering water quality, the methods are somewhat different in that
it is important to deliver only the "required or sufficient" amounts of
water, rather than to deliver "excessive" amounts of water to the farmer's
turnout. This objective is no different when crop yields rather than water
quality is the objective. However, changes that are necessary for water
quality improvement may place severe restrictions upon present canal opera-
tion policies that require new feasible alternatives be developed for
operating these systems.
Many kinds of canal management schemes are used due to each state's
variations and interpretations of water rights and their diverse physical
settings. To outline all the solutions that can improve management and
operation of canals for water quality improvement would be difficult, To
significantly alleviate water quality degradation, including salinity, from
irrigated agriculture requires, in almost every case, a stronger capability
for controlling water deliveries. An expedient first step to strengthen
water control is to improve the physical facilities and operational policies
of the canal system. Some important facets of water delivery subsystem
management and operation are described below.
Call Periods—
A very effective means for increasing irrigation efficiencies is to
establish a "water demand" delivery subsystem. Since demand usually can-
not be satisfied instantaneously, the canal officials usually require a
"call period." A call period is the minimum length of time that an irriga-
tor is expected to place an order with the canal operator for a specified
quantity of water prior to the next irrigation. The minimum length of
time•for the call period is dependent upon the response time of the particu-
lar water delivery subsystem to any changes in flow conditions. This prac-
tice allows water masters and ditch riders to plan in advance the regulation
of the canal. This information is essential to operate the canals in an
optimal manner since it allows the ditch riders to coordinate the internal
water management of the water delivery subsystem with the main diversion,
52
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Call periods facilitate increased on-farm irrigation efficiencies by
encouraging farmers to spend more time planning irrigations, thereby making
better use of the water. Many canal systems in operation today do not
attempt to manage deliveries to the farmers beyond assuring that each irriga-
tor has an equitable share. For example, some irrigation systems deliver
water to a farmer on a periodic rotation basis, or each irrigator gets a
certain percentage of the diversion. These conditions are prevalent in
canal systems depending on direct diversions from fluctuating rivers or
streams, necessitated by the high flows in the spring and low flows in the
late summer. From a water quality management standpoint,, these conditions
promote wasteful use of water during high flow periods and result in water
shortages during low flows, either can reduce crop yields, The irrigators
on fixed rotation water deliveries must apply the water when it comes,
whether it is needed or not, resulting in overirrigation during times when
irrigation water supplies exceed the "water demand" of the crops.
Another important aspect of instituting call periods is the scheduling
of canal maintenance work. This often requires a period of discontinued
canal operation, during times when water demands are minimal, such as during
the late maturity or harvesting of a major-crop in the area. These con-
siderations are important to farmers whose crop yields can be severely
damaged if they cannot get water during critical growth periods. Often,
only a one-or-two-day period is necessary for completion of maintenance work,
and by knowing the irrigating schedules of the farmers, canal shut-off
periods can be more easily scheduled.
Scheduling Water Deliveries—
Recent advances in the development of predictive evapotranspiration
models provide an important water management methodology. Irrigation
scheduling services combine meteorological data and soil moisture levels to
forecast future irrigations, both in terms of the depths of water application
and the timing of the irrigations. Almost all recent research in this
important area has been performed by governmental agencies, but some studies
have been completed by private commercial groups who provide irrigation
scheduling services for farmers. Canal companies and irrigation districts
are among the parties who should be interested in providing at least part
of this information to the irrigators they serve. Most of the existing
commercial services combine pesticide and fertilizer scheduling along with
irrigation scheduling.
The importance of canal companies and irrigation districts being
involved with irrigation scheduling cannot be over emphasized. Canal
operators generally have better relations with farmers since they deal in
the allocation and delivery of water. Also, the canal system can benefit
by having water demand forecasts available. In an efficiently
operated delivery system, it is essential that irrigators improve the
operational practices by providing continual feedback to the canal operators.
Canal Maintenance
Inadequately maintained canals add to water quality degradation in rivers
and streams that receive irrigation return flows. The effects of poor
53
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maintenance are often visually apparent in the irrigation channels, and at
the various diversion and control structures located along the water delivery
subsystem. Keeping irrigation canals in good condition must be accomplished
so adequate water supplies can be made available to farms when required.
Canal cross-sections must be kept free of silt deposits or protected from
scour. Aquatic vegetation, in and along canals, needs to be controlled
periodically to maintain the canal capacity, reduce evapotranspiration losses,
and prevent clogging at control structures. Canal banks must be repaired when
damaged by burrowing rodents or when breaks occur. Structures employed in
canal systems must also be properly maintained against leakage, settling, or
general wear. Because many irrigation projects have more than adequate water
supplies, there has been little initiative to implement effective maintenance
programs unless serious drainage problems or frequent canal failures occur.
The water quality detriments caused by canals in poor condition requires
improved maintenance programs.
Seepage Losses--
When aquatic vegetation is not prevented, the canals must be operated
at higher water surface elevations, even during low demand periods. The
increased wetted perimeters are accompanied by higher seepage rates,
resulting in increased water quality deterioration. Dilapidated and leaky
control structures allow water to move into unused ditches or wasteways
where further additions to the groundwater occur. If bank vegetation be-
comes excessive, evapotranspiration losses in a canal section may become
significant.
Good water delivery characteristics cannot be continued when weeds, moss,
sediment, and leakage develop, or when linings and control structures
are in disrepair. These conditions cause wide fluctuations in water
levels throughout the canal length, which makes it almost impossible for
farmers to apply water uniformly. Without workable canal systems, water
management and thus water quality management cannot be successful.
Managing System Storage—
In well-managed irrigation systems, water storage in internal regulating
reservoirs, if available, and the storage capacity of the canals provide
operational flexibility and more efficient water use. Water storage, how-
ever, should be minimized in order to keep seepage and evaporation losses
as small as possible. One of the common facets in canal operation is the
"checking" of flows to raise the water elevation to provide adequate head on
the canal turnout. To accomplish this, a considerable quantity of water
may be needed as dead storage. Seepage rates, as well as additional water
used by weeds along the canal bank, are increased. In many cases, when the
irrigator concludes an irrigation, it is necessary to dump the dead storage
into wasteways since the next irrigator downstream may not be able to handle
the large surge of water that would develop, A feasible alternative would
be to let the next downstream irrigator draw from the upstream storage
shortly in advance of the conclusion of an irrigation; thereby utilizing
the dead storage water along the system. Another effective alternative is
to provide a limited storage capacity along a canal to collect and regulate
waste flows. The ability to minimize seepage and aquatic weed effects in a
canal system requires a high level of management and the lining of canal and
storage facilities.
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Storage within the canal system can be beneficially minimized in areas
where canal diversions are received from reservoirs that serve to reduce
fluctuations in the river and stream discharges. If water storage is
available throughout the river basin, it is profitable for water users to
store water high in the basin so evaporation losses are reduced, and flexi-
bility in diversions is increased. In arid river basins, such as the
Colorado River Basin, water quality is significantly aggravated by the con-
centrating effects of evaporation from the large storage reservoirs located
in the lower reaches of the river.
Managing Waste Water—
Seasonally, many irrigation systems have a, larger volume of water
available than is necessary to adequately supply crop demands, When this
happens, water management is often lax and water quality problems can occur.
One of the undesirable practices often found in these systems is regulation
of canal capacities by spilling the surplus water supplies rather than
regulating the diversions at the river or reservoir, For example, in canals
that are relatively long, because of the slow response time, it is common
to operate the canals at capacity at all times, with unneeded water spilled
into wasteways. Usually, canal flows must be controlled at several locations
along the canal. Spillage often adds to the water quality deterioration in
the rivers and streams to which the flow returns, mostly as a result of
soil erosion and salt concentrating effects, Wasteways tend to be neglected;
consequently, they usually nourish large populations of phreatophytes. In
addition, water table elevations in the vicinity may be lower than water in
the wasteway, resulting in additional seepage to underlying groundwater
aquifers in the area. Where it is necessary to reduce groundwater flows,
canal spillage is an undesirable water management practice from a water
quality standpoint. Under improved operating conditions, this practice is
usually eliminated because it wastes water that can be more beneficially
used elsewhere.
IMPROVING THE FARM SUBSYSTEM
The most significant improvements to reduce water diversions and control
waterlogging and salinity problems potentially come from improved on-farm
water management. This is particularly true for areas containing large
quantities of naturally occurring salts in the soil profile. Poor irrigation
practices on the farm are the primary cause of excessive water diversions,
as well as being the primary source of irrigation return flow quality
problems.
Cultural Practices
When the soils to be irrigated are finely textured with low infiltration
rates and low permeability, and the water supply delivered to the farm is
saline, cultural practices such as tillage and planting are extremely
significant if crops are to be grown successfully. This situation is
further aggravated in irrigated areas having high summer temperatures. Under
these conditions, the management alternatives include one or more of the
following: (a) planting more salt tolerant plants, which are usually lower
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in cash value; (b) using special soil tillage practices, which may cost
more; (c) leaching during the off-season; (d) leaching the field one year
and planting a crop the next; (.e) preparing the seedbed more carefully;
(f) planning for proper location and accurate placement of seed; and (gX
controlling the timing and amount of water application. Usually, these
problems must be faced in the lower regions of a river basin where accumula-
tive effects of upstream water quality degradation, along with having
finer "heavy" soils, create difficult management decisions.
Generally, the deeper water stored in the soil is removed more slowly by
evapotranspiration because the roots of most crops are more profuse near the
upper portion of the root-zone profile. Soil structure, texture, and strati-
fication primarily control the distribution of water storage in the soil. In
extreme cases, deep tillage may be required to permit greater water storage
capacity\ as well as deeper root penetration into less permeable soil layers.
At the same time, excessive or unnecessary tillage can be detrimental to
stored soil water, increasing evaporative losses at times when the crop needs
moisture the most, while reducing soil porosity and aeration. Therefore,
cultural practices can play a significant role in overall farm water
management.
Irrigation Scheduling
Many studies show that in irrigated areas the amount of water applied is
often unrelated to the amount needed by the crop at that time. Usually,
when the farmer finds the field is dry, irrigation will be initiated. The
irrigation application, however, may be more than is required by the crop.
A twofold problem results. If the plant was stressed, the potential yield
probably has been diminished. The second problem is due to more water being
applied than necessary. In extreme cases, this might lead to reduced aeration
of the soil, loss of fertilizer (nitrogen), and reduced crop yields.
The purpose of computerized irrigation scheduling is to advise a farmer
when to irrigate and how much water should be used (Jensen, 1969; Jensen,
et al., 1970; and Jensen, 1975). A farmer may rely on visual indications of
crop response to decide when to irrigate, or the farmer may irrigate on a
fixed water rotation arrangement dictated by the calendar. Irrigation
scheduling is geared towards taking soil moisture measurements and computing
potential consumptive use for the crops being grown. The scheduling process
then determines when to irrigate and the quantity of water to be applied.
The collection of climatic data and the calculation and measurement of
consumptive use are discussed in Section 8.
Irrigation scheduling relies upon two primary variables, evapotranspi-
ration and available root-zone soil moisture. Field capacity and wilting
point 'for the particular soils in any field must be determined. More im-
portantly, depending on the irrigation method used, infiltration character-
istics of the soils must be measured. Only by knowing how soil intake rates
change with time during a single irrigation, as well as throughout the
irrigation season, can meaningful predictions be made of: (a) the quantity
of water that should be delivered at the farm inlet for each irrigation; and
(b) the effect of modifying deep percolation losses. With good climatic and
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soils data, accurate predictions of the next irrigation date and the quantity
of irrigation water to be applied can be made.
Results from irrigation scheduling are extremely promising and farmers
are realizing significant benefits. Yields have increased since water was
applied when needed rather than after the crops were stressed, or because
excess water was applied by irrigating too early. Presently, irrigation
scheduling has not resulted in reduction in total diverted water, although it
would seem likely that a decrease in water diversions would occur with time
as farmers gain more knowledge of what is occurring in the soil profile.
Another benefit to farmers from scheduling is the capacity to anticipate
dates when irrigation is to occur. This allows farmers to schedule the
irrigation and to more effectively schedule other duties that must be
performed on the farm.
Results of studies by Skogerboe et al. (1974a) indicate that irrigation
scheduling programs have limited effectiveness for controlling salinity
unless scheduling is accompanied by flow measurements at all major division
points, farm inlets, and field tailwater exits. It is necessary for canal
companies, irrigation districts, or government agencies to assume an
expanding role in delivery of the water, These studies show that scheduling
is a necessary, but not sufficient way to achieve improved irrigation, The
real strides in reducing salt pickup resulting from overirrigation will come
from irrigation scheduling in conjunction with improved on-farm irrigation
practices. In fact, irrigation scheduling cannot be effectively accomplished
without efficient irrigation systems unless the distribution of water and
thereby the amount of infiltration is known,
Structural Rehabilitation and Replacement
If the problems of on-farm water management are properly conceptualized,
the structural remedies to a salinity problem evolve into one or two
alternatives:
1. On-farm conveyance networks should be lined or replaced with
pipeline to prevent seepage; and
2. The uniformity of water application should be increased by altering
irrigation practices or converting to more effective irrigation
methods.
On-Farm Seepage Control—
Many farmers have lined their farm conveyance networks to reduce seepage,
maintenance and labor. The most common lining methods include a concrete
slipform lining, buried plastic membrane, compacted earth, or converting the
ditches to plastic, concrete, or aluminum pipelines. These alternatives are
relatively inexpensive and are accepted methods for improving on-farm water
management. In order for the irrigator to operate his system effectively,
flow measurement devices must be located so inflows to each field are known.
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Improving Water Application Uniformity—
To adequately irrigate the least watered areas in a field without
overirrigating other parts, uniformity of the application should be maximized.
Since different irrigation methods have different uniformity problems, it
is useful to segregate this explanation into the following irrigation
methods: (a) surface irrigation; (b) sprinkler irrigation; and (c) trickle
irrigation.
Surface irrigation—The predominant form of surface irrigation in the
western United States is furrow irrigation. Other methods include border
and basin irrigation which are not specifically discussed in this manual.
Most surface irrigation is applied to relatively "tight" soils and on compara-
tively flat slopes, usually less than 1 to 1.5 percent. Infiltration in most
soils generally follows an exponential function decaying with time. Since the
water is conveyed over the surface of the soil in any surface irrigation
method, the uniformity of water application is maximized when the "intake
opportunity time" at both ends of the field are equal (Figure 12) . Because
of the time necessary for the conveyance of water from the head end to the
tail end of the field, opportunity for times of equal intake along the field
is not completely possible. If the slope is uniform and the runoff unre-
stricted, the least watered area of the field is usually at the lower end.
Under existing surface irrigation management practices, the maximum irriga-
tion efficiency which is acceptable to the farmer occurs when the moisture
deficit in the least watered area is refilled. This concept is illustrated
in Figure 13.
There are two wastes that usually occur as a result of surface
irrigation; namely, water percolating below the root zone (deep percolation
losses) and runoff from the lower end of the field (tailwater runoff). Ef-
forts to reduce one type of waste water versus another can be competitive.
For example, measures to minimize runoff will often increase deep
percolation with the reverse also true. Since salinity is directly associated
with deep percolation, local improvement programs will cause high runoff if
not coupled with some system modification. In some irrigated areas, erosion
is a major problem and reducing tailwater runoff would be the highest
priority for alleviating water quality degradation.
Surface irrigation uniformity and efficiency can be substantially im-
proved by three alternatives. First, irrigation scheduling should always
be based on sampling at the least watered area of the field so the minimal
intake opportunity time or irrigation set can be determined. The flow in
the furrow or the unit flow in a border or basin should be adjusted so the
time required for the flow to advance to the end of the field is about 25
percent of the minimal intake opportunity time, assuming a uniform slope.
If these practices are implemented and adhered to strictly, deep percolation
volumes could be cut up to 50 percent. Individual field length and dis-
charge can usually be adjusted to satisfy these criteria.
The second method for improving surface irrigation uniformities and
efficiencies is applicable primarily to furrow irrigation. Under this
method, the head ditch is sufficiently automated so a large "wetting" furrow
stream is introduced to quickly advance the flow down the furrow. The flow
58
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o
o>
o
CO
0)
o
c
CO
0)
E
Start of recession, when
water into the top of the
furrow or border is stopped
Recession Phase
Time water is available to
infiltrate, also referred to as
intake opportunity time
Irrigation commences
Advance Phase
Distance from Top of Field, x
Figure 12. Schematic representation of intake opportunity time and typical
advance and recession curves for surface irrigation.
is then "cutback" to a "soaking" flow rate to finish the irrigation. This
methodology, illustrated in Figure 14, has been utilized with good results in
numerous areas. It has about the same unit salinity control cost-
effectiveness as lining the on-farm conveyance channels. In addition to
substantial labor savings due to inexpensive automation, cutback irrigation
has a notable advantage over simply improving the existing system in that
field tailwater runoff is also greatly reduced. This method is applicable
only if sufficient cross slope is available.
The final alternative for improving surface irrigation uniformities is
a tailwater reuse system which utilizes large flow rates without causing
soil erosion. The system collects the tailwater runoff in a small reservoir,
and then pumps this water back to the head of the field for reuse (Figure 15)
This technology represents a very efficient surface irrigation system when
operating with respect to a known soil moisture deficit. This design com-
pletely eliminates on-farm surface wastes. Another efficient surface irri-
gation method used in the southwestern United States is level borders that
may have application efficiencies greater than 90 percent.
59
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X L
f X
(c)
L= Field Length, m
£ = Advance Distance During
Irrigation Interval, m
1= Depth of Infiltration,cm
Root Zone Soil Moisture
Holding Capacity, cmVcm
Figure 13. Definition sketch of surface irrigation application uniformity
for ta) the case where part of the field is underirrigated, (b)
the case of zero underirrigation, and (c) conditions of signi-
ficant ovenrrigation (Gerards, 1978).
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,Water Surface
,Check Dam
Check Dam
Boy 3
^Drop
Water Surface
Check Dam
O O ' ~ I
OOOO OOOOOOOO
'Water Surface
Figure 14. Elevation drawing of cutback furrow irrigation system with
spiles in the ditch sidewall. In (a) bay 1 is delivering the
initial furrow flow. In (b) the check dam has been removed
from bay 1; bay 2 is delivering the initial furrow flow and bay
1 is delivering the cutback flow. In (c) the check dam has
been removed from bay 2; bay 3 is delivering the initial
furrow flow; bay 2 is delivering the cutback furrow flow and
bay 1 is shutoff (Carton, 1966).
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Qt - Supply Inflow
Supply Set No. "I
Number of
Furrows
N= Qt/Q
per Supply
Set
Runoff-Drains
Reuse Inflow- Q-fr
Reuse Set No.
Storage
Pond
1 2
Runoff-Drains
,Pump
Reuse - Pipe
Figure 15 . Schematic illustration of a tailwater recovery and reuse system for surface irrigation
(Gerards, 1978).
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Sprinkler Irrigation—A conversion by farmers from surface methods to
sprinkler irrigation could be highly beneficial in terms of more efficient
water use. Sprinkler irrigation, properly designed, installed and operated,
is advantageous to both water quantity and quality. Uniform water application
is generally possible on various types of soils, thereby minimizing deep
percolation losses; and, of course, no tailwater runoff should result. The
division and shape of lands in many of the irrigated valleys of the West are
such that the most effective sprinkler system would be the familiar mobile
side-roll, center-pivot, or other movable systems. In orchards, tow-line
or solid-set sprinklers are more advantageous.
Apart from the water quality benefits, there are many other advantages
to farmers in converting to sprinklers. The labor savings are particularly
noticeable in comparison with surface irrigation methods. With permanent
set systems, labor is negligible and the systems are easily adapted to
automation and other water application purposes such as frost control and
cooling.
Besides reducing nutrient losses as a result of lessening deep perco-
lation, further fertilizer economics can be achieved by the use of sprinkler
systems to apply fertilizers during the time(s) required by the plant. Cer-
tain water soluble fertilizers can be applied through the sprinklers. The
timing and amount of application can be controlled more nearly to meet the needs
of the plant. The ability to schedule fertilizer applications to plant
needs, rather than to cultural operations, reduces the opportunity for
leaching nutrients below the root zone. This does have disadvantages;,, for
example, if an unusually large rain should satisfy the plant needs, the sys-
tem may have to be operated in order to meet the fertilizer needs. The
excess water then leaches some of the nutrients, negating the benefits of
the system.
Water soluble herbicides and insecticides can also be applied through
sprinklers. The amount of water applied by a sprinkler irrigation system
can be accurately controlled to meet the needs of the crop. Small depths
of water can be applied early in the season when crop water requirements are
low because of mild weather, or because the roots of the plants are not yet
developed. Being able to apply small depths of water avoids excessive deep
percolation losses that commonly occur under surface irrigation methods
during early season irrigations.
Sprinklers, like most physical systems, do have disadvantages. Damage
to some crops has been observed when poor quality irrigation water has been
applied to the foliage by sprinklers (Harding et al., 1958). Poor quality
water can leave undesirable deposits or coloring on the leaves or fruit of
the crop. Sprinklers are also capable of increasing the incidence of cer-
tain crop diseases, such as fire blight in pears, fungi or foliar bacteria.
A major disadvantage of sprinklers is the relatively high capital cost,
especially for solid-set systems in comparison to surface irrigation methods.
When gravity cannot supply sufficient head to operate the system, sprinklers
can require large amounts of energy when the water has to be pumped to supply
the necessary pressure. In many cases, it is often more economical to use
conventional surface irrigation systems. The advantages and disadvantages
63
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of sprinkler systems must be assessed economically with other irrigation
methods. Likewise, individual types of sprinkler systems should be compared
to one another.
Trickle Irrigation—Trickle or drip irrigation is a recently developed
irrigation method and appears to be particularly suitable for orchard or
other high value crops. This method of irrigation gained attention during
recent years because of the potential for increasing yields while decreasing
water requirements and labor. The concept of trickle irrigation is to con-
tinuously provide the plant with the optimal soil moisture environment. This
is accomplished by conducting water directly to individual plants through
laterals running along each row, instead of providing water to the entire
field as with surface or sprinkler irrigation methods. In widely spaced
crops, losses are reduced because only a small portion of the soil surface is
susceptible to evaporation.
A wetted profile, the shape of which is largely dependent on soil
characteristics, develops in the plant's root zone beneath the "trickier"
or "emitter." Ideally, the area between plants and crop rows is dry and
receives moisture only from incidental rainfall. Trickle irrigation saves
water because only the plant's root zone is supplied with water and little
water should be lost to deep percolation or soil evaporation under proper
management (Figure 16). The only irrigation return flow is due to the
leaching fraction necessary to prevent excessive salt buildup in the root zone.
In fact, salts are often allowed to accumulate outside the wetted profile.
There is no surface runoff and very little nonbeneficial consumptive use of
water by weeds. Water savings are effected through the ease with which the
correct amount of water is accurately applied. Trickle irrigation usually
requires very skilled technical assistance for nutrient balances and
fertilizer applications.
Another advantage is that trickle irrigation systems are easily
automated. The multitude of lateral lines are supplied by manifold lines
connected to the main line, which, in turn, connects to the water source.
Generally, a control head is provided at the water source to regulate
pressure and flow, and to filter suspended solids from the water. A fertili-
zer injection system is often incorporated into the control head (Figure 17).
For irrigation of widely spaced crops such as orchard crops, the cost
of a correctly designed trickle irrigation system can be relatively low in
comparison to that for solid-set or other permanent sprinkler irrigation
systems. In orchards, the unit salinity control cost-effectiveness of a
trickle irrigation system is comparable to that for a solid-set or permanent
sprinkler system with the same level of automation. Where clogging is not
a problem and emitter line maintenance is minimal, operation and maintenance
costs 'of the trickle irrigation system are usually quite low. In plantings
of row crops or vines, where the average distance between emitter lines must
be less than 3 meters, the cost of trickle irrigation is relatively high.
64
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ORCHARD
CROP
WETTED
PROFILE
ROOT ZONE
Figure 16. An illustration of the concept of trickle irrigation in which
only a small part of the field, the wetted profile where crop
roots are growing, is irrigated by emitters bringing water
to each individual plant or group of plants (Smith and
Walker, 1975 ).
65
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FERTILIZER
INJECTOR
CTi
PRESSURE
REGULATOR
SOLENOID
VALVE
CONTROLLER
Figure 17. A schematic diagram of a typical trickle irrigation system control head (Smith and Walker,
1975).
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IMPROVING THE WATER REMOVAL SUBSYSTEM
Surface Runoff
The water removal subsystem removes the surface runoff from agricultural
lands, unless it is captured and pumped back on the farm, and the deep
percolation losses from irrigation. These subsystems are often very complex.
For example, the surface runoff or tailwater from one farm may become all or
part of the water supply for an adjacent farm. Or, the runoff may flow back
into the water delivery system at some downstream location or be transported
back to the river via natural or man-made open drains. Before surface return
flows reach the receiving stream, there are essentially three alternatives
for alleviating waterlogging and preventing or minimizing the quantity of
pollutants discharged into the river. The first alternative could be a bypass
channel constructed at some location where surface return flows could be
discharged or treated without returning to the river.
A second alternative would be to store the return flows in shallow
storage reservoirs and allow the water to evaporate, leaving behind the
pollutants. Seepage must be controlled in bypass channels or storage
reservoirs; otherwise, the groundwater may become contaminated. The dis-
advantage of the second alternative is that pollutants are collected rather
than discharged to the ocean, which may eventually create another kind of
disposal problem.
The third alternative for minimizing quality degradation in the
receiving stream due to return flow would be to treat the return flow.
Desalination processes could be used to restore the water supply to a desired
quality level. Methods for disposing of brine wastes, however, must be
considered.
Subsurface Drainage
Waterlogging and salinity pose a serious menace to many irrigated
areas. Any expansion upslope from existing irrigated lands becomes a direct
threat to the waterlogging of downslope areas. For example, many of the
fertile lands in the San Joaquin Valley of California are now threatened by
upslope irrigation development, and some areas in the Yuma Valley of Arizona
have been rendered unproductive by irrigation development on the Yuma Mesa.
Equally dangerous threats exist from the salt balance problem of these areas.
Recirculation of water by pumping or the reuse of subsurface return flows
results in a buildup of salinity. Concomitant with increased salinity are
increases in the leaching requirement and drainage needs. Irrigation
development, including impoundment, conveyance, and application, upsets the
natural hydrologic cycle of an area. Recognition and the solution to
drainage and salinity problems in such areas require intensive application
of control measures based on sound scientific knowledge.
Tile Drainage—
If the underlying strata are sufficiently permeable, tile drainage is
a very effective means of lowering the water table and facilitating move-
ment of salts from the root zone. Two types of tile drainage are utilized
67
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in irrigated areas. The first is called field relief drainage which
primarily controls the water table elevation by intercepting and removing
deep percolation from the overlying root zone. Relief drainage is not able
to completely remove all of the water moving below the root zone unless the
water table is lowered below the natural groundwater outlet. The second
type is called interception drainage in which the primary source of ground-
water under a field comes from lands above. Interception drainage collects
the groundwater flows at the edge of the field and conveys them downstream,
thereby removing their potential to aggregate high water table conditions.
Usually, some water will still pass by the drains into the groundwater
reservoir and return to the river, but the quantity of such groundwater
return flows can be reduced considerably by tile drainage. The quality de-
gradation to receiving streams by tile drainage outflow can be minimized by
treating the outflow (U.S. EPA, 1971b). This stresses another advantage of
tile drainage. Tile drains allow the collection of subsurface return flows
into a master drainage system for ease of control and treatment. Pollution
aspects of tile drainage are discussed by USDI (1972a) and the California
Department of Water Resources (1971).
Pump Drainage—Pumps have been used effectively for lowering the water
table in many areas. If the water is not too saline, it can be reused
directly or by mixing it with the surface water supplies by discharging the
flow back into laterals for irrigating croplands. The salinity of groundwater
supplies varies widely, with some too saline for irrigation. For good
quality groundwater supplies, pumping serves the dual purpose of alleviating
waterlogging and providing additional irrigation water supplies.
In other situations, pump drainage has been used to remove highly
saline groundwater in order to alleviate waterlogging and increase crop
production. The primary difficulty is the disposal of these highly saline
flows. Potential solutions include ponding and evaporation, conveyances and
disposal in the ocean, deep well injection, or desalination as in the
Wellton-Mohawk area in Arizona.
COLLECTION, TREATMENT, AND DISPOSAL OF IRRIGATION RETURN FLOWS
In many cases where salt pickup is a particularly severe problem,
subsurface return flows from irrigated lands may be so brackish that no
further use of the water is possible. Such flows significantly degrade the
quality of a river, stream, or groundwater resource. An alternative to
expenditures aimed at reducing the volume of these flows by improving ir-
rigation efficiency is to collect the subsurface return flows before they
enter receiving waters. The collected flows can then be directed to a
desalination plant that removes most of the salts and returns the water to
the stream or directly to a disposal area. Major disposal alternatives in-
clude deep well injection and evaporation ponds. A schematic diagram of a
desalination system is shown in Figure 18. One of the applicable desalina-
tion processes for the range of salination encountered in irrigation return
flows is reverse osmosis. Another process that could be applied is electro-
dialysis. These methods are discussed in more detail by Walker (1978a).
68
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filtrate
and
gas
Feedwater
Pretreatment
Desalination
Subsystem
feed
:eedwater Collection
and
Conveyance
Feedwater
Subsystem
Desalting
Processes
brine
product
Product Post-
Treatment
Brine Conveyance
and
Disposal
Brine
Subsystem
product water
Figure 18. Schematic diagram of a typical desalination system (Walker, 1978a)
-------
The costs of collection, desalination, injection wells, and evaporation
ponds are described for planning purposes by the United States Department
of Interior (1972b). A mathematical description of the same information is
given by Walker (1978a). In general, up to a point, the costs of the
collection, desalination, and brine disposal for salinity control exceeds the
costs required to achieve the same level of salt reduction by improving
irrigation efficiencies. However, this is not true in all cases, particularly
when compared to lining large conveyance systems or implementing highly
automated irrigation systems. The desalination alternative is relatively
free of the institutional complications involved in improving an entire
irrigated area, but is an intensive user of energy.
NONTECHNICAL ALTERNATIVES
Controlling salinity in a major river basin is a difficult task because
of the mixture of diffuse and point sources of salinity. Generally, the most
practicable solution lies in combining the strong features of several control
measures and applying each alternative where it is best suited. Salinity
control technology in this regard remains to be developed since few investi-
gations have managed to integrate the alternatives. If the control program
is to be based on "best management practices," then this integration of
alternatives should be optimized in accordance with a specific criterion
for selecting one measure over another. Two nonstructural alternatives which
should be investigated are taxation and land retirements. Legal alternatives
such as influent standards or water markets are discussed in Section 9.
Taxation
Taxation is strictly a linear application of estimated downstream
damages and does not adequately incorporate the costs of treating the problem
by amending local irrigation practices. Plans calling for more than this
level of control could not be adequately financed by taxes alone. In other
words, there is a point where the costs of alleviating salinity are greater
than the downstream damages. Taxation would have to be based on a sliding
scale representing the minimum cost strategy for reducing salinity. These
kinds of taxes are simple repayment fees and are not generated from con-
sideration of the entire economy.
Land Retirement
Land retirement is a viable and competitive salinity control measure
in some areas such as the extensive citrus groves near Yuma, Arizona, where
land retirement is being implemented. These lands do not yet have an alter-
native use which requires water. If these lands should develop nonagricultural
water users, such as urban areas, land retirement could only be a short-term
solution since urban landscapes would then be irrigated, continuing the
old problem.
To determine whether land retirement can bo l^vsihio on economic
efficiency grounds, two sources of information are required: (a) the direct
and indirect costs of removing land from irrigation; and (b) the benefits or
70
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incremental reduction in damages that would occur as a consequence. Direct
costs should accurately reflect the incomes foregone from farming of the
retired irrigated lands. Included in the indirect costs should be net effects
of costs and benefits issuing from resource reallocation, social transition,
impacts on environmental amenities, and other consequences in the affected
region.
An interindustry (input-output) model serves as the underlying structure
for the analysis reported by Leathers and Young (1976). The input-output
model is an analytical accounting technique commonly used in the evaluation
of "total" economic impacts of exogenous (or outside-induced) changes in an
economy. Because of the interdependence among industries in a well developed
economy, which may include small or large regions, secondary or indirect
impacts are often thought to be just as important as the primary or direct
impacts of an induced change. For this reason, the basic approach adopted
in the study by Leathers and Young (1976) is an indirect impact analysis.
Land retirement mechanisms might include one or more of a number of
options and can be either voluntary or involuntary, depending on the level
of public acceptance and participation in the program. The objective is to
discontinue irrigation of selected acreages, thus eliminating all future
salt loading from these sources. Specific program options evaluated by
Leathers and Young (1976) involve a permanent withdrawal of water supplies.
In general, the incremental costs of salt removal for land retirement
programs (in $ per metric ton), using the provisional estimates of salt
loading, appear to be competitive with other more expensive controls such as
canal lining, drainage, and desalting. The cost-effectiveness of the program
is quite sensitive to assumptions regarding estimates of the quantity of
salts removed and the value placed on the salts removed. Accordingly, it is
important that these assumptions be considered very carefully in comparing
alternative salinity control programs.
The future uses of the land which would be retired is also an important
consideration. For example, in the Grand Valley, there is substantial
urbanization occurring and lands retired would sell for high prices to be
developed as housing sites. Many areas in the Valley already converted to
subdivisions utilize the water previously diverted for agriculture. Every
local indication implies the urban irrigator requires more water to irrigate
a smaller vegetative area. This is due to lower water use efficiencies
resulting from existing horticultural practices for landscape maintenance
in the areas. It does not appear that land retirement would be a long-term
solution to the salinity problem in that area.
71
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SECTION 5
INFLOW-OUTFLOW ANALYSIS
Irrigated agriculture is generally a small part of the hydrologic
system in major watersheds as illustrated in Figure 19. The first step in
evaluating salinity resulting from irrigated sources is to delineate the
irrigated segment from the hydrology. This is usually accomplished with an
inflow-outflow analysis which is the first level of modeling for irrigated
agriculture.
Inflow-outflow analysis is used to determine if there is a significant
salinity problem. It is conducted on a macroscale and shows aggregate
pollution as a result of activities in an area without identifying individual
sources or which control measures would be appropriate. The application of
input-output analysis on intervals of less than one year is difficult to
accurately accomplish since internal storage changes within the system are
difficult to quantify in smaller time increments. Because inflow-outflow
analysis is relatively simple, computer programs are not generally required
for these calculations.
Due to the simplicity of inflow-outflow analysis, these procedures are
very useful to 208 planning efforts. Inflow-outflow computations can indi-
cate the priority and the types of initial investigations that should be
conducted on local areas, subdrainage areas, or river basins. These analyses
are usually done with existing data and, therefore, they can be expeditiously
completed.
PROCEDURE
Assessment of inflows and outflows in a region determines the salinity
loads above and below the irrigated areas. Differences in salinity loads
represent the total salt load discharged by the area, including background
and point sources, as well as irrigation return flows. Salt loadings from
irrigation are determined by subtracting the contributions from background
and known point sources from the total difference of salinity loads above
and below the irrigated areas.
The types of data usually needed for this analysis are river inflows to
the area, tributary inflows, subsurface inflows, surface outflows, subsurface
outflows, reservoir operations, and the associated water quality parameters
of each of the above. The basic equation, referring to Figure 19, and not
considering natural precipitation in the area is:
72
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Groundwater
Inflow
--j
U>
- Stream
Gaging Station B
Groundwater
Outflow
Figure 19. An illustration of a watershed which includes an irrigated component
-------
SIRF
1 ~RB CRB ~ ^TC TC ~ ^RA RAj BG PT (1)
where S = salinity load in irrigation return flows
IRF
O = streamflow (and groundwater outflow, if known)
below the irrigated area from gaging Station B
Q = streamflow (and groundwater inflow, if known)
above the irrigated area from gaging Station A
Q = combined (net) streamflow of any tributary streamflows
into the irrigated area between the points of measure-
ment, including all exports and imports of water from
gaging Station C
C = the concentrations of total dissolved solids (TDS) in the
T5O
stream below, above and from the tributaries, respectively,
C in parts per million (ppm) or milligrams per liter (mg/£)
S = salinity contribution of the background (naturally occurring
salinity in the area, if it is known)
S = salinity contribution of point sources in the area
a = conversion constant that depends upon the units used in the
other variables (if Q is in acre-feet) , a = 0.00036, S is
in English tons; if Q is liters/sec, a = 0.0864, S is in
Kg/day; if Q is in cfs, a is 5.39, S is in Ibs/day)
Equation 1 is a form of the basic mass balance hydrologic equation,
QA-QB= As (2)
where Q& is the inflow, QB is the outflow and As is the change in storage
within the confines of the gaging stations. For purposes of discussing
simplified areas, two irrigated tracts are shown in Figure 20. The inflow-
outflow analysis is represented by the difference in the two stream gaging
stations, A and B. Tributary and groundwater fluxes over the system boundaries
are neglected for this case.
The inflow-outflow analysis between stream gaging stations A and B does
not yield any information regarding the differences between the two tracts.
Installation of -an additional stream gaging station on the river between the
two tracts would enable analysis of the inflow-outflow of only one tract of
irrigated land. However, this depends upon the geology and its effects upon
subsurface return flows .
74
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INFLOW
\ r
}•
I
/STREAM GAGING
STATION A
\
\
(
/;
\
\
i - o = As
where
I = Inflow
0 = Outflow
AS = Change in Storage
\
\
STREAM
GAGING STATION B
S
OUTFLOW
Figure 20. Schematic example of inflow-outflow analysis.
75
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The flow of salts into and out of the area can thereby be represented
by:
Q C - Q C = AS (3)
*A A *B B s
in which C& and CB are the salinity concentrations at the respective gaging
stations and Ass equals the change in salt storage within the systems. If
ASS is considered as the solution phase, mass balance for the salts may not
be achieved with Equation 3 because of the salt pickup or precipitation
phenomenon. If ASS is positive, salts are accumulating in the soil profile
of the irrigated lands, whereas if Ass = 0, a salt balance is maintained.
If AS is negative and salt pickup is occurring, it is difficult to
determine the source of the pickup. This also indicates the shallow ground-
water flows would be highly saline. Providing data is available, a long-term
history of salt pickup will disclose whether the pickup rate is increasing,
remaining fairly .constant, or declining. A declining salt pickup rate indi-
cates that natural salts in the soil profile from either irrigated or water-
shed areas are being dissolved and transported into the groundwater reservoir
enroute to the river. A relatively constant salt pickup rate indicates a
large source of salts in comparison with the amount of water percolating
through the soil profile and moving through the groundwater reservoir. Ad-
ditional analysis, such as hydro-salinity analysis, is required to determine
the magnitudes of the salt pickup resulting from irrigated agriculture and
natural runoff. Also, saline water from mineralized springs could signifi-
cantly contribute to the accumulation of salts. An increasing salt pickup
rate indicates the existence of man-made activities, such as increasing
irrigated acreages, or poorer water management on existing irrigated lands.
DATA COLLECTION
These types of .analyses are usually done with existing data, and there
is little need for field data collection unless there are no stream gaging
stations above or below the irrigated area, or sufficient salinity concen-
tration data have not been collected. For this reason, the input-output
analysis is often done only with surface water records since they are usually
readily available, although groundwater inflows and outflows should be
included if they can be quantified.
Flow and chemical concentration data can be obtained from several sources.
The most common sources are the annual United States Geological Survey
summaries of water flow and quality data for each state. The United States
Geological Survey also publishes a catalog on available water data (United
States Geological Survey, 1975). This type of data is often gathered by
other Federal agencies such as the United States Army Corps of Engineers,
the United States Department of the Interior (Bureau of Land Management and
Bureau of Reclamation), the United States Department of Agriculture (Soil
Conservation Service, Science and Education Administration, Forest Service)
and the United States Environmental Protection Agency. Other sources for
flow data are the state agencies such as the State Engineers Office, natural
76
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resource groups, state health and environmental protection agencies, and
universities and colleges conducting research in the area.
Local organizations such as the local health departments, environmen-
talist groups, fisheries and wildlife personnel, consulting engineers or
industries, power plants and mining operations also collect data in con-
junction with EPA discharge permit requirements or other programs. Some-
times the data are collected for purposes other than salinity, but can often
be utilized in these studies.
Climatic data are published by the National Weather Service, as well as
some state and local research and extension agencies. Land use data are
collected annually by the United States Department of Agriculture,
Agricultural Stabilization Conservation Service. The periodic Census of
Agriculture can also provide valuable information.
Since most of these data are general, care must be taken to understand
their limitations and the resulting degree of accuracy that can be expected
in the analysis. The frequency of sampling is very important and the
Validity of the sampling techniques should also be considered. Many of the
United States Geological Survey gaging stations have ratings that serve as
a guide to their estimated accuracy.
The type of data collected is also important. For example, mineral
concentrations are usually given in terms of electrical conductance (EC) and
temperature. Laboratory analysis is required to establish the relationship
between EC and the total dissolved solids (TDS), as well as the ionic composi-
tion of the water. For most surface waters of fairly good quality (TDS <
1000 ppm) , a ratio of EC to TDS is usually about 0.65 when EC is in units of
micromhos/cm (lomhos/cm). For saline groundwater or leachate, this ratio
varies considerably between 0.65 and unity. The United States Salinity
Laboratory uses TDS - EC x 928 where EC is in millimhos/cm (mmhos) for this
case. The relationship is not linear and depends upon the chemical proper-
ties of the water, particularly on the concentrations of sulfates. Hem (1970)
presents more information on the interpretation and analysis of the chemical
characteristics of natural waters.
In the western United States, streamflow records are usually very good and
readily available; however, water quality data are often lacking and must
be estimated or computed. In addition, the length of time records were kept
for water quality data may be very short. The Water Resources Council (1966)
has provided a review of techniques and limitations of various hydrologic
data analyses. For quick and very rough estimation purposes, publications
such as Geraghty et al. (1973), Rainwater (1962), or Todd (1970) can be
helpful.
Computational Methods
The type of computational methods utilized depends upon the kind,
variability, and frequency of the data to be used. If much data are missing
and must be estimated, there are many statistical and stochastic methods
available (Yevjevich, 1972, and Riggs, 1968). The usual procedure is to
77
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generate daily streamflows with corresponding dissolved mineral concentrations,
If that is not possible, a discharge weighted average analysis is sometimes
appropriate. This method uses composite sampling in an attempt to minimize
errors and variance and to approximate the composition of all the water
which passed the station that year. This method is usually used in combina-
tion with good streamflow records and sporadic water quality records.
Another method for computing salt loads, which is essentially the same
as above except for the time scale, is to multiply the daily TDS by the
daily streamflows, then sum the values to get the yearly total of salt that
passed the station. This is a good technique when very good streamflow and
chemical quality data exist.
If only infrequent data are available, the use of linear or nonlinear
regression equations are often used to predict mineral quality parameters.
The most common regressions are of discharge versus EC, and EC versus TDS in
conjunction with daily flows. The daily values are summed to give the
annual salt loading. This method has been used by the United States Geologi-
cal Survey with fair success in the Upper Colorado River Basin (Brennan
and Grozier, 1976) and in other areas.
78
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SECTION 6
HYDRO-SALINITY ANALYSES
The evaluation of the hydrologic and salinity parameters for an area
requires a large amount of data and lengthy computational procedures. Hydro-
salinity modeling is necessary to determine the magnitude and effect of each
segment of the hydrologic system. A water and salt budgeting procedure is
typically used that is essentially a mass balance or conservation of mass
approach. Numerous computer models have been formulated for water and salt
budgeting. Walker (1978b) reviewed 43 of these models, some of which are
described in Appendix A.
The hydro-salinity procedure integrates various aspects of the local
hydrology. For example, measured flows diverted from the river into the
main conveyance system are segregated into measured seepage and measured
lateral diversions. Operational losses are calculated by the difference
between the main diversion, seepage and lateral diversions. The lateral
diversions are likewise delineated into seepage and root-zone diversions
(measured). Root-zone diversions are further separated into root-zone soil
moisture storage (measured and/or computed), deep percolation (usually by
difference, but sometimes measured) and consumptive use (measured or
computed).
Evaluation of the salinity sources is conducted on several levels with
objective procedures that systematically and continuously refine the various
elements of the hydro-salinity flows to the required degree of accuracy.
There are basically four steps for this evaluation procedure:
1. Definition of the components of the hydrologic system as to their
function, such as water delivery, water use, or water removal.
2. Establishment of a large-scale instrumentation network to monitor
the quantity and quality of each component of the hydrologic system
for the area.
3. Establishment of criteria for classifying the existing performance
of the various subsystems for both time-specific events and seasonal
or annual analysis.
4. Definition of the relationships expressing the existing system
performance as functions of management and/or methods of operation.
79
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Defining the components of the hydrologic system is necessary in order
to properly design a monitoring network that evaluates pollution effects and
contributions from each subsystem. The next step is to establish the net-
work of instrumentation to monitor the surface and groundwater hydrology and
to define the hydrologic components of the area in question. A monitoring
network usually consists of flow measurement structures that collect infor-
mation on canal, headgate, on-farm diversions, and surface tailwater flows.
A network of observation wells and piezometers is required to collect infor-
mation on groundwater gradients, flows, and elevations for subsurface returns
to the rivers or lakes. Water quality information is also collected from
all of these sources simultaneously with the other measurements. A gen-
eralized monitoring network for hydro-salinity investigations is presented
in Figure 21. From an anlysis of these data, the total on-farm component
of the irrigation hydrology is determined. A smaller study area of several
farms should then be selected for farm efficiency investigations, and the
individual segments of this subsystem evaluated. An implementation program
can then be initiated on this study area and any changes in conditions caused
by the small-scale implementation program should be reflected in the larger
monitoring network.
DEVELOPING WATER BUDGETS
The process of hydro-salinity modeling is basically one of formulating
a series of water budgets for each hydrologic segment. These budgets are
expansions of the mass balance hydrologic equation presented in the previous
section. This is the first step of hydro-salinity investigations.
The most important consideration for developing water balances is to
select the system boundaries to use the available information to the best
advantage. This boundary selection procedure will also assist in designing
the monitoring network and in determining the extent of the field investi-
gations. Many of the variables such as canal diversions are known, but it
is necessary to develop as many water balance equations as there are unknown
variables. The time frame, monthly or yearly, of the budgets must be deter-
mined at the start of the process.
Establish Water Budget Equations
The following example is developed for purposes of illustration of the
procedures and consideration involved in the hydro-salinity process. In
specific cases, there may be more or less variables than are presented in
this example; and many variables may be insignificant or zero. In this
simplified case, four equations were developed for the selected hydrologic
subsystems. These equations are:
1. Water Balance on a Channel
One segment of the hydrologic system is just the physical boundaries
of the river channel, and a water balance equation can be
written for this subsystem. For example:
80
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00
I mile
Figure 21.
Roads 'lUESs* River
FarmEfficiency Wasteway
j-n_r Canal — Natural
Drdinage Way
Delineation of a study area and a possible monitoring network for a hydro-
salinity investigation.
• Well and Piezometer Location
® Flow Measurement Station
O Weather Station
-------
0 +Q +Q +0 +0 +0 +Q -Q -Q -Q -Q -As =0
A SRF IRF *RP XRR *R XCS *ID *SD XRE KB RS
where Q = river inflows at point A
A
Q = river outflows at point B
B
Q = seepage returns from surface storage
SRF
Q = subsurface irrigation return flows including
ditch seepage
yRP = precipitation on river
Q = natural surface runoff from precipitation
RR
to river
Q = surface runoff from irrigation
R.
Q = canal spillage returns to river
(— 'O
Q = irrigation diversions
Q = diversions for reservoir storage
oU
Q „ = river evaporation
Iv£j
As = change in storage in river channel
Ko
2. Water Balance on Surface Storage
If an area has surface reservoir storage facilities, a water
balance can be written for this hydrologic subsystem.
where Q = diversions for reservoir storage
Oly
Q = precipitation on storage
SP
Q = surface evaporation
SE
Q = reservoir releases to irrigation
oH.
Q,,™ = seepage returns to river from storage
oKr
As = change in surface storage
oo
82
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3. Water Balance on Soil Surface and Root-Zone
One hydrologic subsystem which can be defined is the soil surface
and the root-zone as a unit. Neglecting bank storage, the mass
balance equation which can be written for this system is:
ET-DP-R-SM= (6)
where Q = precipitation on croplands
QTn = irrigation diversion
QSR = reservoir releases to irrigation
Q = pumped water
QET = evapotranspiration
Q = deep percolation
Q,, = surface runoff
r\.
AS = change in soil-moisture storage
. Water Balance on Aquifer
In writing the mass balance for this hydrologic subsystem, it
was assumed that groundwater inflows and outflows at point A
and B are equal, and that there is an impermeable substrata.
The equation is therefore:
DP+^SRF~^PET~^PW~^BSG~^IRF~ A~
where Q = deep percolation
Q-^,, = seepage from reservoirs
SRF
Q = phreatophyte evapotranspiration
Qpw = pumped water
= contributions from bank storage and groundwater
O = subsurface irrigation return flows to river
*IRF
AS = change in aquifer storage
£\
In some cases, this equation can be simplified by assuming that QSRF is
not transient and equals QBSG over a one-year period and cancels. Due to the
manner in which the boundaries of hydrologic subsystems were initially se-
lected, it is possible to further reduce the number of variables by adding
83
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the mass balance equations of 6 and 7 above. In which case the resulting
equation is:
\S -ASA=0 (8)
where the Qpw has been eliminated. With justification, additional assumptions
can be made to simplify the equation, such as setting QR=O and ASSM - 0 over
a month. QET can be divided into QETp +QETi where QETp is the evapotranspira-
tion from precipitated water and QETI i-s the evapotranspiration from irriga-
tion diversions. If it is assumed that precipitation is 100 percent effec-
tive, then Qp can be equal to QETp- The equation now can be written as:
Q +Q -Q -Q -Q ^-As =0 (9)
XD S R. ET PET IRF 3.
For purposes of salinity control, the most important term in this equation
is QlKFr and the others can be calculated or measured through the information
collected by the monitoring network. The subsurface irrigation return flows
can be calculated analytically by methods proposed by Glover (1975) and
others, or by use of a suitable numeric hydro-salinity computer model. The
results are then applied back through the other water balance equations
until all the unknowns are determined. From the simplified water budgeting
(water balancing) example presented above, it can be seen that the proper
selection of the hydrologic subsystem boundaries can greatly simplify the
calculations and the analysis.
DELIVERY SUBSYSTEM
The delivery subsystem consists of canals and laterals. Each segment
contains three surface water components that must be identified. These are
(a) diversions, (b) internal water accretions, and (c) operational losses
that include spillage and flow measurement errors.
The first step is collection of time distributed records on canal
diversions from the river or reservoir. These records are usually available
from state or federal agencies and the irrigation companies. The irrigation
companies can usually supply time distributed headgate diversion records,
and sometimes have on-farm diversion records as well. However, in most
cases, the farm diversions must be collected by project personnel.
Usually, the internal accretions to a delivery subsystem are not
measured by any of the aforementioned agencies, but must be collected to do
proper mass balance analyses. These additional sources of water can come
from drainage wells or supplemental irrigation wells, return flows from
higher lands, groundwater inflows, if the canal passes through a high water
table area, and intermittent streams, washes and drains that intersect the
canal system.
Operational losses, often called administrative losses, primarily
consist of spillage, but also include all measurement errors that cannot be
84
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accounted for by other means. Spillage is water returned to the river as a
result of canal or lateral discharges for the purpose of regulating water
levels in the delivery subsystem. In many areas, total spillage is about
5 to 10 percent of the total diversions, while in extreme cases it can be as
high as 30 to 50 percent.
Surface hydrology of an area is usually easy to define. Flows can be
measured by standardized methods with minimal error. However, to adequately
account for all surface flows in a large area may require extensive instru-
mentation and/or personnel to collect and analyze data. It is primarily for
this reason, and due to the nature of concurrent subsurface investigations,
that a small area is usually heavily instrumented and the results extended to
the larger area.
Seepage from canals, laterals and other channels must be determined.
For example, the effectiveness of existing linings, proposed lining, or
conversion to closed conduits for canals and laterals cannot be assessed
without proper seepage measurements. Seepage measurements are also normally
used to define conveyance efficiency functions that are used for calculations
in the hydro-salinity model. Seepage measurement techniques are discussed
in Section 8.
FARM SUBSYSTEM
Evaluation of existing on-farm water application methods is one of the
most important parts of any hydro-salinity investigation. The irrigation and
cultural practices of a grower affect many of the parameters of irrigation
return flows. For example, cultural practices can influence the infiltration
and erosion properties of the soil. The irrigation practices, including the
type of irrigation system, time duration of individual sets, and field size
affect the application efficiency, the uniformity of water application over
the entire field, volume of surface runoff, soil moisture storage, and deep
percolation.
Evaluation of on-farm water application practices must be done with
respect to the individual irrigation event and the seasonal or annual
irrigation practices. Each irrigation of the season needs to be individually
evaluated to determine the seasonal impact of irrigation for that field.
These evaluations must be reduced into the subcategories of irrigation
management and the irrigation method considerations. These relationships
are quantified in terms of the irrigation performance. This procedure and
interrelationships of the on-farm subsystem are illustrated in Figure 22.
Irrigation performance, as used in this manual, is defined as the
aggregate result of the water application characteristics of the individual
irrigation system and the quality of irrigation management provided by the
grower. The evaluation of irrigation performance at each of the two levels
should define the performance in relation to irrigation management practices
and physical operational capabilities or conditions of that system. These
relationships identify the specific effects and results of irrigation
on the soil and the area.
85
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Irrigotion Monogement
Analysis
Irrigation Management
Analysis
Seasonal Management and
Irrigation Performance
Relationships
irrigation Performance
Did this Irrigotion
Satisfy Pollution
Control Requirements?
Formulate
Acceptable
Alternatives
Seasonal Irrigation
System Analysis
Seasonal irrigation
System Performance
(Cumulative Effect of All
Irrigations of the Season)
Does Seasonal Irrigation
Performance Meet Pollution
Requirements?
Formulate Acceptable
Alternatives
Implementation
Program
Continued Monitoring
And Evaluation
Irrigation Method
Analysis
Operational and
Irrigation Performance
Relationships
Test Alternatives
(Subsequent Irrigations)
irrigation Method
Analysis
Seasonal Operational
and Individual Irrigation
Performance Relationships
Test Seasonal Alternatives
(Next Season)
Figure 22.
Schematic representation of the investigative procedure for evalu-
ation of specific irrigation events and seasonal irrigations.
86
-------
When the irrigation performance has been established, it is then
necessary to evaluate the system in terms of pollution control objectives.
If the system does not meet these requirements, the parameters or other
potential management and/or operation conditions that cause the system to
conform must be defined, and acceptable alternatives for their implementa-
tion must be formulated. These alternatives should then be tested on sub-
sequent irrigations or seasons using the same procedures as outlined above.
The results are utilized to provide direction for a full-scale implementation
program.
An evaluation of each irrigation event throughout the irrigation season
is necessary because several environmental factors can change with time. For
example, infiltration functions generally decrease as the irrigation season
progresses because of soil compaction and soil sealing effects of sediment
and chemical action. In addition, salinity and ionic balances change due to
concentrating effects and nutrient movements and off-season leaching. The
elevations and the respective chemical composition and concentrations of
the groundwater vary significantly throughout the year. A detailed schematic
of the factors to consider in the individual irrigation analysis is presented
in Figure 23, and for the seasonal analysis in Figure 24.
When evaluating an irrigation system, the fields and crops should be
representative of conditions encountered in the entire area, and all tests
should be run at a time corresponding to when the crop is to be irrigated.
Or, if the land is not irrigated at all, tests such as infiltration deter-
minations should be conducted at a time when the soil moisture is relatively
low. Extra effort should be made to ensure that conditions of the tests
closely approximate the actual conditions of irrigation. For example, in-
filtration tests should use the same water which is used for irrigation.
The evaluation of on-farm irrigation systems for salinity control is
primarily concerned with developing alternatives for the control of the
volume of deep percolation. Figure 25 illustrates many of the parameters
that must be considered in the formulation of these alternatives. The
collection of these data is discussed in Section 8.
WATER REMOVAL SUBSYSTEM
The surface hydrology aspect of the water removal subsystem is concerned
with conveyance channels that return excess water (tailwater) from the ends
of the fields to wastewater channels that carry all other sources of excess
water, including canal and lateral spillage, to the river. These wastewater
channels often function as open drains and carry some intercepted groundwater
flows. Since this is a conveyance system similar to the delivery subsystem,
there are flow quantities, internal accretions, and outflows that must be
measured or indirectly determined. The surface hydrology component of the
water removal subsystem has basically the same criteria as the surface hy-
drology component of water delivery subsystem. Therefore, data requirements
and evaluation methodology are similar.
87
-------
Formulate Acceptable Alternative
Operation A Management
Condition* Which Would
Achieve the Desired
Irrigation Performance
Seasonal Irrigation
System Analyst
Evaluation and Improvement
Figure 23. Detailed schematic for individual irrigation event on-farm
subsystem analyses.
88
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Individual Irrigation
Analysis —
Evaluation and
Improvement
Seasonal Irrigation
System Analysis-
Evaluation and
Improvement
Quantify Relationships
Between Seasonal Irriga
lion Management and
Irrigation
System Performance
Seasonal Irrigation System
Performance
(Cumulative Function of
All Previous Irrigations)
Quantify Relationships
Between Seasonal Irriga-
tion Performance and the
Individual Irrigations
Does the Existing Seasonal
Irrigation System Performance
Satisfy Pollution Control
Requirements P
No
Define Seasonal Irrigation
Performance Guidelines Which
Would Achieve the Pollution
Control Requirements
Formulative Acceptable Seasonal Alterna-
tive Operation and Management Conditions In-
cluding Each Individual Irrigation
Implementation of Pollution
Control Program for Entire
Irrigated Area Using the Most Cost
Effective Measures
Continued Monitoring and
Evaluation of Area
Figure 24. Detailed schematic for seasonal on-farm subsystem analyses.
89
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River
Open Supply
Ditch ( Lateral)
Tailwater
Ditch
-------
The subsurface component of the water removal subsystem and the hydro-
salinity model generally is much more difficult to define than the surface
portion. Much of the data needs to be evaluated indirectly rather than by
direct means such as flow measurement. There are three main components of
the subsurface water removal subsystem budget that must be determined. These
are groundwater flows to the area, ditch seepage, and deep percolation
contributions from agriculture, and consumptive use by phreatophytes. Deep
percolation losses and consumptive use are considered in Section 8.
In some areas, septic tank leach fields and urban landscape irrigations
contribute substantially to the groundwater flows. Some municipalities
and industries may purposely inject water for storage or disposal. In areas
where groundwater is pumped for municipal or irrigation purposes, the hy-
drology can become very complicated in attempting to define both the surface
and groundwater segments. Groundwater inflows are usually determined by the
use of wells, piezometers and various aquifer tests to determine the rate
and quantities of these flows. There are numerous references on groundwater
hydrology that include McWhorter and Sunada (1977); Welton (1970); Todd (1960)
and Glover (1974) and others.
Hydro-Salinity Models
Salinity problems from irrigated agriculture generally result from
subsurface return flows. The capability of a hydro-salinity model to provide
necessary information for arriving at technological solutions is, therefore,
dependent on the accuracy of groundwater field data and analysis. A problem
often encountered during preparation of water and salt budgets is the
reliability of the measured data. Usually, the precision of measurement
varies with the scope of the investigation and the area under study. Since
the hydrologic system is difficult to monitor and predict, it is impractical
to expect models to operate without applying some adjustments in order that
all components will balance. The budgeting procedure is defined as a
weighting of the contributing factors in water and salt flows until all
parameters represent the closest possible approximation of the conditions
of the area. A schematic diagram of a general hydro-salinity model is
shown in Figure 26. Oster and Wood (1977) discussed the sensitivity of some
hydro-salinity models to various input parameters.
One of the early steady-state hydro-salinity models was developed by
Walker (1970) and applied to the Grand Valley of western Colorado. The
following description of the respective components of this model sufficiently
describes the principles used in similar models, a few of which are
referenced later.
Cropland Diversions—
Diversions to the croplands are accounted for by simple numerical
budgeting procedures. The water flows are divided into several categories
depending on the physical constraints of the system. For example, gravity
irrigation supply is usually diverted by means of diversion dams. It is
then conveyed through irrigated lands with water being lost by seepage,
spilled into wasteways, evaporation, and discharged through turnout structures
91
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Irrigation System
Water Supply
(Reservoir and/or
River System)
Main Delivery Subsystem
Canal
Diversions
LL
1
1
1
Canal
Seepage
••
i
i
i
Lateral
Diversions
Lateral
Subsystem
1, M— -
— « 1 Lateral On-Farm
; 1 Seepage Diversions
•
. .. -L
Farm
Subsystem
Root-Zone
Supply
i
i
I
1
1
<
Deep Consumptive £
Percolation Use
-------
into laterals or farm supply ditches. Canal regulations by spillage may
be facilitated by wasteways such as natural washes and man-made drains
which may be located throughout the irrigated lands. These wasteways may
also serve as outlets for subsurface return flows. Diversions into the
lateral system are also reduced by seepage. Evaporation from canal and
lateral surfaces is usually insignificant.
Root-Zone Flows—
The goal of irrigation is to recharge the soil-moisture of the root-zone
with sufficient water to meet the needs of the crop until the next irrigation.
Irrigation also serves to maintain an acceptable salt concentration in the
root-zone. As described previously, overirrigation produces high water
tables (waterlogging) and salinity problems in many areas. The root-zone
submodel makes a detailed examination of the various flows occurring within
the root-zone in order to quantify the salinity problem.
The important water movements within the root-zone are evapotranspiration
and deep percolation,.with water storage changes also occurring. Separation
of these flows to take measurements on a large scale is impractical. Con-
sequently, empirical computational methods are employed. The model described
herein accounts for these basic water and salt flows by a budgeting process.
The operation of this model assumes that irrigations are applied uniformly
over each acre of cropland. Phreatophyte vegetation in the area is assumed
to extract water only from groundwater flows or only use natural precipitation
which falls on the area occupied by these plants. A generalized flow chart
of the root-zone budgeting procedure is presented in Figure 27.
Several methods for estimating evapotranspiration can be used in this
model. The locally calibrated Blaney-Criddle Method has provided an
acceptable degree of accuracy for many studies. The Jensen-Haise Method and
the Penman Method are more precise but require more climatological data.
With the evapotranspiration data and field measurements of moisture
holding capacity, infiltration rates, and rooting depths, the budgeting
scheme proceeds with computation of deep percolation losses from the root-
zone. Calculations are initiated by assuming the crops use soil-moisture
at the potential rate until the wilting point is reached, assuming that
there are no adverse effects on the plant due to crop stress. The calculated
potential consumptive use is limited to the water added by irrigation and
the existing available soil-moisture storage. If irrigation water added to
the root-zone is insufficient to meet demands of the crop, but storage of
soil-moisture is sufficient to make up the difference, the need for water
is satisfied. The assumption is also made that no deep percolation occurs
while the available soil-moisture storage level is below field capacity,
and that deep percolation and leaching occurs above this value. If the
total available moisture for the period between irrigations is insufficient
to meet the total demand, the crops use all water available. A term called
"consumptive use deficit" is defined as the difference between potential
and actual utilization.
93
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IS
Area of
Cropland=0?
USEfO_J—
USE=PREC[-
USE=RZS+RZST
IS
(RZS-PCU)>
(RZC-RZST)?
| AGW=(RZS-PCUHRZC-RZST)
RZST=RZST+(RZS-PC
is
Area of
Land Used?
Figure 27.
PCU= Potential Consumptive Use
PREC= Precipitation
RZC= Root-Zone Capacity
RZST= Root-Zone Storage
CUD= Consumptive Use Deficit
AGW= Additions to Ground Water
Illustrative flow chart of the root-zone budgeting procedure
(Walker, 1970).
94
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Salts in the water applied to the crops move into the root-zone where
they become concentrated by evapotranspiration. The behavior of specific
ions is complex and has not been considered in this particular model, but is
discussed in Section 7.
Groundwater Model—
Most of the water in the soils and shallow groundwater aquifers originate
as seepage from canals and laterals, deep percolation from the irrigation of
croplands, and tributary subsurface inflows. Groundwater discharges eventually
reach a local river or stream as surface drainage interception or subsurface
return flows. The flows in the surface drainage system can be measured by
installing flow measuring devices at the outflow points. Subsurface return
flows are estimated from water table elevation data, the hydraulic gradients
and the estimated hydraulic conductivities of the aquifer. It should be
noted that in any hydro-salinity model, these estimated hydraulic conductivi-
ties contain the largest potential for error. Therefore, considerable effort
must be made to properly evaluate the necessary parameters in the groundwater
computations. For purposes of this model, Darcy's steady-state equation is
used,
Q = AK ^ (10)
in which Q is the discharge, A is the cross-sectional area of flow, K is the
saturated hydraulic conductivity, and dh/dx is the hydraulic gradient in the
direction of flow. Transient groundwater flows can be evaluated by other
procedures such as those developed by Glover (1974 and 1975), and Morel-
Seytoux and Daly (1975).
The groundwater analysis used in the Walker model, illustrated in Figure
28, starts by comparing the values for subsurface return flow, obtained from
a mass balance of the area, to values obtained by computer calculation which
uses field data. Therefore, two estimates of the subsurface return flows
are formulated. The model is then adjusted until both methods yield the
same values, thus obtaining a satisfactory alignment between the hydrologic
and salinity parameters if no significant sources or sinks have been over-
looked. Because the model only focuses on the relative magnitude of hy-
draulic conductivities, only the relative cross-sectional areas of the
strata are important. The width can be any convenient value. The values
for cross-sectional areas can be adjusted and used with the known hydraulic
conductivities which are only known for selected points in the aquifer. The
model adjusts the values of strata hydraulic conductivity until both esti-
mates of the flows are equal. Since this is done on a monthly basis, the
model calculates 12 values of hydraulic conductivity yearly for each strata.
When adjustments in the model finally result in homogeneous annual values
of hydraulic conductivity, the model represents the "best fit" between
monitored and estimated data.
Available Models—
In addition to the hydro-salinity model reported by Walker (1970),
several other models may be utilized. Hillel (1977) presented a simulation
model that evaluates precipitation, infiltration, runoff, evapotranspiration,
95
-------
Comparison of
Homogeneity of
Hydraulic
Conductivity During
Water 'fear
i = Refers to ith Strata
Grad. = Hydraulic Gradient
Q =Computed Ground Water Outflow
PHC =Field Values of Hydraulic Conductivity
AHC =Adjusted Values of Hydraulic Conductivity
TGWOF= Total Ground Water Outflow from Mass Balance Analysis
AQ =TGWOF
Figure 28. Flow chart of the groundwater modeling procedure
(Walker, 1970).
96
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deep percolation, capillary rise from the water table, and groundwater
drainage of an agricultural field.
A similar model was designed by Makkink and Van Heemst (1975). The
chemical quality aspects of irrigation return flows are simulated in models
by Shaffer et al. (1977), the United States Department of the Interior,
Bureau of Reclamation (1977) , Crawford and Donigian (1973), and Hill et al.
(1973). A summary of these models including the input requirements, time
and spatial scales, computer structure, mathematical approach, and their
acquisition is given in Appendix A. Most of these models include more
modeling than is necessary for planning purposes. In all cases, salinity
is only one aspect of these models.
97
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SECTION 7
SOIL MOISTURE—CHEMISTRY SIMULATION
Detailed simulations are used to provide refinement to the hydro-
salinity models primarily in the temporal prediction of root-zone and deep
percolation salinities. These calibrated models can locally predict the
quantity and quality of the leachate with respect to time. They can also
quantify the effects of reducing irrigation return flows and the corresponding
reductions in salinity. Their most valuable purpose is to establish the
functions relating groundwater or deep percolation quantities and salt
loading. In effect, such models are microscale hydro-salinity models that
consider individual chemical reactions, the ionic constituents and the water
flow system.
Since almost every area is unique, the models must be applied with care.
Model use depends on a thorough understanding of the model capabilities and
limitations. For example, many models were formulated to handle certain
types of chemical systems; and, if the governing chemical process should
happen to be different in a new area, the model cannot be used without
adjustment. Successful application of the proper model requires an adequate
data base, and the exact form of the data needed will often be different for
each model. The type of data available may influence the selection of the
model to be used. Detailed models will usually handle a combination of
saturated and unsaturated flow conditions as well as the governing chemical
reactions. McNeal (1974) examined soluble soil salts and their relationships
to soil water movement.
Soil moisture-chemistry investigations should include consideration of
the movement of salts or dissolved constituents, as well as the displacement
of the solvent (water). Biggar and Nielsen (1967) stated:
...such considerations become particularly important in irrigated
agriculture when it is desirable to know the concentration and
location of a dissolved constituent in the soil profile, the
reactions of constituents with each other and the soil matrix
during the displacement and transport of water and solutes to
plant roots.
It is important to establish some of the more important physio-chemical
processes occurring in the soil which cause changes in the irrigation water
as it moves through the root-zone. For example, it is necessary to consider
the ion exchange and its relationship to the relative proportions of ions in
the adsorbed phase and the solution phase. The precipitation and dissolution
of slightly soluble salts can also be affected by the composition concentration
98
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of the soil-water solution and the partial pressure of carbon dioxide gas in
the soil. The spatial variability of soils and the transport and mixing of
the resident soil solution with the infiltrated irrigation water will also
have a significant impact on the soil-water chemistry.
Much of the information on soil hydraulic properties required for the
different models is included in the root-zone diversions, infiltration
functions, initial soil-moisture conditions, field capacity, which is also
referred to as residual saturation, permanent wilting point, bulk densities,
and porosity of the various soil layers. It is necessary to establish the
hydraulic conductivity-moisture content-capillary pressure relationships which
are collectively referred to as the hydraulic properties of the soil (Figure
29). Additional chemical information required includes the respective con-
centrations of various ions in the soil-water solution, soil temperature with
depth, and characteristic chemical reactions of the dominant ions which may
include nitrogen, sulfates, carbon compounds and others that are typical of
that particular soil chemistry system. Most of these data must be determined
by thorough laboratory analyses of field soil and water data.
MODELING CONSIDERATIONS
The hydro-salinity models generally describe the existing water and salt
flow conditions in an agricultural area. Many methods for predicting the
reduction in salts returning via the groundwater to the river as an outcome
of salinity control measures assume a one-to-one relationship between water
and salt flows. For example, when subsurface return flows are reduced by
50 percent, it is assumed the salt pickup is also reduced by 50 percent. This
is a poor assumption since this is not always the case. It is usually
necessary to perform a soil moisture-chemistry simulation to arrive at the
correct relationships between the volume of return flows and the mass
emission of salt.
The primary objective of soil moisture-chemistry simulation is to model
the transport of salts through the soils. The first portion of the flow of
water and consequent transport of salts is through the rootrzone which is
usually partially saturated. A transient model of the moisture flow and
chemical and biological reactions occurring in the root-zone was developed
by Shaffer et al. (1977) and is described in Appendix A. An examination of
this model illustrates the principal concepts in the detailed simulation level
of irrigation return flow modeling.
The Shaffer Model consists of three separate programs. The first program
describes soil-moisture movement and distribution with time. The second
program interfaces the soil-moisture movements with a chemical-biological
model to reconcile differences in the soil layers used in the calculations of
soil moisture and chemistry. The third program computes the chemical and
biological activity occurring in the soil profile. Figure 30 shows a block
diagram of the overall model. A brief description of the moisture flow and
chemical-biological models is included as a basis for understanding the data
collection requirements.
99
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• Fine Sand
o Volcanic Sand
1.2
1.0
6 0.8
^
* 0.6
0.4
0.2
"• mx!-Sr'
Km= Hydraulic Conductivity
at Saturation
3<«<4 For Sands
€«4 For Typical Soil
c>4 For Well Aggregated)
Soils
Sr = Residual Saturation
• Fine Sand
o Volcanic Sand
0.2 0.4 0.6 0.8
Saturation-S
(a) Typical Relationship between
Capillary Pressure Head and
Saturation.
I0c 1—r-i
0.8
1.0
V0 0.2 0.4 0.6
Saturation-S
(b) Typical Relationship between Effective
Hydraulic Conductivity (Ke) and
Saturation.
10
O
X
10
•x
E
o
1.0
O.I
0.01
• Fine Sand
o Volcanic Sand
i i i i i 111
100
Figure 29.
1.0 10
Capillary Pressure Head-h,cm
(c) Typical Relationship between Effective Hydraulic
Conductivity (Ke) and Capillary Pressure
Head.
Typical relationships defining the hydraulic properties of a
soil ( Brooks and Corey, 1964).
100
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INPUTS
WATER APPLICATION
AND CONSUMPTIVE
USE DATA
MOISTURE FLOW
PROGRAM
INPUTS
INITIAL WATER CONTENT
AND PHYSICAL PROPERTIES
OF SOIL
OUTPUTS
MOISTURE CONTENT AND
MOVEMENT WITH TIME
INPUTS
FERTILIZER AND ORGANIC-
N APPLICATIONS,
TEMPERATURES,CROP TYPES
BIOLOGICAL
CHEMICAL PROGRAM
INPUTS
INITIAL CHEMICAL AND
PHYSICAL PROPERTIES
OF SOIL
OUTPUTS
WATER, NITROGEN AND
SALTS ENTERING GROUND
WATER
Figure 30. Generalized block diagram of the model (Dutt et al., 1972)
-------
The flow is one-dimensional and was developed using Richard's equation
with a sink term. The model is schematically defined in Figure 31. Mathema-
tically, flow is described by the diffusivity form of the Richard's equation:
fl - a - s (ID
GT 3x
where 8 = volumetric water content
T = time
x = length
K = hydraulic conductivity
S = sink term; and
D = diffusivity.
The sink term S is computed using the Blaney-Criddle equations for evapo-
transpiration although other equations can be adapted to the model. Loss due
to evapotranspiration is distributed through the soil profile by assuming a
specific root distribution for the crop. The root distribution and coeffi-
cients for the Blaney-Criddle equations are supplied by the user. When
known, actual values of evapotranspiration can be used in the sink term.
Salt transport in one dimension is described by the following equation:
3c _ 3 i 9c\ ^Bc
3t 9z V 9zy 9z
where c = solute concentration
t = time
D = hydrodynamic dispersion coefficient
z = depth; and
V = seepage velocity (Darcy velocity divided by porosity).
By assuming the term -%% ID ~3Zis negligible compared to V^"' the
ID ~3Zji
equation reduces to jj.| = ~v|~ Tnis assumption implies that transport due
to diffusion in partially saturated soils is negligible as compared to the
convective transport. The model computes the moisture flow (V) and couples
flow with chemical changes -jp- computed in the bio logical- chemical program
to provide the salt transport rate. This technique is called a mixing cell
concept.
102
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I
o
c
START MOISTURE
FLOW PROGRAM
1
i
PROGRAM MOISTRE
READ CONTROL AND INPUT DATA
COMPUTE MOISTURE CONTENT AND
FLUX FOR EACH DEPTH NODE AND
TIME STEP
WRITE ON MAGNETIC TAPE OR
PRINT OUTPUT
t
SUBROUTINE THEDATE
COMPUTE CALENDAR DATE FROM
DAY NUMBER
i
SUBROUTINE CONUSE
COMPUTE VALUE OF MACROSCOPIC
SINK TERM
1
f
c
STOP MOISTURE
FLOW PROGRAM
u.
_l
<
I
o
<
Ul
§
-------
The chemical exchange model computes the equilibrium chemistry
concentrations for calcium, magnesium, sodium, bicarbonates, carbonates,
chlorides, and sulfates. The nitrogen chemistry, including ammonium, nitrates,
and urea-nitrogen, uses a kinetic instead of an equilibrium approach. A block
diagram of the biological-chemical model is given in Figure 32.
Once the necessary field data are collected, equations can be developed
to predict the variation in chemical quality, including ionic constituents
of water in the soil profile. The salt pickup or salt precipitation resulting
from movement of subsurface irrigation return flows is also determined.
These results, when combined with the hydro-salinity model, allow an evaluation
of various salinity control measures for reducing salinity reaching the
groundwater and returning to the river.
Other Available Models
In addition to the highlj detailed simulation model by Shaffer et al.
(1977), there are several simple models that may be quite appropriate for a
planning study. One of the more complete irrigation hydrology models is
presented by Gupta et al. (1977). This model gives a fairly complete water
budgeting procedure for the region between the crop canopy and the water
table. The most important feature of this method is its flexibility. The
authors provided the capability to use several analytical approaches for
major segments of the program. While the model does not consider water
quality, the model could be coupled with salinity models reported by Oster
and McNeal (1971), Rai and Franklin (1973), Tanji et al. (1972), and Saxton
et al. (1977). Models by DeWit and Van Keulen (1975), Frissel and Reiniger
(1974), Margheim (1967), and Melamed et al. (1977) encompass both the
irrigated hydrology and the quality of return flows on a detailed level.
104
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I
o
<
Ul
/"START BIOLOGICAL-^
VCHEMICAL PROGRAMS
PROGRAM MAIN
READ CONTROL AND INPUT DATA
STORE INITIAL SOIL-CHEM DATA
PRINT CONTROL AND INPUT DATA
(OPTIONAL)
SUBROUTINE EXECUTE
MAKE ANY FERTILIZER AND/OR
ORGANIC MATTER APPLICATIONS
INITIALIZE OR UPDATE SOIL
TEMPERATURES (WEEKLY)
READ MOISTURE FLOW DATA FROM
MAGNETIC TAPE
SUBROUTINE COMBINE
FOR EACH SEGMENT:
CALL EXCHANGE SUBROUTINE
CALL NITROGEN SUBROUTINE
CALL SOLUTE REDISTRIBUTION
SUBROUTINE
CALL PLANT-N UPTAKE SUBROUTINE
SUM CHEMISTRY CHANGES AND
UPDATE VALUES IN STORAGE
PRINT OR WRITE SPECIFIED VALUES
a.
LU —.
UJ
2<
HZ
1 1
-------
SECTION 8
FIELD INVESTIGATIONS
The purpose of field investigations is to determine and/or predict the
results expected from various potential solutions. Evaluating the effective-
ness of existing practices and methods provides insight into more efficient
and economical operational methods which result in savings of labor, money,
and water, and a better balance between crop returns and irrigation costs.
The evaluation procedure provides direction to action programs for the reduc-
tion of nonpoint pollution from that area and similar areas.
Because many of the nonpoint parameters tend to be site specific, every
irrigated area should be evaluated individually. The same conditions en-
countered in one area may produce different consequences in another. If the
uniqueness of each area is recognized, it may be possible to apply data col-
lected in one area to other areas. For example, much of the information
gathered in the Colorado River Basin may be applied to irrigated areas in the
Rio Grande and Arkansas River systems even though irrigation practices,
cropping patterns, topography, soil types, and crops may differ. The method-
ology employed can certainly be made applicable to other areas under
investigation.
This Section is divided into three primary discussion areas t (a) deli-
very subsystem investigations; (b) farm subsystem investigations, and (c)
water removal subsystem investigations. It should be realized that there is
an overlap between these broad topics because many of the techniques apply
to all three categories. Many of the methodologies discussed in this section
apply to the collection of data for both hydro-salinity modeling and soil
moisture-chemistry simulation.
This section is concerned with the analysis of the managerial and
physical inputs to an irrigation system. It is often necessary to evaluate
the economic, legal and sociological aspects of pollution control problems
in the area. These analyses should establish the effects and costs of the
proposed solutions, potential crop yields (increase or reduction), land costs
(initial investments, preparation, seed, fertilizer), irrigation costs (labor,
energy, maintenance), and existing crop yields as functions of an area,
water quantity and costs. Interest rates, taxes, expected life of equipment,
bank lending practices and availability of loans, and many other financial
aspects of the area also need examination. Modifications to the existing
institutional framework or establishment of one to adequately handle an
implementation project must also be determined.
106
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PROCEDURE
There are three basic steps that must be undertaken to evaluate non-
point source pollution in any irrigated area. These are:
1. Establishment of the boundaries of the area to be studied;
2. Determination of whether there is a problem by using inflow-outflow
analysis, where inflow minus outflow equals the change in storage;
and
3. Determination of the source of the problem with a monitoring net-
work and subsequent investigations of the delivery subsystem
(canals and laterals), the farm subsystem (irrigation methods,
cultural practices), and the water removal subsystem (type, reuse).
Obviously, these are broad categories and the depth to which these investiga-
tions are conducted depends on the cost and the degree of accuracy required
for the study, as well as the limitations of the applications of the IRF
Models including time frame requirements, data needs, and applicability to
the situation.
Determination of the sources of the problem is the most time consuming
and costly portion of the investigative procedure. This process is usually
referred to as the hydro-salinity analysis, the methodology of which varies
depending on the types of problems encountered. Regardless of the type of
problem, the usual procedure is to instrument and investigate a small study
area intensively, and to sample many of the important variables on the re-
mainder of the irrigated lands not in the study area. Results are then
extrapolated to the entire area. Careful selection of the area for inten-
sive study must be done to ensure the area is large enough and contains
soils, field conditions, irrigation methods, and other aspects that are rep-
resentative of the larger area.
The boundaries of the study area are usually dependent on natural hy-
drologic characteristics such as hydrologic divides, watersheds, drainage
systems, and the canal or lateral network. In the first step, the whole area
is defined. During subsequent investigations, the boundaries may be incre-
mentally reduced so only one farm or field is considered. In small areas,
hydrologic conditions can be studied in detail, and the various segments of
water and salt flow budgets can be isolated and more accurately measured.
DELIVERY SUBSYSTEM INVESTIGATIONS
Field investigation techniques that are particularly important in
nonpoint source pollution control programs for the delivery subsystem are
primarily quantity of flow and seepage measurements. Often, the importance
of flow measurement is neglected in irrigated agriculture, but accurate mea-
surement of surface water is required for water resource evaluations as well
as for efficient water management, and irrigation return flow studies. Er-
rors in flow measurement tend to be cumulative, and the end result can cause
107
-------
errors in excess of 100 percent of the real values. Accurate flow measurement
is required in seepage measurement and various on-farm investigations.
Flow Measurement of Surface Water
Any person attempting to conduct a hydrological investigation should
have a good understanding of the theory and methodology of various flow mea-
surement techniques, including the limitations, advantages, and expected ac-
curacy for each method. For example, weirs can not always be used in ir-
rigated areas due to high hydraulic energy losses; Parshall or Cutthroat
flumes should not be placed immediately below a culvert or other physical
devices that cause nonuniform flow; and precalibrated propeller meters are
accurate only for specified situations and can be severely affected by
sediment.
Another consideration is that most flow measurement devices, particularly
for open channel flow, give only the instantaneous rate of flow. If the flow
rate changes, as is common with surface delivery systems, the use of an
instantaneous measurement to calculate an irrigation application may easily
result in errors of 20 percent or more. It is therefore usually necessary
to obtain some type of a totalizing record of the flow.
Units of volume and of volume per unit time are the basic units required
for water measurement in irrigated agriculture. The units of volume commonly
used in agriculture are gallon, cubic foot and acre-foot, while the corre-
sponding metric units are liters, cubic meters and hectare-meters. The
common rates of flow in English units are gallons per minute, cubic feet per
second, and Miner's Inch, which is defined by each state's legislation and
varies from state to state. The common metric units are liters per second
and cubic meters per second, or cubic meters per day.
During most hydrologic investigations, it is necessary to obtain a
temporal history of water flow. There are numerous commercial devices avail-
able that provide continuous records for analysis, such as clock-driven
water level recorders. Information on the installation and availability of
such instruments can be obtained through the local office of the Water
Resources Division of the United States Geological Survey or other federal
and state water resource agencies.
There are numerous detailed references for surface water measurement
techniques including the United States Bureau of Reclamation (1974) , United
States Geological Survey (1968-1978), Skogerboe et al., (1967a), Bos (1976),
and Robinson and Humpherys (1967). An excellent theoretical description of
flow measurement is presented by Troskolanski (1960). Thomas (1957) presents
a good discussion on sources of error in flow measurements for irrigation.
For purposes of discussion, flow measurement methods are divided into the
three broad categories of velocity, hydraulic head, and other miscellaneous
methods. Velocity methods include Pitot tubes and current meters, venturi
meters, and propeller meters. Hydraulic head techniques include the
Parshall, Cutthroat and trapezoidal flumes, as well as weirs and orifices.
Other miscellaneous techniques include chemical salt and dye dilution, total
count radioisotopes, magnetic and sonic methods.
108
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Velocity Measurements—
Velocity determination may be made in open channels or in closed conduits
with Pitot tubes (King and Brater, 1963). Pitot tubes can be calibrated to
read the flow rate directly from one velocity measurement. One such device
is known as the Cox meter. Another special form of the Pitot tube which is
very useful in measuring pump discharges is the Collins flow gage (Figure 33) .
This device consists of an impact tube which is a straight small diameter
brass tube inserted through a pipe. The tube is divided into two compart-
ments, each with an orifice 180 degrees apart. One is called the impact
orifice and is oriented to the upstream side, and the trailing orifice,
is located on the downstream side. The differential head indicated on'the
air-water manometer is twice the velocity head, from which the discharge
rate can be determined.
The determination of mean velocity and the calculation of discharges in
open channel flows are usually done by methods such as current meter mea-
surements. Current meters are instruments that employ an impeller which
rotates at a speed proportional to the velocity of the flowing water. The
cross-section of flow is divided into a number of subareas varying from 0.15
to 6 meters (0.5 to 18 ft.) in width, depending on the size of the stream
and precision desired. It has been found that mean velocity readings taken
at 0.2 and 0.8 of the depth below the surface is an accurate estimation of
the average velocity in the vertical direction. Discharge is then determined
by application of the continuity equation. Current meters are often used
as the standard by which many other methods are calibrated, and depending on
the type of equipment, skill and care of data collection, the current meter
values are accurate. Any structure that has a constant cross-sectional area
can be rated for use as a flow measuring device, and these ratings are
usually established with a current meter. A typical set of rating curves is
shown in Figure 34.
When a horizontal pipe, for example from a pump, discharges into the
atmosphere, the discharge can be obtained using the trajectory of the jet
which is a function of the velocity of discharge. The discharge of Q is
given by the equation:
Q = cc
where C is the units conversion constant, C is the coefficient of discharge,
A is the cross-sectional area of the pipe, and X and Y are the horizontal
and vertical coordinates. Israelson and Hansen (1967) discuss this method
in more detail as shown in Figure 35b.
A vane or pendulum type meter is a velocity measurement with a variable
width vane that extends into the water of an open channel. Discharge is
measured by calibrating the angular displacement of the vane.
Precalibrated propeller meters are made by a variety of manufacturers for
velocity measurement in conduits where cross-sectional areas of flow remain
constant with time (Figure 35a). These meters can be calibrated to read
directly in cumulative volume and/or flow rate. Easily portable installations
are also often used for pump or farm efficiency studies (Figure 36) . Impeller
meters can also be used, but tend to be much more expensive.
109
-------
\
i
x
1
h-
?
!
^-Air Water
Manometer
2222>^
To
Manometer
Figure 33. Schematic representation of the Collins flow gage.
Manometer
110
-------
Discharge, second-feet
^0 20 40 60 80 100 120 140 160 180 200220
I 23456
Velocity, feet per second
Figure 34, Typical rating curves for current meter measurements.
Ill
-------
a) Sketch of a precalibrated propeller meter
for use in closed conduits.
b) Trajectory method of determining discharge.
Figure 35- Trajectory method and propeller meters for measuring
discharge.
112
-------
-Pump
Flexible Plastic or
Canvas Tubing
Propeller Meter
Figure 36. Portable installation of a-pfcopeller meter used to measure
discharge from.a pump for farm or pump efficiency studies,
-------
Head Measurement Techniques
Hydraulic head measurement techniques employ the use of structures by
which discharge is a function of the depth of water or head on the device.
In other words, they have a consistent relationship between the hydraulic
head and discharge. Most of these devices used in irrigated agriculture have
standardized dimensions and known hydraulic characteristics. In open channel
flow, weirs and flumes cause flow to pass through critical depth, and in
closed conduits orifices are used.
A weir is a barrier that is placed in a channel to constrict the flow
and cause it to fall over a crest (Figure 37). Weir openings can be rec-
tangular, trapezoidal, triangular or other shapes to give specialized head-
discharge relationships. Standard references such as King and Brater (1963)
can be consulted for tables and specific discussions on weirs, Weirs working
in submerged conditions are discussed by .Skogerboe et al. (1967d) and
Chesness et al. (1973).
Generally, any specially shaped and stabilized channel section that
constricts the flow is called a flume. Flumes are generally less inclined
to catch floating debris and sediment than are weirs, and for this reason,
they are well suited for measuring runoff or other waters with high sediment
loads.
The most commonly used flume in the western United States is the Parshall
flume (Figure 38b), which has been described by Parshall in several publica-
tions (1941, 1959, 1953); Robinson in 1959 and 1965; Caplan (1963); and
the United States Bureau of Reclamation (1974). Submerged flow calibrations
for the Parshall flume are presented by Skogerboe et al. (1967b).
Another type of flume gaining wide acceptance due to its inexpensive
construction is the Cutthroat flume which can be built in a wide variety
of throat width and flume length combinations to suit specific needs
(Figure 38a). Design and installation information for free flow and sub-
merged conditions is presented by Skogerboe et al. (1967c) and Skogerboe
et al. (1973). The Cutthroat flume and Parshall flume have equivalent
accuracy and require very low head losses for operation,
A flume adapted for widely fluctuating runoff measurements is known as
the H-flume (Figure 38d), which is discussed by Frevert et al. (1966) and
the United States Department of Agriculture (1962). Another type of flume
which is well suited for small flows such as individual furrow flows is the
trapezoidal flume (Robinson and Chamberlain, I960; American Society of
Agricultural Engineers, 1977), which is shown in Figure 38c.
King and Brater (1963) define an orifice as an opening with a closed
perimeter through which water flows. An orifice with prolonged sides (2
or 3 pipe diameters in length) is called a tube. For orifice flow conditions
to occur, the water surface upstream of the orifice must be well above the
top of the opening. If the upstream depth drops below the top of the
opening, it then conforms to the conditions of weir operation (United States
Bureau of Reclamation, 1974). If the orifice is level with the floor of
114
-------
H-
Ul
Strip
»«o»o««unr Mir Cip*llettl Vtoir 9O*V- NetcH W»lr
Stanford ConlracOd W*ir«
Foot)
tr Goto
r
TM CipalUttl or
V-Notch Wtirs n«y
fe* Sinilvlt lMt«ll*4.
t, V
Orifice and Weir Formulas
Measuring
Davic*
(AHSnorp
Cr*«tml)
Orlfiea
Raetongular
W*lr
(without
Contraction)
Rectangular
Wair
(with
Contrastion)
Trapazoldal
W«ir
(Clpollatti)
»C*
Triangular
Wtlr
VMWS
y
^Hj-
Front View
1
~
Top Vim
ru.
t£—
I' (L-0.2H)
%p Vi«w
1 . ,
"fep-^rar^jH
End Vi»w
"5^5"
End Vi««
Tl
^
Sid* View
JZ,
"*:>
*J
Sid* Vi«*
Formula
0=0.6IA-HV*
0=3.37LHS/Z
0'2.4»H8/*
Figure 37. Schematic representation of various weirs used in agricultural water management.
-------
i_
Plan
L -f- 2L
3
Converging
Inlet Section
Q
.ho
1 2L
3
Diverging
Outlet Section
•
5L
hb
Section T-T
a) Cutthroat flume.
D
Plan
0
Converging Inlet
Section
Q
m Flume Crestx
Throat
Section
Hb>
VHn HJ P5^^
Diverging
Outlet
Section
J
Section T-T
•Water
Level
Recorder
b) Parshall flume.
Figure 38. Schematic representation of some common types of flumes
used in agricultural water measurement.
116
-------
•L
Plan
c) Trapezoidal flume.
K- 1.9 D -H
Well
Opening Wafer Leve|
Recorder
I- I.9D -I
Front Elev.
-U35D h
Side Elev.
*For D
-------
a structure, it is called a gate or sluice; however, often these must be
individually calibrated by use of a current meter. Orifices can be submerged
with the exit jet under water or with free discharge directly to the
atmosphere. These devices are often troubled with debris problems in
agricultural situations.
A commonly used gate for controlling and measuring irrigation diversions
to a lateral or farm is called the metergate. Basically, these are standard
headgates that are calibrated to yield discharge as a function of the
difference in static head between the canal and a point located 1/3 the pipe
diameter behind the gate. These are sold commercially by several steel
headgate manufacturers and calibration tables are usually furnished by
these suppliers. Two common methods for measuring flows from headgates are
illustrated in Figure 39.
Miscellaneous Measurement Techniques--
There are a multitude of flow measurement techniques that directly
utilize velocity or head for discharge determinations, These include chemical
dye and salt dilution methods, total count radioisotope, and magnetic and
sonic techniques.
Salt dilution methods consist of adding a concentrated salt solution
of known strength to a stream. By chemical analysis, the diluted concentra^-
tion is determined after it has mixed completely with the water (United
States Bureau of Reclamation, 1974), Dye dilution techniques are quite
similar and the diluted concentration is determined by colorimetric analysis
or fluorescent analysis (Liang and Richardson, 1971; Wilson, 1968), These
methods are often used when flow conditions for making current meter mea-
surements or volumetric calibrations are unfavorable such as closed conduits,
ice covered reaches, and turbulent mountain streams, The general procedure
for dilution measurement is illustrated in Figure 40,
Radioisotope methods are a variation of the dilution techniques that
use a radioactive substance as a tracer material. The degree of dilution
is indirectly obtained by comparing gamma ray emissions from the concen-
trated isotope solution and the diluted solution, Geiger counters or
scintillation counters are required for this purpose.
One type of acoustic or sonic flow meter operates on the principle that
difference in the time of arrival of two simultaneously created ground
pulses traveling in opposite directions through the water can be related to
the velocity of flow (United States Bureau of Reclamation, 1974). The two
sources are placed on opposite sides of the channel, but one is placed a
sufficient distance downstream to yield meaningful time differences, Another
type of sonic meter is used to measure the stage height by measuring the
distance to the water surface from the meter.
Magnetic flow meters utilize a section of nonmagnetic closed conduit
with two magnetic coils on opposite sides. In effect, water acts as a
rotor in a generator and the induced voltage measured by electrodes in the
wall indicates the rate of water flow, Troskolanski C1960) discusses this
method in more detail.
118
-------
Meter Totalizer or
Transmitter for -x
Remote Recorder 1
Concrete »
Minimum Headwall_j7
^ f Water Surface
Longitudinal Section
Drop
.-Pipe
End View
a) Propeller meter installation for measuring headgate diversions,
Amount of Opening is Shown by Distance Between
Notch on Lift Rod and Top of Lift Nut
.!.-=•
. 7™%
Difference ^<
in Head AH ^\
not Greater
tha
Bottom of
Outlet Ditch-
Tap must
be on Top
of Pipe
C O
es
i —
j
/'Bottom of
\Supply Ditch
\ V'mir
, Minimum 12" or other Minimum
Distance which would Insure Complete
Submergence of the Outlet Pipe in
Order that there will always be a
Positive Water Measurement in the
Metergate Tank.
b} Metergate installation for measuring headgate diversions,
Figure 39. Two closed-to-open conduit methods for measuring headgate
diversions.
119
-------
Tracer
q= Injection Rote ,.-^>
C|= Tracer Concentration
QC0+qC, = (Q+q)(C2)
Q = q 71—T2- second - f eet
C2-C0
100% Tracer
Dispersion
Figure 40. Schematic representation of methodology for chemical dilution
techniques of flow measurement (United States Bureau of
Reclamation, 1974).
Methods of Measuring Seepage Rates
There have been numerous methods developed for measuring seepage from
canals and laterals involving both field and laboratory investigations, Each
has its own unique characteristics that make it useful under certain con-
ditions. The objective of this discussion is not to describe different
methods of measuring seepage rates, but to provide a review of several methods
that could be used as a part of salinity investigations.
The factors influencing seepage rates are many and complex. Among the
more important ones are soil characteristics of the channel bed, time of
year the tests are made, length of time the channel has been operating, depth
to groundwater, sediment load in the water, depth of water in the channel,
temperature of both the water and soil, barometric pressure, biological fac-
tors and salts contained in both the water and soil (Robinson and Rohwer,
1959; United States Department of Interior, United States Bureau of Recla-
mation, 1952; United States Department of Interior, United States Bureau of
Reclamation, 1963; Rohwer and Stout, 1948; Brockway and Worstell, 1968).
Although the literature contains much information about the relationship
between these parameters and seepage rates, the seepage process is so com-
plex that individual field tests are required. Worstell (1976) provides an
excellent review of seepage measurement techniques.
The most common methods of measuring seepage can be categorized as those
that yield results indicating an average seepage from a length of channel
and those in which information simply gives the permeability of a sample of
the channel bed. If the latter type of measurement is employed, additional
120
-------
information on hydraulic gradients is necessary in order for the actual
seepage to be computed. In most salinity investigations, those methods that
indicate actual seepage rates prove to be most valuable. Three of the most
employed methods include the inflow-outflow method, ponding method, and
seepage-meter method.
Inflow-Outflow Measurements—
When the seepage rates from relatively long lengths are to be measured,
the inflow-outflow method is a reliable and commonly used technique. The
method consists of measuring the inflow and outflow to the canal or lateral
section under investigation. It includes all diversions from the channel
and return flows to the channel. By computing the net discharge loss in the
channel, the actual seepage rate can be determined. Since the usual units of
seepage rate are feet per day (ft3/ft2/day) , the conversion from the total
loss in the section requires a knowledge of the length and wetted perimeter
(average) of the channel. The seepage rate can thus be expressed as:
_ (Inflow - Outflow) x 8.64 x 1Q4
SR - A(14)
in which SR is the seepage rate in ft/day (average for canal section) , 8.64
x 1C)4 is the number of seconds per day, A is "the average wetted area of the
channel in square feet, and the difference between the inflow and outflow is
expressed in cfs (ft^/sec). Use of this method is discussed by Bourns (1955).
The canal reach must be of sufficient length so that the seepage loss is much
greater than the measurement error or the measured seepage is meaningless.
Although this method does give an indication of seepage rates under
actual operating conditions, there are several factors that should be care-
fully observed or large errors will be introduced into the results. The
maintenance of constant flow depths in the canal during the tests is essential
to eliminate the effects of bank and channel storage. Also, an accounting of
all return flows from high lands and diversions or leaks from the canal must
be made. Occasionally, if the seepage rates are small, it may be useful to
note rainfalls and evaporation, although these latter factors are generally
inconsequential. Finally, but probably most important, is the consideration
of flow measuring devices to be employed. In the absence of measuring
structures in the system, flows generally can be measured by small flumes
or weirs when small, and by current water within at least 5 percent if
operated correctly. Current meter measurements require careful attention by
experienced personnel to maintain accuracies below 5 percent.
Ponding Method—
Although an objection is often raised that still water may seep at a
different rate than flowing water, the difference is probably small in com-
parison to errors associated with other measurement methods regarding seepage.
Basically, the ponding method involves measuring the rate of fall of the
water surface in the pool created in the canal section (Figure 41) . Then,
121
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/•Dam /Heodgate
Spillage
Water Level
Measurement
Stations
Water Supply
Wasteway for
Excess Water
Control
(Optional)
Plan View
-Heodgate
Dam
Section View
( Not to Scale )
Water Stage Recorder
Steel Headgate
Temporary Earthen Dam
\ Water Level,
Downstream
Water LeveK
Temporary Dam and Water Measurement Station Detail
Figure 41. Schematic representation of the ponding test method for
seepage measurement.
122
-------
by knowing the geometric properties of the section which is illustrated in
Figure 42, it is possible to compute the seepage rate according to the
following formula:
AE x SW x 24
a
(15)
WP x T
a
where AE is the drop in water surface elevation in feet, SW is the average
surface width in feet, WPa is the average wetted perimeter in feet, and T
is the time of the run in"hours. Some of the details and layout considera-
tions for ponding tests are shown in Figure 41. Further information on the
analysis can be found in Skogerboe and Walker (1972).
The ponding method usually provides the basis for comparison with other
methods because it can be expected to yield the best results (Robinson and
Rohwer, 1959; United States Department of Interior, United States Bureau of
Reclamation, 1968). It does have certain disadvantages that should be de-
lineated. Construction of the dikes is often expensive and must be completed
during periods when the canal is not in use, or during periods of interrupted
canal operation. Providing water to fill the pools or ponds may represent
a significant problem. If the canal discharges are very large in relation
to the seepage rates, then the ponding method is the best method by which
the seepage rates can be determined. Under such conditions, errors expected
in other methods, such as the inflow-outflow method, may not be able to
discern any seepage loss. Table 10 presents an example of the calculations
involved in ponding method tests.
Seepage Meters—
Seepage meters determine seepage rates under normal operating conditions,
but only for a small area at a time. Nevertheless, by taking readings at
several points along the canal section, a realistic average value can be
determined. One type of seepage meter uses a cylindrical bell that is
pressed into the channel bed. Attached to the bell via plastic hose is a
plastic bag filled with water that is submerged in the channel. Water that
seeps into the channel bed is replaced by water in the bag which is under the
same pressure as the channel flows. Seepage from the bell is determined by
weighing the plastic bag before and after the test, and the elapsed time of
the test. The seepage rate then may be determined by:
SR = Q (16)
A x T
in which Q is the amount of water that seeped through the canal bank in
ft3, A is the area of the bell in ft2, and T is the elapsed time in days.
123
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TABLE 10. TABLE FOR COMPUTING SEEPAGE LOSSES (USBR, 1968)
1
Date
9/16
9/16
9/16
9/17
9/17
9/18
9/18
9/18
9/18
BASIC
(cfu)
2 3
Elapsed
Time time ,
hours
9: 00AM
6.0
3 ; 00PM
8.0
11 : 00PM
10.0
9: 00AM
8.0
5: 00PM
8.0
1:OOAM
-REFILL
9: 00AM
3.0
12N
5.0
5 ; 00PM
EQUATION :
4
Water
surface
elevation
1910.70
1910.52
1910.24
1909.89
1909,64
1909.43
1910.42
1910,32
1910.16
Length of Pond x Drop in
Length of Pond x Average
5
Drop in
water
surface
feet
.18
.28
.35
.25
.21
0.10
0.16
6
Water
surface
width
feet
13.8
13,3
12.4
10.5
10.1
9.7
12.8
12.6
11.9
Water Surface x
Wetted
7
Average
water
surface
width
feet
13.55
12.85
11.45
10.3
9.9
12.7
11,75
8
Product
of
Columns
5 S 7
2.44
3.60
4,01
2.60
2.08
1.27
1.88
9
Wetted
perimeter,
feet
15.9
15.2
13.9
12.4
11.8
11.5
14.4
14.0
13.7
Average Width of Water Surface x
Perimeter x Hours
of Run
10
Average
wetted
perimeter
15,55
14.55
13.15
12.1
11.65
14.2
13.85
24
11
Product
of
Columns
3 S 10
93.4
116.4
131.5
96.8
93.2
42.6
69.3
12
Seepage
rate
cfd
Col. 8 x 24
Col. 11
0.63
0.74
0.73
0.65
0.54
0.72^
0.65
I/ Note that an error of 0.01 in gage readings in this calculation would influence the seepage rate 10%.
The time interval is too short.
-------
Q
-------
Farm Efficiency Studies
The amount of water diverted is also very critical in establishing the
water-salt budgets for a farm or a field. All of this water must be mea-
sured and accounted for, and it is allocated to any one of three main
categories. The surface hydrology categories are (a) evapotranspiration,
(b) infiltration and (c) tailwater runoff (Figure 43). In areas with a high
water table, there can be a substantial amount of water that moves upward via
capillary action from the water table and is used by the plants, This capil-
lary water will usually contribute significantly to soil salination problems,
The subsurface hydrology categories are head ditch seepage and deep perco-
lation losses. The variables that must be considered in an on-farm irrigation
return flow investigation are schematically illustrated in Figure 44.
Many of the parameters such as drainage discharges, lateral diversions,
water quality, and precipitation can be measured directly. Others must be
investigated indirectly. These indirect measurements of parameters are
related mostly to groundwater movement and soil hydraulic characteristics
and can be monitored using techniques such as piezometers, wells, and soil
sample analyses.
Because so many of the parameters in the water and salt budgets cannot
be evaluated directly on a large scale, peripheral investigations are
usually made in which a portion of the area is examined in detail. Such
investigations include farm efficiency studies that indicate the relative
proportion of evapotranspiration, deep percolation, and soil moisture storage;
vegetative land use mapping of the entire irrigated area so that the total
consumption of water for the area can be calculated; and other studies per-
taining to specific conditions of water and salt movement. There is no
substitute for good field data collection. The United States Geological
Survey (1968 to 1976) has published 34 manuals on techniques used for water
resource data collection, many of which are very useful for irrigation return
flow studies.
Basic Data
Four types of basic data are required for on-farm water use
investigations. These are crop parameters, soil parameters, water quality
information, and climatic data, such as evapotranspiration and precipitation
data.
Crop parameters are important to many of the irrigation method
decisions and the plant sensitivity and ionic toxicity response to salinity,
growth rates, and evapotranspiration demands. Crop responses to salinity
are discussed by Bernstein and Hayward (1957), Bernstein et al. (1954) , Black
(1968), Maas and Hoffman (1977), Robinson (1971), and others. Climatic data
requirements, evapotranspiration and growth rates are reviewed later in this
section.
Soil parameters considered as basic data include field capacity,
permanent wilting point, and bulk density for each soil layer with depth.
Since field soil-moisture sampling procedures are gravimetric, the bulk den-
sity is needed to relate gravimetric to volumetric moisture content which is
126
-------
Diversion
Structure
Flow Measurement
Structures
Field Evaluation
of Soil Moisture
Flow Measurement
Structure
Weather
Station
V^Tailwater
Runoff
MM
Deep Percolation
Losses
Figure 43- Schematic of instrumentation required for on-farm hydrology investigations.
-------
c
o
Isl
D
w
3
D
c
•o
o
O
(O
<
•>
o
Nl
0
1 ^
\
T
\*
1 M M M M 1 1
I
Upward Flow by Capillarity
_i__J t t
Deep
Percolation
Water Table
Groundwater Flow
apillary Fringe
of Water Table
Figure 44. Schematic of the hydro-logic variables to be considered in an
on-farm subsystem investigation.
128
-------
used in most analytical procedures. An attempt should be made to obtain
several samples from numerous locations to approximate the average conditions
of the field (Warrick, 1977; Karmeli et al., 1978). Basic soil chemistry
reactions, electrical conductivity and ionic content of the soil solution
should also be determined. Black et al. (1965), Food and Agricultural
Organization of the United Nations (1975), Quirk (1971), Richards (1954),
United States Department of Agriculture (1964a), Bolt and Bruggenwert (1976),
Chapman (1966), and others describe the soil-chemistry-plant relationships
and procedures for data collection and analysis.
Quality of the incoming water can greatly influence management and
operation of an irrigation system. If the water is of poor quality, it can
limit many of the alternatives for pollution control. Ayers (1976), Ayers
and Westcot (1976), Christiansen et al. (1976), Kemp (1971), Kovda et al.
(1973), Wilcox and Durum (1967) and others present information regarding
water quality in irrigated agriculture.
Evapotranspiration—
A review of the alternative approaches to estimating the volume and rates
of water evaporated from wet crop and soil surfaces or transpired by the
plants can be found in several sources (Jensen, 1973; Doorenbos and Pruitt,
1977; Horton, 1973) . As far as this technology is applicable to the manage-
ment of irrigation return flow quality through irrigation scheduling,
Skogerboe et al. (1974a), Jensen (1975, 1976) provide good summaries.
There are many methods by which evapotranspiration (ET) can be
calculated. The three most common approaches to estimating evapotranspiration
are: (a) the Blaney-Criddle method (United States Department of Agriculture,
Soil Conservation Service, 1964b) ; (b) the Modified Jensen-Haise methods;
and (c) the Penman Combination method. These methods represent the range of
sophisticated techniques available today, varying in detail from a tempera-
ture dependent analysis (Blaney-Criddle) to an analysis of energy balance
and convective transport (Penman).
Depending on the estimating formula used, data required for consumptive
use studies can include climatic data such as daily solar radiation, air
temperature, dew point temperature, relative humidity, wind speed, and pre-
cipitation. Some consumptive use formulas require information on the
monthly percentages of daylight hours, latitude, altitude, crop height,
depth of root zone, crop and phreatophyte growth stage coefficients, and
the areal percentage of plant cover.
Evapotranspiration is a very important part of any water-salt budget
since it can account for the majority of the water delivered to an irrigated
area. The accuracy of these measurements and resulting calculations can
seriously affect the validity of the results. It is necessary that this
value be determined as accurately as possible, and it is imperative that^
the method of estimating evapotranspiration be calibrated for local condi-
tions. Attempts to base conclusions on uncalibrated consumptive use equa-
tions would be extremely presumptious. These equations are usually cali-
brated by the use of field measurements. A detailed discussion of the
calibration procedure and comparisons of the three main estimating formulas
mentioned above can be found in Evans et al. (1978b).
129
-------
It is usually necessary to apply the results of the study area to the
entire area, which includes extending the water-salt budgets. A very im-
portant part of consumptive use studies is vegetative mapping of the areal
extent of each crop. This should be done each year in the study area, and
at least once for the entire irrigated area. Since each crop uses varying
amounts of water at different times, it is advantageous to know the acreages,
planting and harvesting dates, as well as the dates of irrigation. Most of
the data available from the Agricultural Stabilization and Conservation
Service or the Census of Agriculture do not have sufficient resolution to be
used for hydro-salinity investigations. It is often necessary for project
personnel to map the agricultural land use of these areas.
Measurement of Evapotranspiration—Tanner (1967) and the World
Meteorological Organization (1966) provide an excellent review of the pro-
cedures and methodologies used for the measurement of potential evapotranspi-
ration in the field. Measurement of evapotranspiration should include the
means for the actual measurement of consumptive use and a complete weather
station to measure air temperature (including maximum and minimum daily
temperatures), dew point temperature, relative humidity, precipitation, wind
run, solar and net radiation, and evaporation (Class A pan), Doorenbos (1976)
presents an excellent discussion on the establishment and operation of a
weather station and the calibration of empirical evapotranspiration indices
to actual evapotranspiration measurements. The World Meteorological Organi-
zation in 1970 and 1971 presented much information on the collection and
analyses of hydrometeorological data.
Probably the most accurate measurement of evapotranspiration is obtained
by the use of lysimeters. A lysimeter is a device that is hydrologically
isolated from the surrounding soil. This device contains a known volume of
soil, is usually planted to the crop under study, and has some means to
directly measure the consumptive use of water. Lysimeters must be representa-
tive of the surrounding conditions and the soil types if they are to provide
useful evapotranspiration measurements. Lysimetry establishes a datum for
evapotranspiration calculations because it is the only method of measuring
evapotranspiration where the investigator has complete knowledge of all the
terms of the water balance equation. Harrold (1966) presents a comprehensive
review of the use of lysimeters for measuring evapotranspiration.
Two types of lysimeters, which have worked quite well for calibration
purposes, are the constant water table and the hydraulic weighing lysimeters.
The constant water table lysimeters are usually planted to grass, such as
Kentucky Bluegrass or other crops with shallow root systems. On the other
hand, the hydraulic weighing lysimeters are usually planted to deeper rooted
crops, such as alfalfa or corn.
Construction of a constant water table lysimeter is shown in Figure 45.
They are usually about 1 meter square and about 40 to 60 cm deep. The
amount of water used is calculated by using an area ratio of the lysimeter
to the reservoir. The evapotranspiration rate is very sensitive to the depth
of the water table in the lysimeter which is usually kept at about 15 cm
below the grass surface. The crop must be trimmed periodically to ensure
130
-------
Soil
Ive
<
n
^tr\"f
x^"
Water S
Rarnr H t
- — Reservoir
\^Welded Aluminum Lysimeter
Tank Im x Im x 0.46 m
4 Tubing
Figure 45. Schematic of constant water-table lysimeter.
-------
vigorous growth; and any vegetative growth extending over the sides of the
lysimeter should be trimmed back. Construction of one of these types of lysi-
meters costs from $250-300 each, not including the water level recorder.
Construction of a hydraulic weighing lysimeter is shown in Figure 46,
and a typical calibration curve is shown in Figure 47. This type of
lysimeter is irrigated for the purpose of maintaining low-tension soil
moisture conditions in order to approximate the potential evapotranspiration.
A neutron probe access tube or other method can be installed to assist in
monitoring soil moisture distribution. A method to extract the surplus water
and to provide a leaching mechanism should be installed. One bar ceramic
candles connected to a vacuum system work well for this purpose. Depending
on the degree of sophistication, this type of lysimeter can cost from
$1,000-3,000 each to construct (Hanks and Shawcroft, 1965).
Land Use Mapping—
Significant.agricultural land use surveys have been conducted by the
United States Department of Agriculture, Soil Conservation Service for many
irrigated areas in the United States. Detailed soil survey information has
been developed for almost all irrigated areas.
The type of land use data required for the preparation of a water budget
consists of delineating the various types of vegetation and land requirements
that utilize water in excess of normal precipitation. This cataloging pro-
cess is an expensive and time-consuming effort that includes separating the
agricultural areas from the wetland phreatophytes, the urban areas and the
industrial areas, as well as the open water surfaces. These types of
studies are not only necessary for budgeting procedures, but they also pro-
vide an excellent data base for future studies in an area for many disci-
plines. These data must be collected by field investigations.
Aerial photographs are an excellent resource for vegetative land use
mapping. The most current photographs available should be used since land
use changes are usually minimal, field boundaries and ditches remain the
same, farmsteads and urban areas are well defined, and adjustments and up-
dating are easily accomplished.
Aerial photographs with almost any scale are available for most areas in
the western United States and can be ordered from the United States Department
of Agriculture, Agricultural Stabilization and Conservation Service, Aerial
Photography Division, Western Laboratory in Salt Lake City, Utah. It is
important to select a scale for the photographs that corresponds to other
base maps or design maps that exist or will be used for the project.
The range, township, and section numbers are marked on the photographs
that are then taken into the field. The existing land use is marked on the
appropriate photographs for each field. A suggested land use mapping index
is presented in Table 11. Other indices are used by the United States Bureau
of Reclamation, the Soil Conservation Service, and other agencies. Whatever
index is used for mapping purposes, it should be compatible with other
studies undertaken in the area or river basin. A typical photograph from
which the land uses were labeled in accordance with the water related land
132
-------
2cm
118 cm
2cm
Flexible Plastic
-Water Seal
Manometers
(filled with antifreeze)
£L
1.2cm Diameter
I Bar Ceramic Candle
Pressure Line
(6.5 mm Imperial Tubin
Drainage Line
(6.5mm Imperial Tubing)
Nylon Reinforced
Butyl Rubber Tubing
ample
Collection Bottle
Dummy Lysimeter
for Temperature Correction
Figure 46. Schematic of the construction of a hydraulic weighing lysimeter.
-------
120 r
100
g 80
o
•o
o
0)
0»
E
o
c
o
60
40
20
I cm. Manometer = 0.2702 cm ET
= 0.10637 in ET
0 50 100 150 200 250 300 350 400 450 500 550 600
Load, Kg
Figure 47. Typical calibration curve for hydraulic weighing lysimeter.
134
-------
TABLE 11. SUGGESTED LAND USE MAPPING INDEX
A.
Irrigated Cropland
1. Corn
2. Sugar beets
3. Potatoes
4. Peas
5. Tomatoes
6. Truck crop
7. Barley
8. Oats
9. Wheat
10. Alfalfa
11. Native grass hay
12. Cultivated grass and hay
13. Pasture
14. Wetland pasture
15. Native grass pasture
16. Orchard
17. Idle
18. Other
B. Dry Cropland
1. Alfalfa
2. Wheat .
3. Barley
4. Beans
5. Cultivated grasses
6. Fallow
7. Other
D. Industrial
1. Power Plants
2. Refineries
3. Meat Packing
4. Other
E. Open Water Surfaces
1. Major storage
2. Holding storage
3. Sump ponds
4. Natural ponds
F. Phreatophytes
1. Cottonwood
2. Salt Cedar
3. Willows
4. Rushes or Cattails
5. Greasewood
6. Sagebrush and/or rabbit-
brush
7. Wildrose, Squawberry, etc.
8. Grasses and/or Sedges
9. Atriplex
P. Precipitation only
C. Other Land Use
1. Farmsteads
2. Residential yards
3. Urban
4. Stock yards
5. School yards
135
-------
use index is shown in Figure 48. For example, a field marked Al on the
aerial photograph indicates that during that year, corn was grown in that
field. Although it is realized that certain changes occur from year to
year, it is usually safe to assume that the total acreages and the general
distribution of crop acreages over a large area varies slowly with time.
Due to scale distortion, which is always present in aerial photographs,
an effort should be made to prepare land use base maps with accurately
placed section lines. To assist in accomplishing this, maps should be pre-
pared using a grid based on geodetic coordinates. This is usually not a
problem since most agricultural areas in the western United States have roads
and field boundaries corresponding to these coordinates. The scale of the
base maps should correspond directly to the scale of the aerial photographs.
United States Geological Survey quadrangle maps can be used for control where
available. In addition, there are several computer techniques that correct
for distortion if adequate ground control is established,
Results of these investigations, including the base maps and tabulation
of the data for each section or subgroupf would be organized and made avail-
able for public distribution. This type of information is very valuable
and is needed by many state and local planning agencies, public interest
groups, environmental impact assessments, and for other groups and purposes,
This information also provides a good basis for comparison of future land
use related investigations. Examples of these types of examinations can
be found in Walker and Skogerboe (.1971) and Evans et al. (J.973) ,
The various water related land use areas are then transferred from the
aerial photographs to the base maps which also depict the individual field
boundaries (Figure 49), The irrigation conveyance system should be added to
the base maps in order that lands served by each canal or lateral can be
established.
Many sections are not exactly 640 acres (.259 ha) , and they can often
vary by as much as ± 10 percent of this value Cas much as 90% on correction
lines). It is necessary, therefore, to establish the area of each section.
One method is to use graphical computer techniques or a planimeter on each
section from the quadrangles to arrive at the acreage for that section.
Another method is to check the land survey maps in the local County Recorder's
office. The acreage of each land use within that section must also be
determined from the base maps by similar methods, The acreage of each land
use is then summed for each canal, lateral, or watershed in order to develop
the necessary water budgets.
Infiltration—
Infiltration, which refers to the rate at which water will move into the
soil profile, is a very important and necessary component of the farm subsys-
tem evaluation. Information on infiltration is necessary to predict the
maximum application rates, the quantity of water to be delivered, and the
effect of modifying deep percolation losses. Irrigations cannot be ef-
fectively scheduled unless the temporal and spatial distribution of water is
known. Infiltration characteristics should be determined at numerous loca-
tions in a field.
136
-------
Figure 48. Typical areal photograph used for land use mapping showing the
land use mapping index used in Table 11.
137
-------
AI3o'rCPi A13
Figure 49. Finished map corresponding to the areal photograph shown in
Figure 48.
138
-------
Infiltration rates vary throughout the irrigation season as well as
from year to year and are due to many factors. In addition, infiltration rates
are dependent on soil moisture content and crop type and whether it is annual
or perennial. Drainage problems such as perched water tables, restrictive
soil layers (plow layers, geologic conditions, mineral depositions such as
caliche layers), sodium problems, soil sealing due to compaction and/or soil
crusting, and surface roughness and shape can also significantly affect
infiltration.
There are several methods for measuring soil infiltration characteristics.
The most common is probably the cylinder infiltrometer (Raise et al., 1956;
McCulloch, et al., 1967). This method utilizes a steel cylinder or pipe
(usually 30-40 cm in diameter) which is forced into the soil to a depth of
10 to 20 cms. The area around the cylinder and the cylinder are filled with
water an,d the change of the depth of water with time is recorded.
Davis and Fry (1963) discuss the measurement of infiltration in furrows.
The blocked-furrow infiltrometer (Bondurant, 1957), utilizes two steel plates,
usually 1 meter apart, which are forced into the soil along a furrow. The
volume of water necessary to maintain the water level between the plates at
a specified level, which should approximate normal flow depth in the furrow,
will indicate the infiltration rate. Karmeli, et al., (1978) presents an
analysis of this type of data. Methods for evaluating border infiltration
are presented by Finkel and Nir (1960), Gilley (1968) and others.
Another method of measuring the infiltration characteristics in surface
irrigation are the advance-recession tests (Merriam et al., 1973). Methods
of analysis of advance-recession infiltration data are discussed by Wilke
and Smerdon (1965), Fok et al. (1971), Christiansen et al, (1966), Fok and
Bishop (1965), Gerards (1978), Phillip and Farrell (1964), and Karmeli et al.
(1978) . This procedure uses the rate of water advance and of recession down
a furrow or border to indicate the infiltration. The mathematics, however,
can become very complicated.
Probably the most common mathematical description of infiltration is the
use of the Kostiakov equation (Kostiakov, 1932) which is commonly given as
I = AtB, with I defined as the total infiltrated depth, t is the time, and
A, B are empirical constants. The instantaneous infiltration function follows
a decaying exponential function with time and asymptotically approaches a
value often referred to as the basic infiltration rate. The basic infiltra-
tion rate is often used as the application rate for conservatively designed
sprinkler systems.
Tailwater runoff—
In analyzing field tailwater runoff, it is necessary to obtain the
runof,f hydrograph for each irrigation and for each field. A common error in
tailwater investigations is that conclusions are often extrapolated from one
or two daily, or even one or two weekly, flow measurements.
Figure 50 shows an actual tailwater hydrograph obtained from a clock-
driven water level recorder on a 6-inch Parshall flume. The "valleys" shown
in the figure indicate, with a certain time lag, the time the water for each
139
-------
0.50
0.40
« 0.30
o>
o
0.20
0.10
0
.Start of Irrigation
0
24
End of
Irrigation
48
144
72 96 120
Time In Hours
Figure 50. Typical hydrograph of field tailwater for surface irrigation.
168
192
-------
individual set was turned off. The time lag is due to the recession phase
of irrigation, or the time that it takes for the water to travel the length
of the field. The area under the "hills" in Figure 50 is the volume of
runoff from each set. The height of the "hills" indicates the maximum rate
of runoff during an irrigation set. Therefore, it can be seen that unless
continuous hydrographs are obtained, tremendous error could be introduced by
"spot" measurements.
Irrigation Method Analysis
Generally, there are four main types of irrigation systems.
1. Surface irrigation systems which include furrow, corrugation, graded
borders and basins; (Bishop et al., 1967; Booher, 1974; United States
Department of Agriculture, 1974a);
2. Subsurface irrigation (Griddle and Kalisvaart, 1967);
3. Sprinkler irrigation systems (Christiansen, 1942; Davis and
Christiansen, 1967; Fry and Gray, 1971; United States Department
of Agriculture, 1974b; Pair et al. ,, 1975).
4. Trickle or drip irrigation (Barrs, 1976; Keller and Karmeli, 1975;
Holland, 1972; Geohring, 1976; Jobling, 1974; Shoji, 1977; Smith
and Walker, 1975; Wierenga, 1977; Goldberg et al., 1976).
Since subsurface irrigation systems, not including subsurface drip
irrigation, are relatively rare, require high quality water, and are not
found in areas that significantly contribute to nonpoint pollution, they
are not described in this section.
Procedures for evaluation of sprinkler and surface irrigation systems
are described by Merriam et al. (1973), Merriam (1968), and Griddle et al.
(1956). Further examples of the complete analyses are presented by Evans
et al. (1978b), and Karmeli et al. (1978).
Managerial or irrigation policy factors that influence the irrigation
are only indirectly dependent on the irrigation method. These decisions
include: (a) the irrigation requirement, determination of evapotranspiration
needs, soil moisture deficits, and leaching requirements based on water
quality; (b) irrigation scheduling decisions regarding the depth of water to
be applied and the date of irrigation; and (c) irrigation cost factors that
may be constraints to efficient irrigation, such as labor, energy, or capital.
Other factors that are managerial decisions include cultural practices, such
as planting, fertilization, cultivation, and harvesting operations which
can influence timing and depths of applications.
For purposes of analysis, it is necessary to divide the irrigation
method evaluation into two categories: (a) application characteristics that
are basically functions of operational procedures; and (b) physical opera-
tional parameters of the application system that are functions of design,
water availability, and soil hydraulic characteristics.
141
-------
Water Application Characteristics—
There are two subcategories to be determined in the evaluation of water
application characteristics. These are the average depth applied, and
spatial and depth distribution of water in the field, The latter subcategory
is further divided into losses such as field tailwater and deep percolation,
and the various efficiency and uniformity parameters necessary to statisti-
cally describe the spatial distribution of water.
Various types of irrigation efficiencies are discussed by Bos and
Nugteren (1974), Israelsen and Hansen (1967), Karmeli et al. (1978), Jensen
(1967), Somehalder (1958), and Willardson and Bishop (1967). The evaluation
and description of spatial distribution of water in the field (uniformity)
is discussed by Karmeli et al. (1978), Hart (1961), Hart and Reynolds (1965),
Karmeli (1977), Seniwengse et al. (1972), and Chaudry (1976, 1978).
Physical Operation Conditions—
Determination of the physical characteristics of the water application
system is a relatively straightforward process. For example, the dimensions
of pipes or ditches, sprinkler spacing, topographic information such as the
field slopes, lengths, widths and areas, the quantity of flow available,
pumping lifts and water pressure are all easily measured by conventional
means. The application for sprinkler or drip systems can be measured
volumetrically by metering devices.
Physical operational conditions or irrigation method factors that
influence irrigation are the physical characteristics and constraints of the
system such as pipe sizes and lengths, water availability (quantity and
timing), pressure, soil infiltration characteristics and physical properties
of the field, including acreage, slope, length, and width. Other factors
include the water application characteristics of the system which are di-
rectly influenced by the management decisions, such as depth actually applied,
soil moisture storage capacity, spatial and depth of soil moisture distri-
butions (water losses, uniformities, efficiencies) and the application rate
when sprinklers or drip irrigation systems are considered. One of the most
critical parameters of the farm efficiency studies is the change in soil
moisture storage throughout the irrigation season.
Soil Moisture Storage—
Soil moisture storage refers to the capacity of the soil to hold water.
When irrigation occurs, the soil moisture reservoir is usually filled to
capacity to replace water which has been lost to evapotranspiration. Excess
root-zone diversions pass below the root zone as deep percolation. Root-
zone diversions are all of the water which has infiltrated into the soil
profile. Field capacity and permanent wilting points should be established
for each soil type in the project area, and bulk density determinations
should be made with respect to depth. Holmes et al. (1967) present a good
discussion on methods and procedures for obtaining soil moisture data.
Methods of measuring and monitoring soil moisture include gravimetric
(weighing—drying—reweighing) water and calcium carbide reaction methods
(for example, "SPEEDY" meter), or by indirect methods such as tensiometers
and gypsum resistance blocks. Information on these methods is contained
142
-------
in Richards (1954), Black et al. (1965), and Oster et al. (1976). Tensio-
meters and resistance blocks should not be used in saline conditions. Soil
salinity can be monitored by soil sampling and laboratory analysis (Black
et al.., 1965) and by the use of salinity sensors (Oster and Willardson, 1971;
Oster et al., 1976). Soil samples should be taken immediately before each
irrigation and again when soil is at field capacity, which is usually
twenty-four to forty-eight hours after the irrigation event. It may often
be necessary to take a series of soil samples after an irrigation event until
the soil moisture "levels" off to establish field capacities.
Irrigation Management Analysis—
Irrigation management can greatly influence the pollution contribution
from a given field. Frequently, the majority of pollution can be alleviated
by improved irrigation management practices. Van Schilfgaarde et al, (1974)
discussed' irrigation management for salinity control.
Evapotranspiration and the soil moisture deficit, or soil moisture
tension, dictates when an irrigation event should occur. Considering the
leaching requirements as a function of the quality of irrigation water and
soil salinity, these parameters indicate the proper depth to be applied,
and hence the time duration of each irrigation set.
The adequacy of the irrigation is a comparison of what should have been
done with what actually occurred, and where improvements could be made.
Examples of this type of analysis are presented in Skogerboe et al, (1974a)
and Evans et al. (1978b) . Other considerations to assess the adequacy
of the irrigation include nutrient losses, leaching or surface runoff;
erosion problems; areas of deposition and degradation; tailwater reuse;
power consumption; as well as many other considerations that may be of local
concern.
Irrigation costs can also be a major factor in choosing acceptable
alternatives for pollution control activities. The costs of labor, capital,
energy, crop yields, farm income, and annual farm and irrigation system
operation and maintenance must be considered in selecting any potential solu-
tion. Sheffield (1977), Reed et al. (1977), New (1977), Wilson et al.
(1976), Pitchford and Wilkinson (1975) and others discuss the economic deci-
sions and evaluations of irrigation systems. It should be remembered that
agricultural economics and crop yields are very sensitive to local conditions,
WATER REMOVAL SUBSYSTEM INVESTIGATIONS
Head Ditch Seepage
Since each field or farm must be evaluated individually, the time
distribution of on-farm water use is very critical in the analysis of the
water removal subsystem. These data must be collected in the field if good
data are not available from other sources. For example, water is usually in
a head ditch less than 50 percent of the growing season. The effectiveness
of already lined, or of lining unlined head ditches, or replacing them with
other means such as gated pipe cannot be assessed without knowing how and when
these ditches are in use.
143
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Deep Percolation
Deep percolation is probably the most difficult and elusive segment of
the hydro-salinity study to define. It is usually a residual calculation
and, as such, it contains the errors of all the other measurements. The total
deep percolation is a fairly small quantity, usually less than 10 to 30 per-
cent of the total field diversions, and since the errors from all the other
components of the hydrologic budget tend to be additive, it may be in error
by 100 percent or more. If water balance techniques are to be used, with deep
percolation being calculated, it is critical that every effort be made to
minimize the error of the other terms.
Duke and Raise (1973) developed a method for the direct measurement of
deep percolation quantity and quality using vacuum extractors (Figure 51),
This method is much more accurate than many of the other procedures. These
installations are fairly expensive ($1000-$2000 each), and they should be
installed and used by qualified personnel.
Methods using the concentration of conservative ions with depth in the
soil profile as tracers are fairly inexpensive and usually reliably indicate
the quantity of deep percolation. Suitable conservative ions should be
water soluble, not used by the plants, and unaffected by cation exchange
reaction in the soil. The most commonly used ion for these investigations
is chloride (Cl~) , The analysis and use of this method is discussed by van
Schilfgaarde et al. (1974), Bernstein and Francois (1973) , and Rhoades et
al. (1973). The chemical analysis is discussed by Richards et al. (1954),
Black et al. (1965) and Lusczynski (1961) , The bromide (Br—) ion has also
been used as a suitable tracer (Wendt et al. 1976).
An inexpensive method for the collection of soil-water quality samples
with depth is a pressure-vacuum lysimeter (Figure 52). These devices use an
unglazed porous ceramic cup attached to a piece of plastic pipe for a sample
collection area. These units are sealed and a vacuum is maintained to col-
lect a sample. To extract the sample, positive pressure is applied and the
sample removed. These lysimeters are usually buried in several locations
across a field at various depths. More information on this method can be
obtained from Parizek and Lane (1970), Wagner (1962), Grover and Lamborn
(1970), Wood (1973), and Wengel and Griffen (1971),
Drainage Investigations
There are basically three types of drainage systems for agricultural
use. These are interceptor drains, relief drains, and pump drains. All are
installed to lower the water table to provide proper aeration in the crop
root zone and/or provide an effective leaching mechanism for the control
of soil salination. As such, drainage system effluents are very often quite
saline. Rhoades (1974) and Skogerboe et al. (1974b) discuss the importance
of drainage for salinity control.
To determine the effectiveness of existing drainage systems or to
determine the design requirements of new systems, it is necessary to collect
sufficient field data. Data required for most drainage evaluations vary
with the computer models to be used, but will usually include drain spacing,
144
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^^
Note: Air, Vocuum, ond
Sample Dump Lines
Con Run to the Edge
of the Field or to
a Manhole.
To
Adjustable
Vocuum
Source
Soil
Solution
Sample
Collection
Bottle
Disturbed
Soil
Undistur
Soil
Ceromic Candle (
6.5 mm
Imperial Tubing.
\ r
| t l f" C
1 Air Pressure
\ / /
\ Soil
- 1
/ r
^KLDn Butyl Rubber ' .
ooKr-a Disturbed , Undisturbed Air Pillow Shee1
~~j£ Soil J^J-*- Soi,
' A Sample
I Dump
_ !j i
/ '
Metal
Trough
(Lysi meter Pon|
Im
(a) Schematic of Vacuum Extractor Installation
1.2 cm Diameter
Ceramic Tube
Fitting
_Vinyl Tubing
Connectors
(Glued }
(b) Ceramic Candle Detail
Top View
V
Side View
is Open)
End View
Collector Line -
Lysimeter Pan
-Ceramic Candle
^Outlet to
Collector Bottle
"^—Air Pillow
-Lysimeter Pan
-Air Pillow
(c) Lysimeter Pan Detail
Figure 51. Schematic of vacuum extractors installation with detail.
145
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2-Way Pump
Pressure-
Vacuum inlet
Bentonite—
3/l6"(5mm) Copper
or Polyethylene Tubing
2 (5cm) Plastic PVC Pipe
24" (608mm) Long
Ceramic —
Porous Cup
Sample
Bottle
Discharge Tube
152 mm (6")
Borehole
Sealed by Cap
Sand Backfill
Silicon or Epoxy
Sealant
-Bentonite
•Tubing to Surface
Connectors
Pipe-Thread Sealant
PVC Pipe Cap (Glued)
Schedule 40 PVC Pipe
( w 5 cm Dia.)
PVC Cement
Polyethylene Tubing
~ . ii_. ii
Branch T
Female Elbow
Connectors
Poppet Check Valve
Epoxy Cement
olyethylene Tubing
Ceramic Porous Cup
(K 5cm Dia.)
Cross Section of a Typical Pressure - Vacuum
Lysimeter Installation ( Parizek ft Lane, 1970)
Modified Pressure-Vacuum Lysimeter
Installation ( Wood, 1973)
Figure 52. Schematic view of pressure-vacuum lysimeters for soil water quality sample collection.
-------
depth, size, envelope thickness, soil hydraulic characteristics, time
distributed effluent, and influent water quality and quantity. It is often
necessary to obtain soil hydraulic properties including initial moisture
conditions, apparent saturated hydraulic conductivity, and infiltration
functions. Corey (1977), Boersma (1965), Klute (1965), and Bouwer and
Jackson (1974) are good references on the direct and indirect procedures and
methods of determining these data.
Groundwater Monitoring Techniques
Todd et al, (1976) presents an excellent discussion on the methodologies
of groundwater monitoring techniques, A general indication of the costs of
various groundwater monitoring and evaluation techniques is presented by
Everett et al. (.1976). Tinlin (1976) presents several illustrated examples
of monitoring groundwater pollution sources, one of which is concerned with
subsurface agricultural return flow.
The most common devices for hydrologic measurements of groundwater are
the combined use of observation wells and piezometers. Observation wells are
used to provide information on the water table depth at a location. Wells
are perforated throughout the saturated thickness of the aquifer, and as
such, provide a vertical integration of the aquifer properties and water
quality. Piezometers, on the other hand, are not perforated and are only
open at the extreme ends. Piezometers are used when vertical gradients in
an aquifer are needed and when it is desirable to obtain the hydraulic pro-
perties at a point. Clusters of piezometers can also be used to collect
water samples with depth so that concentration profiles may be determined.
The depth from the top of the casings to the water in the wells and piezo-
meters should be obtained at least weekly, and water samples should be ex-
tracted at least once a month for water quality analyses, The observation
wells are usually a small diameter (5 to 10 cm) and are cased with either
steel or heavy plastic perforated pipe (Figure 53) , Benz et al- (1963) ,
Donnan and Bradshaw (1952), Myers and van Bavel (1962), the United States
Department of Interior, United States Bureau of Reclamation (1964), United
States Department of Agriculture, Soil Conservation Service (1973), and
Johnson Division of Universal Oil Products (.1972) describe the general
placement and evaluation procedures for observation wells,
Observation wells are usually installed on a grid system. It may be
necessary to have a professional well driller install these wells. To
facilitate identification of the types and composition of the individual
strata, their thickness, and the thickness of the total aquifer, a driller"s
log is a minimum requirement. More complete data is often needed for
analyses than can be obtained from a driller's log, in which case "self -
potential" (SP), apparent resistivity and gamma rays or other types of logs
can be. required (Figure 54) .
In stratified aquifers it is often necessary to install observation
wells in a cluster arrangement. Several wells can be installed in one large
bore hole or the wells can be individually installed (Figure 55). Shallow
wells, less than 5 meters, can also be installed by project personnel when
the well casing can be driven using a well point or a small drilling rig.
147
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Concrete
Plug
Construction
.Cop
Borehole
Low Permeobility
Backfill
Water Table
Spiral Welded Steel Pipe
7ga.or Schedule 40 PVC
Casing
Brass or PVC
Well Screen 1/8"
Opening
Gravel Pack
Ground
Surface
Driller's
We 11 Log
Depth
(m) (ft)
O-r-0
5-
-10
-20
10--30
15-
20-1-60
-40
-50
Figure 53. Sketch of typical Observation well construction and the
associated driller's log.
148
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DESCRIPTIVE
LOG
ELECTRIC LOG
SELF POTENTIAL
- "SP"
APPARENT
RESISTIVITY
GAMMA-
RAY LOG
CASING
y-:-:-:-;DRY SAND;-:;
::: FRESH-WATER
-WATER.':\V-
.VA/.SAND vX-:
-CLAY :-:
5 C L AYE Y t^St
%*& FRESH-
S^s AN D 5£3gjg
-I-CLAY-=-
••-.BRACKISH-v.
WATER SAND'
I CLAY
•-•. SALT-WATER .•
/:••;;:•: SAND ::;•/'
=:CLAY
Figure 54. Representative geologic, electric, and gamma-ray
logs that are used for the identification of the
hydrologic properties of the geologic substrata.
(Johnson Division of Universal Oil Products, 1972)
149
-------
PLAN VIEW
Depth
12 meters
(40ft)
O
I Depth
24 meters
(80ft)
30meters
(100ft)
01
o
a>
8
SECTION
VIEW
a>
ffi
Land
Surface^
0 n
15m
(50')
3Om
doo'i
O Depth '
18 meters (60 ft)
O Depth
6 meters
; (20ft)
W(
t7!
jter Tab
8
Screened
Interval
Well Casings
Land Surface
Aquifer
^0.1
'
No.2
Aquif,er
No.3
V.
Large Dia. Borehole
ivxa Low Permeability
l^vvj Material
pf-P] Sand Backfill in
fofr '':l Screened Interval
Figure 55. Schematic representation of observation well clusters for stratified aquifer situations
-------
Obviously, driller's log information would not be obtainable from this type
of installation. Campbell and Lehr (1973) present more information concerning
water well drilling techniques and well development.
After the wells are installed, data can be collected on the hydraulic
conductivity, specific yield and storage coefficients, transmissibility,
and other values. Each well should be provided with a threaded cap to keep
debris from natural sources or vandalism from destroying the usefulness of
the wells. The wells should be periodically flushed to ensure representative
flows and water quality information.
Piezometers are usually small diameter nonperforated pipes about 1 to 2
cm (3/8 to 3/4 inch) in diameter (Figure 56) . Piezometers measure the
hydraulic gradient of the aquifer at the depth of the end of the piezometer.
Piezometers are also used to .collect water quality samples for laboratory
analysis-1 Piezometers are also installed on a grid system that is usually
close to the observation wells and in a cluster arrangement. Under good con-
ditions with few large rocks, piezometers can often be installed by project
personnel by use of a jetting rig. The jetting rig forces water through the
piezometer pipe as it is pushed into the ground (Mickelson et al., 1961;
Donnan and Bradshaw, 1952). The force of the water jet removes unconsoli-
dated particles. Piezometers may often be driven into place. Information
on the installation measurement and evaluation of piezometers is presented
by the United States Department of Interior, United States Bureau of Reclama-
tion (1964), Reeve and Jensen (1949) , Bornstien and Alberts (1963), Myers
and van Bavel (1962), and Donnan and Bradshaw (1952).
When selecting sites for observation wells and piezometer installations,
they should be located where vehicular traffic, farming equipment, or road
maintenance equipment will not disturb or remove the upper portions of the
pipes. A good location is very often found in a fence row.
The water levels for piezometers and wells are usually measured from
the top of the pipe to the water level. To tie together all of the data
from all of the wells and piezometers in a grid system, it is necessary to
determine the elevations of the tops of each well and piezometer casing, and
thereby the respective water level elevations for each well and piezometer.
Another method often used in groundwater studies is radioactive tracers
such as bromide isotopes (Jester et al,, 1977; Jester and Uhler, 1974) to
indicate flow velocities, quantities, and direction of flow. It is often
very difficult to interpret these data, and it is not recommended for use
except by experienced personnel.
Equipment for water sampling and depth to water determinations are
commercially available from organizations such as Soiltest Inc. (2205 Lee
Street', Evanston, Illinois 60202) . It often becomes necessary to construct
equipment to meet the specific requirements of the project installations.
Some specialized equipment for water sampling are presented by Hansen
and Harris (1974) and Cherry (1965).
151
-------
Ul
Elevation of the Tops of Each Piezometer and
their Respective Lengths must be Known
Ground
Surface
Caps
Undisturbed
Soil ~--
I to 5cm Dla.^f
Hole
(Drilled or
Jetted« -
Exact Size)-
I to 5cm Dia,
PVC or Steel
• Pipe Casing
(Unperforated)
Water Table
For Stratified
or Uniform
Aquifer Conditions
' , i, * . ' ' ' '
Impervious Layer
Pressure-Vacuum
Line _
Low Permeability-
Material
Borehole
(« 5cm Dia.)
25 to 30 cm Length
Slotted PVC Pipe
(«5cm Dia.
Check Valve
>*"
BackfilK
Discharge Line
Land Surface
Polyethelene
End Cap
"T"and Elbow
Fittings
Sample Collection
Chamber
End Cap
Details of One Type Low-cost Piezometer Modified
For Collection of Water Sample
Series of Typical Piezometer Installations
Figure 56. Typical piezometer installations that can be used for vertical water gradients
and water quality information.
-------
Very often local consulting engineering firms, local government
engineering service agencies, state engineers' offices and the local United
States Geological Survey office will have information on the hydraulic
properties of the aquifers in the area. The United States Geological Survey
also has numerous publications such as Johnson (1966) that are beneficial
in estimating hydraulic parameters for an area, and extending the results of
a project area to cover the entire irrigated portion.
153
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SECTION 9
DEVELOPING BEST MANAGEMENT PRACTICES
ALTERNATIVE MANAGEMENT PRACTICES
Results from hydro-salinity and detailed simulation models calibrated
and verified by research and/or demonstration programs provide considerable
insight as to which technologies might be most appropriate in reducing sub-
surface return flows. Goals considered in reducing subsurface return flows
may include: (a) lowering groundwater levels to alleviate waterlogging,
thereby allowing the leaching of salts in the root-zone to increase crop
production; (b) reducing downstream water-quality degradation resulting from
salt pickup; and (c) improving on-farm water management to increase crop
production. Specifically, the array of potential practices noted in Section
4 that can be applied in an irrigated region to improve the quality of
irrigation return flows or reduce the impact of these discharges on receiving
waters, might include structural changes (canal and lateral linings, drainage,
land leveling, and conversion to more suitable irrigation methods) and
improved irrigation practices. For instance, providing the irrigator with
an irrigation scheduling service to help maximize the water application
efficiency would diminish the volume of irrigation return flows. Various
other improvements in designs or practices might be noted, some of which
would be more applicable than others to the multitude of conditions existing
in irrigated areas in the western United States. Various nonstructual
changes such as land retirement, taxation, influent or effluent controls
should also be considered and the results quantified.
Environmental Protection Agency Rules and Regulations (40CFR 130.1 (q))
define best management practice as "a practice or combination of practices
determined by a State or other management agency after problem assessment,
examination of alternative practices and appropriate public participation
to be the most effective, practicable means of preventing or reducing the
amount of pollution generated by nonpoint sources to a level compatible
with water quality goals." "Practicable" is defined as being feasible,
based on technological, economic and institutional considerations. Best
management practices are intended to meet both temporary and permanent
pollution control needs to minimize erosion, salinity, sedimentation and
other types of water quality degradation. The particular best management
practices must be determined and developed according to site-specific
requirements.
154
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ANALYSIS OF FIELD DATA
Costs associated with water quality problems and the measures necessary
for their resolution are generally high, particularly when the problem re-
sults from irrigated agriculture. It is important to implement only the
management alternatives that promise the most cost-effective treatment of
the problem, subject to environmental and political constraints. The
ultimate use of research and demonstration data is in determining the best
management practices to be implemented for the solution to the specific
salinity problems identified for the basin area under study.
If the project is such that no improvements or demonstration programs
are constructed prior to the large-scale implementation, materials and
installation costs must be obtained from other sources. Reliable cost
information for larger sizes (10 cfs or greater) of various types of canal
lining,, closed conduits and water control structures can be obtained from
agencies such as the United States Bureau of Reclamation and the United
States Corps of Engineers. Fairly accurate and representative costs for
smaller delivery system components, as well as various on-farm and drainage
improvements, can be obtained from local irrigation equipment suppliers
and manufacturers.
To extend the costs to the entire irrigated area to be treated, it is
necessary to collect information for the area concerning farm sizes, field
areas, lengths, and widths, field slopes, lateral lengths and acreages
served by each lateral, and canal and lateral capacities and cross-sections
at several points in the delivery systems. To delineate the types of on-
farm improvements that could be implemented, it is necessary to collect
information on infiltration rates, cropping patterns, depth of top soil,
and localized problems such as high water tables or soil fertility levels.
Some of this information can be taken from aerial photographs or detailed
farm maps, but much of the information must be collected in the field via
actual measurements and personal interviews with farmers and local irrigation
company officials. Open personal communications and periodic project reviews
by as many local persons as possible can lend valuable insight into possible
economic and sociological ramifications of the project.
Collection of the information necessary to extend the costs for a study
area or an entire irrigated system, which is basically a preliminary design
analysis, is facilitated by the use of aerial photographs for the area. The
United States Agriculture Stabilization and Conservation Service maintains
a fairly current inventory of aerial photography for most irrigated areas.
Commercial aerial photography services may also be a source of photographs.
Also, if vegetative mapping is to be done, it may be necessary to have two
sets of photographs for the areas. Both sets of photos should have the same
scale in order to facilitate data analysis.
Preliminary design information may be gathered by personal inspection
of the farm lateral systems. Data may be collected by marking on each
photograph information concerning land ownership, diversion points for the
canals, laterals, and farm distribution systems; ditch lengths and dimensions;
locations of special hydraulic structures such as measurement devices,
155
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flow division locations, culverts, siphons, and bridges; the location and
quantities of return flows; locations of pumps; special problem areas such
as buried utilities, trees that might have to be removed, hazardous areas
requiring special safety considerations, areas requiring fill dirt, and areas
that may require flood protection facilities. When these data have been
collected, an inventory or tabulation method must be devised to summarize
all of the information into a usable form.
COST-EFFECTIVENESS ANALYSES
Cost-effectiveness functions should be developed for each appropriate
technology that can be identified. Then, cost-effectiveness functions should
be developed for each combination of appropriate technologies. These cost-
effectiveness functions that relate subsurface return flow reductions to the
desired goals resulting from a specified investment, would then be assessed
in an optimizational format to arrive at the least-cost combination for
achieving a desired level of the stated salinity reduction goals. This
analysis details the optimal strategy for implementing various levels of
individual technological improvements into a comprehensive technology
package.
On a basin-wide scale, a salinity problem is the combined effect of many
irrigated areas, saline springs, diffuse natural inflows, and other
miscellaneous sources. These salinity sources not only occur sequentially
due to the geographic structure of a hydrologic area, but are often governed
by differing administrative structures. The problem of determining an
"optimal" strategy for a large river basin rapidly becomes too large and too
complex for direct analysis. One of the various mathematical techniques for
optimizing complicated systems is to decompose the problem into a series
of subproblems with solutions coordinated in a manner that produces an
answer to the larger problems. One method applied to analysis of water
quality improvements in the Utah Lake drainage basin of central Utah provides
both a simple and effective method of decomposition (Walker et al., 1973;
Walker, 1978a). The structure of the decomposition methodology referred to
above is shown schematically in Figure 57. Individual levels of modeling
define water quality cost-effectiveness analyses at that level enroute to
the most cost-effective water quality control program on a basin-wide scale.
Conceptual Salinity Control Model
The conceptual model illustrated in Figure 57 represents an additive
approach for determining the minimal cost salinity control strategy in a
river basin. A number of levels or subdivisions having similar characteris-
tics can be defined to correspond to various levels of hydrologic or admini-
strative boundaries in a region. Within each level, the alternative mea-
sures for salinity management are characterized by cost-effectiveness
relationships. A more detailed review of the structure of cost-effectiveness
functions and their interdependence assist in understanding the application
of the conceptual model.
156
-------
Cost
Cost of achieving desired
salinity control at
level 4
LEVEL 4 SALINITY CONTROL
COST-EFFECTIVENESS FUNCTION
: Optimal investment in
I alternative 2at levels
1 Optimal investment in
alternative I at level 3
Cost
Optimal level 3
Costs from level 4
ALTERNATIVE I, LEVEL 3
Desired salinity control
at level 4
Cost
level 2 investments
level 2 investments
/ALTERNATIVE 2,
LEVEL 3, COST-
EFFECTIVENESS
FUNCTION
Cost
ALTERNATIVE 1, LEVEL 2
ALTERNATIVE 2, LEVEL2
^5H COST-EFFECTIVENESS
eve I I
costs
FUNCTION
/
r ALTERNATIVE 3,LEVEL 2
UOSt COST-f
level I
costs
. . , -,
Optimal level 2
Cost from level 3
Effectiveness
Optimal level 2
Cost from level 3
Effectiveness
Effectiveness
/ I
„ ALTERNATIVE 4, LEVEL]
UOSt COST-EFFECTIVENESS
FUNCTION
level I costs
Effectiveness
Effectiveness
Figure 57- Conceptual decomposition model of a regional or basin salinity control strategy (Walker,
1978a).
-------
Description of Cost-Effectiveness Functions—
The alternatives for managing salinity on a basin-wide scale fall into
two categories: (a) those that reduce salinity concentrations by dilution,
or minimizing the loss or pure water from the system by evaporation; and (b)
those that improve water quality by reducing the mass emission of salt.
Examples of the first category include weather modification to enhance stream
flow, evaporation suppression, and phreatophyte control. Many of these
approaches may be more costly and difficult to apply than is justified by
the salinity control achieved. In the second category, such measures as
saline flow collection and treatment, reduction in agricultural return
flows, and land use regulation can be used to reduce the total emission
of salinity entering receiving waters.
By letting the spatial scale of the problem correspond to successive
layering or additions, the multilevel approach is congruent to the subbasin
breakdown of major hydrologic areas. The smallest spatial scale considered
in this kind of analysis should be a subbasin containing an irrigated valley,
or a stream segment delineated by inflow-outflow analysis. In a major river
basin, a number of subbasins may combine to form the basin itself so there
are actually three levels to be considered in a river basin. Vertical
integration of subbasins yield the aggregate river basin. In this analysis,
the river basin, river subsystem, and subbasin divisions are designated as
levels 4, 3, and 2, respectively. Level 1 also encompasses the subbasin
scale as described later.
Associated with each level of the model are cost-effectiveness functions
describing each alternative for controlling salinity. The structure of the
cost-effectiveness functions includes two parts. The first is the function
itself. To compare the respective feasibility among various salinity control
measures at each level, the mathematical description of each alternative must
be in the same format. Since most planning studies involve evaluating the
minimal cost strategy for reducing salt loading, each salinity control
measure's feasibility for inclusion in the eventual strategy is based on its
resulting reduction in salt loading. The second part of the cost-
effectiveness functions is what might be called a "policy space." To
appreciate this aspect of the model, it is helpful to first discuss the
determination of the optimal basin-wide strategy.
Evaluating the Optimal Strategy—
Providing the optimal policy for controlling salinity in a river basin
was determined with a minimum cost decision criterion. The analysis
provides two pieces of information. First, it details the cost associated
with a range of reductions in salinity, and second, it delineates how much
of these costs would be expanded in each river subsystem. In other words,
the evaluation of the optimal strategy at level 4 involves systematic com-
parison of level 3 cost-effectiveness functions; and once the strategy has
been determined, it also yields the optimal costs or expenditures in each
level 3 alternative or river subsystem. Similarly, level 2 costs and policies
are determined during the level 4 analysis, and so on. Thus, the cost-
effectiveness function for any alternative within a level is:
158
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1. The result of optimization of respective cost-effectiveness
functions at a lower level, and therefore, a minimum cost
relationship at every point; and
2. The sum of costs from optimal investments into each alterna-
tive at a lower level. The "policy space" is, therefore,
a delineation of lower level cost-effectiveness functions.
The preceding paragraphs note the detailing of a salinity control
strategy once the optimal is known. Determining the basin optimal, on the
other hand, begins at level 1. A comparison of level 1 cost-effectiveness
functions describing each alternative at that level produces the array of
level 2 functions. Similar steps yield each succeeding level's optimal
program. Thus, the multilevel approach described herein involves a vertical
integration through the levels to determine the optimal policy and a back-
wards trace to delineate its components.
The most detailed cost-effectiveness function referred to above is the
level 1 analysis of individual salinity control measures which can be applied
in a subbasin. In this report, the primary emphasis is on salinity problems
due to salt pickup in irrigated agriculture. Consequently, it should be
helpful to mention a few items relative to the level 1 analysis of irrigated
agriculture.
The major first level salinity control options in an irrigated area are
the: (a) construction of conveyance channel linings to reduce or eliminate
seepage; (b) improvement of irrigation practices to minimize deep percolation;
(c) collection and desalination of drainage flows; (d) selective retirement
of irrigated lands to eliminate deep percolation; (e) employment of economic
incentives such as subsidies or taxation to induce salinity reduction mea-
surements; and (f) imposing of legal effluent or influent limitations to force
more efficiency in irrigation water use. Each of these alternatives has
one or more specific components that require evaluation. For instance, the
most cost-effective conveyance channel lining, or the best on-farm control
measure, must be determined. The first cost-effective modeling level is also
the result of a lower level optimization or the zero level. At this level of
analysis, it is difficult to generate adequate cost-effectiveness functions
without pilot field demonstrations and evaluation of the alternatives. During
planning studies, field investigations are generally not conducted and the
investigators must rely on reported distributions of costs and effects.
Walker (1978a) gives a dimensionless distribution of costs and salinity
reductions for the first level alternatives (Figure 58). These curves can
be utilized in planning studies to arrive at the Level 1 cost-effectiveness
functions without actual field studies. These curves, however, were developed
for the Grand Valley in western Colorado and will introduce errors if con-
ditions vary greatly from those for which the curves were formulated.
To use Figure 58, it is first necessary to estimate the total cost
of implementing each measure to its maximum effectiveness. The salinity
reduction associated with this level of investment must also be estimated.
Then, an expression describing curves in Figure 58 can be used to calculate
the range of costs for reducing salinity by each measure up to the
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1.0
U)
Of
•
c
.8
.6
.4
C
O
1 .2
0 .2 .4 .6 .8
Fraction of Total Individual Salinity Reduction
Figure 58. Dimensionless level 1 cost-effectiveness curves for the
Grand Valley (Walker, 1978a)
1.0
maximum possible. Figure 58 represents the expected distribution of cost-
effectiveness for the salinity alternatives included.
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SECTION 10
IMPLEMENTATION OF BEST MANAGEMENT PRACTICES
INTRODUCTION
The program for implementing best management practices should promote
social and economic benefits through cooperative action. Every effort should
be made to prevent polarization between state and federal water pollution
control agencies, organizations providing technical assistance during imple-
mentation, and local water users. A necessity exists to develop credibility
among all institutions involved with the individual farmers. In order for a
comprehensive salinity control program to be successful, active farmer
participation is required.
There are many legal and economic consi'derations involved with
implementation, as well as questions regarding organization. Besides the
actual construction of improvements, there are aspects of training, technical
assistance and farmer participation that are important for facilitating
implementation. Lastly, since all solutions are never known, it becomes
essential to continually evaluate and refine the implementation processes.
LEGAL CONSIDERATIONS
Beneficial Use
A major legal problem that is universal throughout the 17 western states
is the failure to enforce the beneficial use provisions in the law. The
reason is twofold. First, the definition of beneficial use is nebulous and,
thus lacks appropriate direction for administrators to follow or the courts
to interpret. The second reason is derived from a lack of social conscious-
ness on the part of water users so the burden of proving nonbeneficial use is
upon the state, which is an administrative impossibility. Generally, the
system of water law places emphasis upon the right to use water, not the
duty to use it appropriately.
Court cases in Colorado and other western states reflect the difficulty
of enforcing the general concept of beneficial use. It is suggested that
State Engineers Offices develop and adopt criteria for enforcing beneficial
use as an agency regulation, particularly for those irrigated areas where
water quality problems are significant. Regulation should define standards
for the conveyance and application of water relative to the quantity of
diversions, water use, and quality of discharge. The State Engineer must
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also provide the authority for shifting the burden of proper water use upon
the benefactor (both purveyor and user), and identify the quantity of water
necessary or the water duty for delivery, use, and removal of water.
Influent Standards
Along each irrigation canal, turnout gates discharge water into a lateral.
This turnout gate is a critical control point in the irrigation system
because it represents the termination of responsibility for many irrigation
companies. The control point for each irrigation company is the point of
diversion from a river. The responsibility for these river diversions belongs
to a water commissioner or river commissioner who is usually a state employee.
The amount of water discharged at each turnout gate along a canal is the
responsibility of water masters or ditch riders who are employees of the
irrigation company or district.
Proper operation of an improved lateral subsystem results in significant
reductions in the discharge requirement at the turnout gate. Since the
lateral turnout gate is an important control point, standards for water use
could be enforced at each turnout gate, which could be classified as an
influent pollution control standard. An initial influent standard should be
the intended water duty for the irrigated lands as a result of implementing
best management practices. This standard should be measured at each farm
inlet, which can then be translated back to the lateral turnout gate taking
into consideration some small administrative losses and lateral seepage
losses, which essentially could be ignored if the laterals were lined or con-
verted to pipelines. An important consideration would be to use either a
volumetric water duty as a standard or a variable flow rate which is
dependent upon the changing water requirements of the crops during an
irrigation season.
By using influent standards, the salinity problem is alleviated by
improved water management practices, rather than end-of-pipe treatment, which
would be the result of using effluent standards under a permit program. The
success of an influent approach is dependent upon: (a) use of numerous flow
measuring devices; (b) adequate technical assistance for working with and
advising farmers on improved irrigation practices and methods; and (c)
availability of funds for making the necessary structural improvements.
ECONOMIC CONSIDERATIONS
Cost Sharing
Historically, the Agricultural Conservation Program, administered under
the U.S. Department of Agriculture, with technical assistance provided by
the Soil Conservation Service, has provided cost-sharing funds to farmers
and irrigation districts for irrigation system improvements. The program
had water quality benefits which were never verified or documented. This
program has been relatively inactive in recent years because of insignifi-
cant funds. However, with recent congressional action (P.L. 95-217, Section
208 (j)) this program should be very important in implementing the best
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management practices for salinity control as part of a cooperative effort
among federal-state-local interests in alleviating water quality degradation
from agriculture.
Most western states have revolving funds or low interest loan programs
for water resources planning and development. Generally, these programs
require that the applicant be an irrigation district or other corporate body.
Where such state programs exist, legislation and/or regulations for partici-
pation qualification should be changed to allow: (a) individual irrigators
to qualify; (b) a broader use of funds to include on-farm improvement
practices, as well as improvement of delivery systems; and (c) the inclusion
of program objectives to specifically improve water quality. When states
have no such programs, a low or no-interest loan program containing the three
components above should be adopted in order to cooperatively assist the
federal government and local water users in achieving improved water
management practices.
Dissemination of information about other state and federal agency
incentive programs should be carried out by state water agencies. Emphasis
should be directed to irrigated areas identified as producing salinity
problems. Cooperation must be extended to ensure utilization of such
programs.
By providing incentives for water users in designated salinity problem
areas, farmers will have an opportunity to voluntarily improve their water
use practices, which in turn will result in improved quality of irrigation
return flows. This is consistent with the philosophy of encouraging voluntary
compliance versus forced or involuntary compliance. If states are to create
standards and criteria for beneficial use, some mechanism should be made
available to the farmers that will facilitate compliance with the new
criteria. Without it, irrigators have legal grounds to continue exercising
their water rights as they have in the past. The legal cost to the state
and water users in litigation to change their traditional practices may be
much more expensive than devising procedures that increase the efficiency of
water use, while still protecting downstream users.
Development of a Water Market
Irrigation water supplies are allocated among farmers on the basis of
the rights they established in the past. Thus, allocation of water is based
on legal rather than economic grounds. Because of this, users tend to apply
or at least divert all the water to which they are entitled throughout the
irrigation season, even though it is not needed by the crops. Salinity
problems in irrigated agriculture are largely the result of excessive irri-
gation return flows, which in. turn are attributable to water allocations and
applications being much greater than crop water requirements. In these same
cases, the farmer pays only for the conveyance cost of the water which is
too low to encourage efficient use. The price of water is a poor reflection
of the "opportunity cost" or the value of the water with respect to alterna-
tive uses. One alternative is a water market where water could be sold,
traded, or rented which would reflect these opportunity costs.
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The establishment of a water market would alter the present institutional
arrangement. From a practical standpoint , it would seem desirable to alter
that arrangement as little as possible to assure its acceptance. To minimize
the disruption to the present institutional arrangements for allocating
irrigation water, a water rental market could be established. Under such an
arrangement, the present structure of water rights and allotments would be
maintained, but would permit rental of surplus water to other users in the
irrigated area, or upstream, without jeopardizing these rights and allotments.
There is a legal requirement that water transfers not injure other water
rights. In most cases, transfers must be restricted to the amount previously
consumptively used by the crops; however, the question of whether or not
reduced diversion requirements can be transferred to other locations is
dependent upon whether other water rights are affected as a result. Irrigation
return flows from many irrigated areas are already part or all of the water
rights of downstream users. In most western states under the existing laws,
the water surpluses should go to "junior" appropriators. A water rental
would have to address these problems and, in some cases, the "junior" rights
would have priority. The ability to initiate a water market, particularly
for transferring water outside the irrigated area, becomes "site specific."
An impediment of water rights is the transfer restriction of rights
within an irrigation system to other uses, or out of the basin. This con-
straint may exist in the substantive water law or as a result of the organi-
zational and administrative system of the state. Only a few states prevent
the sale and transfer of water rights for other uses. States restricting
transfers rely upon the appurtenancy concept to prevent such transfers. The
law should be modified to reflect state encouragement in the renting, leasing,
transferring or selling of water rights to other uses and places so long as
the vested rights of others are protected. For example, although there are
no restrictions on the transfer of water rights in Colorado, the organizational
red tape—delay and expense—acts as an impediment. Changes in the admini-
strative and judicial system should be made to facilitate exchanges of water
rights. Recognition of water right exchanges and a change in the concept of
beneficial use to include recreation, aesthetics, fish and wildlife, and
other beneficial uses would serve to nullify the fear of losing that portion
of the water right not exercised by permitting the transfer of unneeded
portions to other uses within the system.
Removing rigidities in the law to give the water right holder greater
freedom and flexibility will eliminate many of the irrigation water quality
problems perpetuated by the appropriation doctrine. A legal solution would
be to merge the economic benefits from a more liberal transfer policy into
legal guidelines that still provide protection to existing water rights
holders. This would require the adoption of an incentive mechanism to
encourage water users to "market transfer" some of their water through the
irrigation companies. Water could be rented or leased to upstream water
users or for new water demands, with the revenues used to improve the
irrigation system.
A substantial change in water laws affecting the administrative
organization of the state should be enacted to enable greater cooperation
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between the state agency and water users. At the same time, changes in the
water laws should permit the state to concentrate on development of a
desirable state water plan while specifically enacting legislation that per-
mits state water resources agencies or other public organizations to acquire
water through appropriation, purchase, abandonment, or condemnation. This
type of legislation will grant the state greater freedom in carrying out its
responsibilities and negotiating agreements with its water users and other
states.
There is a need for a means of allocating and reallocating water within
the irrigation system by an organization cognizant of the needs of water
users within the system, the state water development plan, the basin and
international impacts. The development of a centralized state brokerage
system is a possibility that would be operated as a market center for the
exchange and sale of water rights, or renting of water available under the
rights held (Radosevich, 1972). This brokerage system could be organized as
a public or private institution. Water users would be permitted to divert
only that amount of water necessary for their operation, without fear of
losing the unused decreed quantity, and lease or rent the difference to
other users. Hence, there would be an economic incentive to implement more
efficient irrigation water management practices in an attempt to reduce the
quantity of water applied.
A brokerage system created as a public entity could be established in
the Office of the State Engineer or Water Planning and Resource Department.
These offices, or their respective divisional offices located throughout the
state, would list all available water for rent, lease, exchange, or sale.
The location of available waters would determine the impact upon other
vested rights, but the responsibility for delivery and protection of such
rights would rest upon either the water right holder or water acquirer.
Uniform prices for units of water could be established, or the available
water could be marketed to the highest bidder. The adoption of such a system
in state organizations would require changes in agency law. Likewise, it
would be imperative that the state should have the power to purchase,
condemn, or receive water rights. This would allow the state to take action
against appropriators who refuse to implement efficient practices. The
state should have the power to acquire their unused rights and retain them
for future use while renting or leasing water during the interim.. A percent
of the transacted price would be retained for the operation and maintenance
expense of the brokerage system (Radosevich, 1972) .
ORGANIZING FOR IMPLEMENTATION
Local Entity
Based upon monitoring and analysis for identifying significant
irrigation return flow salinity problem areas within the state, the state
water pollution agency should: (a) designate the boundaries of the problem
area, which may be the boundaries of an irrigation system, or subsystem, or
watershed; (b) designate an entity that is a legally constituted body
representing water users within the area to undertake responsibility for
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working with the water users, collecting data, and disseminating information;
and (c) ensure that the local entity is carrying out the best management
practices developed for the area.
From a practical point of view, this entity may be a newly formed
organization, an existing organization such as the irrigation district that
assumes the program, or a federation of several existing organizations. The
area entity would utilize a representative board of commissioners that would
be responsible for carrying out monitoring, discussing with water users ways
to alleviate unreasonable salinity degradation by irrigation return flows,
and encouraging voluntary improvement of irrigation practices by those users
identified as contributing to the area's problems. For those users who
refuse or fail to respond as recommended, entities representing users within
an area would notify the state water quality control agency of the specific
noncompliance. The state would proceed under existing federal and state law
to initiate control and enforcement action under the general provisions of
the water pollution laws prohibiting discharges of pollutants and violation
of stream standards. The area entity is thus responsible for assisting in
managing the agricultural practices within the designated area, but control
and necessary enforcement are appropriately left to the state.
Water User Organizations
A crucial element in implementation of an effective salinity control
program is gaining participation of the users. The basic unit of organization
should be at the lateral subsystem level because it is a natural hydrologic
subunit where farmers know each other and interact on a daily basis. In
some irrigated areas, the jurisdiction of the irrigation companies or dis-
tricts does not include the laterals; so an organizational vacuum for water
users on these laterals results. In other cases, the irrigation companies
are also responsible for the operation and maintenance of the laterals; so
there are no real organizational problems with the individual farmers along
each lateral.
The goal should be to gain participation by all water users on each
lateral. This may not always be possible due to human problems.' While the
organization could be on an ad 'hoc or informal basis, experience indicates
that it is probably best to aim for a formal organization with rules
developed by the members. A formal organization with its own rules and
regulations also makes it easier for the implementing agency because all
parties have a knowledge of the structure and mechanisms involved. When
leadership is defined, this facilitates the work of the implementing agencies.
Water users usually have been formally organized as nonprofit mutual
irrigation companies under their particular state laws. If each lateral must
also be formally organized, members :of these associations may encounter
problems with lawyer fees for incorporation. This can be partially overcome
by providing example sets of bylaws and other provisions to farmers con-
sidering organization. In fact, alternative examples can be provided to
farmers to help decide whether the set of rules and regulations meet their
special needs for the most effective means of operation and maintenance of
their particular water delivery and irrigation systems. These examples could
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be described and illustrated in a manual or booklet and made available to
interested farmers. The booklet should explain the benefits of formal
organization, how to organize legally, and the types of bylaws and provisions
required. It is important that such a booklet be well illustrated and in
easily understood language, rather than containing much legal jargon which is
not easily understood.
Technical Assistance
The most likely candidate for providing the necessary technical assistance
to implement the best management practices for salinity control is the Soil
Conservation Service. The Soil Conservation Service has the responsibility
for this area, with cost sharing funds provided by Congress to the United
States Department of Agriculture for alleviating water quality degradation
from agriculture. At the same time, there are state programs that can also
provide funding and technical assistance personnel. The cooperation and
coordination of state and federal technical assistance, including the
Cooperative Extension Service, is highly desirable.
TRAINING
Training Field Personnel
The primary agency providing technical assistance to farmers for a
salinity control program will likely be the Soil Conservation Service. The
Soil Conservation Service would cooperate with other state agencies in
supplying the required technical assistance. Given the levels of manpower
needed to work with farmers and the current shortage of trained manpower
with on-farm water management experience, special short courses for training
personnel would be essential. As a complement to technical competence,
personnel working directly with farmers should know how to develop good
working relationships with their clients. Additionally, personnel should
have definite skills and knowledge related to organizing farmers into water
user associations for action programs. Personnel must also have the
capabilities required for assisting farmers in increasing the efficiency of
existing irrigation practices and maintaining and improving conveyance
systems.
Technical assistance to farmers include convincing them to use
"scientific" irrigation scheduling procedures and other improved irrigation
practices. The focus on improved irrigation scheduling is essential because
the existing piece-meal methods of scheduling are inadequate as an individual
salinity control measure.
Farmer Training Materials
Materials are needed to motivate farmers and help them understand the
importance of improving present water management practices for increased
crop production and control of salinity. Data obtained in problem identifi-
cation and alternative solutions should be utilized in preparing well-
illustrated materials for farmers. These materials should graphically and
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clearly define the problem, explain its consequences, document the
contributing factors, and explain the costs and benefits. Alternative
solutions should be carefully delineated and estimated costs presented.
Techniques for such communications could include slide presentations, a
condensed booklet, and selective use of local electronic media. Since a
comprehensive salinity control program requires changes in attitudes and
behavior, the first major consideration should be the establishment of
definite communication objectives. To make the program successful in
reaching all water users and the community, several complementary communica-
tion methods should be continuously used to reinforce the central messages.
Local print and electronic media must be identified, enlisted, and used
efficiently and creatively to attain optimum results. Essentially, salinity
control is a problem of water conservation which requires much communication
directed to farmers and communities.
FARMER ACTIVITIES
Farmer Participation
One of the unique characteristics of improving on-farm water management
is that the degree of success is highly dependent upon the degree of partici-
pation of each individual farmer, as well as their ability to cooperate
collectively for the good of all water users. The construction of on-farm
physical improvements only provides an increased potential for water use
efficiency, whereas the success achieved is dependent upon the operation,
management, and maintenance of the physical improvements. This, in turn,
is dependent upon the level and ability of technical assistance provided,
farmer attitudes, and the degree of credibility between all individuals
involved.
Credibility and acceptance by the farmers begins when the basic training
and motivational materials are initially used to describe the problem. Efforts
to organize the water users provide an opportune time to develop early rapport
with the farmers. Credibility and acceptance of the technical personnel by
farmers during the planning and implementation of individual farm plans for
improved water management is essential to the long-range goals of a control
program. Credibility and good communication must exist during the collective
negotiations in determining the physical improvements to be made. Farmer
participation is crucial during these stages in order that a plan of develop-
ment evolves that is acceptable to the water users and also satisfies the
goals of the salinity control program.
The final step in this process dictates the real success of the entire
program. After spending vast sums of money to construct physical improvements,
the test of effectiveness centers largely around the operation, management,
and maintenance of these improvements. This is the phase of the work where
the rapport developed with the farmers pays high dividends. Unfortunately,
this step is very time-consuming and most frequently neglected. Considerable
evaluation is required to perfect or maximize these new improvements so they
operate at their full potential. This is the key to the success of any
nonpoint source pollution program.
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Farmer Recognition
Many experiences have demonstrated the importance of farmer recognition.
Farmers usually can sell a program to other farmers more successfully than
public officials. Where possible, farmers should be given special recognition
because the success of any salinity control program ultimately rests with the
degree of participation by farmers themselves. Several methods employing
farmer recognition can be effectively utilized to motivate other farmers.
Proper use of radio and television announcements and newspaper articles can
help considerably in fostering enthusiasm for the program. Local newspapers
are usually willing to provide coverage on news related to natural resources
and agriculture. Local television and radio stations can often be expected
to cooperate with the project by disseminating news related to the salinity
control activities.
In addition to reports about current activities of the salinity program,
the news media are also interested in covering human interest stories. If
human interest reports and farmers' testimonials are well prepared, they can
excite other farmers about the program. Such publicity is free and probably
can generate better image-building for the state and federal agencies than
these agencies do for themselves.
Awards should be considered for those farmers who have made exceptional
progress in improving their on-farm water management practices. Awards for
providing leadership in the local water user organization(s) should also be
considered. Awards presented to each water user served by the lateral
demonstrating the most efficient use of water would be highly effective in
promoting the goals of an improvement program. These awards would serve to:
(a) make farmers more aware of improved management practices; (b) foster
efficient irrigation practices; and (c) promote cooperation to help each
other improve methods. News media coverage of such awards also provides
additional incentives for improved water management on the part of other
farmers. Framed photographs of farmers engaged in improvement activities
with an inscription could be considered for presentation. Plaques could be
presented to cooperators to show appreciation for their contributions.
An excellent method of employing farmers for promoting wide interest in
a project , once substantial progress has been made in an improvement program,
is the use of Field Days. A Field Day could be held annually that would
involve strong participation by local farmers. Water users and irrigation
company leadership from nearby areas could be given special invitations to
attend the Field Day in order to observe firsthand the implementation of a
salinity control program. In addition, special tours could be arranged
during other times of the year for a group of irrigators from any particular
area to visit the project area and meet with farmers who have participated
in the program. The emphasis should be farmer-to-farmer interaction, with
the local farmers being highlighted rather than technical assistance
personnel. These personnel, however, should play a strong backstage role in
facilitating this interaction.
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MONITORING, EVALUATION, AND REFINEMENT
A monitoring network should be established so that the impact of the
implementation program can be measured. A part of this network would be the
stations used in the inflow-outflow analysis. In addition, some of the
groundwater monitoring stations used in developing the hydro-salinity model
would be incorporated into the monitoring network, along with some new stations
scattered throughout the irrigated lands near the river or point(s) of
outflow.
Evaluation research techniques are available which, if properly
utilized, can be used to determine the strengths and weaknesses of project
implementation. Information from such studies is needed by sponsoring and
implementing agencies to discover the most effective and efficient methods
of working cooperatively with farmers. Continual and periodic evaluation
mechanisms are needed in order to continually refine the implementation
program. Feedback is needed both from farmers and technical assistance
personnel for improving: (a) the delivery of the salinity control technologi-
cal package; (b) the operation and management of the physical improvements;
and (c) the cost-effectiveness of the entire program. Credibility with the
farmers will be strengthened by continually refining and improving the
implementation program.
Extension communication strategies should be designed into the project
work plans so that various techniques can be effectively evaluated. While
technical expertise for such programs is usually adequate, there is a general
weakness in designing and evaluating extension communication strategies.
Technical assistance personnel should be given short courses in skills
needed for working effectively with farmers. As stated often in this report,
the key variable in achieving successful program implementation and long-
term effective management of improved systems are the farmer clients
themselves. Consequently, communication techniques used for working with
farmers as individuals and groups should be designed into the implementation
program and evaluated to the same degree as the technical components and
activities.
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191
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APPENDIX A
DESCRIPTION OF SELECTED IRRIGATION RETURN FLOW MODELS
FORMAT OF DESCRIPTIONS
The models that might be employed in salinity control planning were
referenced in Section 6. This appendix contains a brief technical review of
these models developed by Walker (1978b). The format of the descriptions is
divided into the following input segments:
(1) Model grouping;
(2) Descriptive or developmental references;
(3) Scope of the model;
(4) Input requirements;
(5) Spatial and time scales;
(6) Structure of the code;
(7) Basic mathematical or analytical approach; and
(8) General comments.
In describing various models, as much as possible of the information
presented originates in the cited references in order that descriptive errors
are minimized. The phrases used in describing various aspects of a model are
intended to alert the reader's own understanding rather than trying to be
exhaustive in detail. Those unfamiliar with a modeling area are encouraged
to obtain and review the reference material for a more in-depth assessment.
Model Scope
Under the heading of model scope, a few sentences are given to describe
what the model simulates and its expected output, including the use of input
dumps, error flags, and intermediate calculation results to make the model
more useable. Most models print input data to insure that it has been input
properly. Error flags and results of intermediate computations are not
generally included.
Input Requirements
A list of control parameters and basic input data is included to
identify to the potential user the information required to utilize the models
effectively. This list is not exhaustive, but represents input data of most
importance. Occasionally, a series of data is described in more general
terms or classified in order to reduce the length of the descriptions.
192
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Spatial and Time Scales
It is important that the "size" of the modeled portion be identified in
comparison to the problem's real dimensions. The interval or time scale is
also useful in selecting a model to evaluate an IRF problem since detail of
the study is directly related to time resolution. The dimension of the
simulation may also be provided in this section, thereby indicating the
assumptions made about the system's structure.
Computer Code Structure
In these sections, the programming language, core requirements, expected
execution times, internal program linkage, and application comments are made.
Computer words have different byte lengths and execution times depending on
whether or not program compilation is necessary. A major problem occurs when
the code is written in the CSMP format for IBM computers because much of the
program then becomes part of the basic computer software and storage-time
requirements are not known.
Basic Mathematical Approach
It is difficult to express in one or two paragraphs what a complex
simulation model accomplished and in what manner. Key phrases and words are
used as much as possible to convey to the reader a great deal more than is
actually written, and if possible these phrases were taken from the documen-
tation of the code or reference material. The assumptions in a model are at
least partially apparent to the already informed reader from the description
of what is calculated. Often classical equations in the literature are named
without reference. These are noted in the text and generally identify the
analytical approach taken.
_GenerajL
Of general interest to modelers is where the model can be obtained,
verification that has occurred, the stage of development and future applica-
tions, and special problems that might be encountered in its use. For the
most part, this section contains comments on verification and acquisition.
MODEL DESCRIPTIONS
The individual model descriptions are given in the following pages.
193
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MODEL; A-l
REFERENCES;
Bureau of Reclamation. 1977. Prediction of mineral quality of irrigation
return flow, Volume III, Simulation model of conjunctive use and water
quality for a river system or basin. User's Manual. EPA-600/2-77-179c.
Robert S. Kerr Environmental Research Laboratory, Office of Research and
Development, U.S. Environmental Protection Agency, Ada, Oklahoma. August.
295 pp.
MODEL SCOPE;
Simulation is of the water and salinity impacts in a river basin
resulting from urban, industrial, and agricultural water uses. Formulated as
part of a need for evaluating irrigation return flows in a river system, the
model is primarily oriented to this end. In addition to the mass balance of
surface and groundwater in a river section, the model simulates reservoir
operations, and a comparatively complex analysis of the soil salinity system.
Program features both input dumps and error flags. Intermediate results are
not presented.
INPUT REQUIREMENTS;
Control parameters are initially read in to completely dictate problem
structures since enough flexibility has been added to allow simulation of
river systems with a multitude of local conditions. Control parameters
include division of system into segments (nodes), processes being simulated
in each segment, node structure and labels, and time dimensions. Input data
descriptive of local hydrochemical processes are input as follows:
(1) aquifer volume and salinity; (2) reservoir conditions and operating
rules; (3) sediment analyses; (4) irrigated soil chemical and hydraulic
characteristics; (5) hydropower generation parameters; (6) discharge and
chemistry of major surface and subsurface inflows; (7) demands by agricul-
tural, municipal and industrial users; and (8) allocation of flows within an
irrigation system.
SPATIAL AND TIME SCALES:
Simulation is of hydrochemical systems on a river basin scale or segment
thereof. Time period utilized is one month at minimum scale to several years
duration of simulation.
COMPUTER CODE STRUCTURE:
Internal program linkage is accomplished by subroutines and functions
with both common and call statement data transfer (control parameters are
generally passed in the call). Code is set up to read and write results as
output or onto tape or disk systems. Approximate core and execution time
requirements are 140 k bytes and 120 central processor seconds, respectively.
Program language is Fortran.
194
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BASIC MATHEMATICAL APPROACH;
The program utilizes an iterative scheme for balancing mass between
"nodes" or subbasins comprising the area being analyzed. This model includes
substantial simulation of reservoir and stream management. After initializa-
tion of the system, the various demands are compared with the available
stream flow to determine if a shortage is occurring. If insufficient water
is available in the river, a series of reservoir checks and manipulations are
made to adjust flow if possible. Return flow volumes and chemistry are then
computed and checked against initial assumptions. Iterations are repeated
until mass balance is achieved.
Of particular interest in this review are the model segments describing
the irrigation return flow system. Diversions are determined by computing
the consumptive use (using the Penman equation) and then dividing by con-
veyance and farm irrigation efficiencies. Return flow is then the difference
between demand and consumptive use. The chemical simulation of the soil
profile is based on a simplified version of the model presented by Dutt, et
al., 1972.
GENERAL;
Model is very complex in scope and therefore difficult to apply without
some previous experience in hydrosalinity modeling on a large scale.
However, the user manuals are excellent descriptions of the computer system
and can be used to implement the model. This model has been used extensively
by the Bureau of Reclamation in studies throughout the western United States.
The code is available from the Bureau of Reclamation.
MODEL: A-2
REFERENCES:
Crawford, N. H. and A. S. Donigian. 1973. Pesticide transport and runoff
model for agricultural lands. EPA-660/2-74-013, Office of Research and
Development, U.S. Environmental Protection Agency, Washington, D.C.
December. 317 pp.
Donigan, A.S. and N.H. Crawford. 1976. Modeling pesticides and nutrients
on agricultural lands. EPA-600/2-76-043. Office of Research and Development,
Environmental Research Laboratory, U.S. Environmental Protection Agency,
Athens, Georgia. February. 211 pp.
MODEL SCOPE:
Simulation is of runoff, snow accumulations and melt, sediment loss,
pesticide-soil interactions, and soil nutrient transformations. Analyse
involves transport of pollutants from both surface and subsurface sources.
The model is intended for watersheds up to two square miles in which in-
channel processes and transformations can be neglected. Parameters simulated
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include mass transport of water, sediment, pesticides, nitrogen and phospho-
rus. The program is set up to operate in either a calibration or production
mode. In general, the output includes water movements (runoff, infiltration,
evapotranspiration, and storage), sediment transport, pesticide transport and
transformation in various soil layers and in sediment, nutrient transforma-
tions in the soil profile, and leaching of nitrate. The only salinity
parameter in this version of the model is chloride. Output can be controlled
to intervals from every 5-15 minutes in the simulation to monthly summaries.
INPUT REQUIREMENTS;
Data necessary to operate the model can be divided into control,
hydrologic, snow, sediment, pesticides, and nitrogen-phosphorus parameters.
The control parameters include type of run, units, output interval, snow,
pesticide, nutrient simulation options, input data checks, time intervals for
calculations, watershed structure, and time limits. For the hydrology sub-
model, input includes soil moisture storage (capacity and initial), length,
slope, roughness, intake, evaporation coefficients, and surface storage of
watershed, groundwater return flow timing, and groundwater distribution and
conditions. The snow accumulation-melt analysis requires a number of coeffi-
cients describing the energy balance description of the solid-liquid phase
transformations, climatological parameters (radiation, temperature, wind),
and topographic description of the watershed. To compute the sediment phases
of the model, the user must supply crop cover, tillage effects (depth,
timing, and fine deposits), soil depths and bulk densities, rainfall splash
coefficients, overland flow exponent and wash-off coefficient, and the
initial soil fines deposit. Pesticides are described using application
timing, solubility, fixing capacity, coefficients in the Freundlich
adsorption/desorption equation, decay rate, and control coefficients noting
how the pesticide is applied and whether or not a single-valued adsorption/
desorption algorithm is to be used. Nitrogen and phosphorous simulation
begins by first specifying time stops, number of fertilizer applications, and
harvest dates. Then a series of nitrogen reaction rates for mineralization,
immobilization, nitrification, denitrification, and uptake are input.
Similar coefficients are needed for phosphorous analysis. Storage of nitro-
gen (in its various forms) and phosphorus is detailed by describing the ad-
sorbed, dissolved, absorbed, and gaseous phases.
SPATIAL AND TIME SCALES:
Calculations are based on a 5 or 15 minute real time interval with
results lumped in hourly, daily or monthly summaries. The spatial scale is
of a 1-2 square mile agricultural watershed.
COMPUTER CODE STRUCTURE:
The basic code structure is one of a main program controlling the order
and presentation of input, calculations, and output with subroutines encom-
passing specific submodel tasks. The code is written in Fortran IV and run
on an IBM 360/67 computer but appears readily adaptable to other machines.
The code is very lengthy and is best operated in compilation and production
steps. The number of pages of output can be several hundred with even
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routine analysis. Neither core requirements nor execution times are noted,
however, both are comparatively large on visual inspection.
BASIC MATHEMATICAL APPROACH;
This model is too large for a thorough description of the mathematical
approach. Consequently, only a brief summary will be presented here with
more detail left to the interested user. The code involves six primary
subprograms: (1) MAIN which orders overall execution; (2) LANDS which
simulates the hydrology and snow; (3) SEDT, the analysis of sediment,
(4) ADSRB, the pesticide adsorption and removal program; (5) DEGRAD,
pesticide degradation; and (6) NUTRNT, which is the simulation of nutrient
transformation and removal.
The LANDS subprogram is derived from the Stanford Watershed Model
presented in various publications. The hydrologic response of a watershed to
inputs of precipitation and evaporation involves a water budgeting procedure
which accounts for evapotranspiration, surface storage and retardance,
infiltration, groundwater storage depletion and return flow, and overland
flow.
The SEDT subprogram is a simulation of sheet and rill erosion involving
the processes of particle detachment by rain drop impact and transport by
overland flow. The particles detached during an interval are expressed as a
power function of vegetal cover, precipitation, and a detachment coefficient.
Overland transport is expressed as a power function of the detachment
coefficient, initial volume of soil fines, and overland flow rate. Attempts
have been made to include the effects of tillage operations by allowing the
user to redefine initial volumes of detachable soil fines. Vegetal cover
effects are specified monthly by the user with linear interpolation during
the month.
The subprogram ADSRB is based on two alternatives. First, the standard
single-valued Freundlich adsorption/desorption isotherm is used to separate
adsorbed and dissolved pesticide concentrations. The second approach
involves a multiple valued adsorption/desorption process taken from the work
of Davidson, et al. (1975) reported elsewhere in this writing.
The degradation and volatilization of pesticides are simulated in the
subprogram DEGRAD. Volatilization is ignored in the model's present version
and degradation is assumed to follow a first-order decay function.
The nutrient simulation model (NUTRNT) attempts to predict nitrogen and
phosphorous losses due to erosion, surface washoff, leaching, and biological
conversion. Phosphorus is assumed to exist as organic phosphorus, solid
phosphate,'dissolved phosphates, and phosphorus adsorbed by the plants.
Transformations are all based on first-order kinetics (temperature dependent).
Nitrogen processes (mineralization, immobilization, nitrification, denitn-
fication, and uptake) are based on first-order kinetics using literature
values of the coefficients. Ammonium adsorption/desorption is also Deluded
in the model. Solution of the coupled differential equations is accomplished
with a simple Euler integration scheme.
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GENERAL:
This model is currently being improved and modified for a more general
The nutrient phases remain unverified at this time although the surface
hydrology and pesticide portions are tested against actual field data. Code
listing is presented in the reference.
use.
MODEL: A-3
REFERENCES:
DeWit, C. T. and H. Van Keulen. 1975. Simulation of transport processes in
soils. Centre for Agricultural Publishing and Documentation. Wageningen,
The Netherlands. 100 pp.
MODEL SCOPE:
The movement of heat, salts and ions, and water in unsaturated soil is
described by a series of CSMP coded computer routines. Transport of these
materials and energy encompasses diffusion, dispersion, and mass flow. The
soil profile is divided into a series of compartments in which a sequential
materials balance is computed. A number of the processes are not mutually
exclusive, and the writers do not attempt to present a clear description
for integrating the system together. Output includes the time and spatial
distribution of the parameters. Plotting functions are also described.
INPUT REQUIREMENTS;
1. Soil heat flow: surface temperature fluctuations with time,
thickness and number of soil depth increments, thermal conductivity of the
soil compartments, and volumetric heat capacities; 2. Salt transport:
initial moisture contents, labyrinth factors, initial solute concentrations,
inflow volume and concentration, compartment thicknesses, a dispersion
factor, and water diffusion coefficients; 3. Ionic diffusion: compartment
thickness, diffusion distance, total depth, water contents, labyrinth factor,
valency and initial concentrations of the ion under study, and diffusion
coefficients; 4. Ionic transport: initial moisture contents, labyrinth
factor, compartment geometry, dispersion factor, exchange capacities, water
inflow, diffusion coefficients, solution-adsorption site equilibrium constant,
and initial distribution of ions in the soil profile; 5. Infiltration:
initial moisture contents, compartment geometries, initial water content at
the soil surface, hydraulic conductivity and diffusivity functions of water
contents.
SPATIAL AND TIME SCALES:
Calculation proceeds in time increments on the order of five to ten
minutes and continues through periods of days. The analysis is limited to
the 1-3 meter unsaturated zone and is one-dimensional.
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COMPUTER CODE STRUCTURE;
Code is written in CSMP with Fortran output. Program is dispersed in
the reference and not presented in a single listing. In addition, the five
segments noted are not described as an integrated model; although, as will be
pointed out next, the basic model structure is the same for each segment.
Neither core nor execution times are available.
BASIC MATHEMATICAL APPROACH:
The soil depth is divided into a number of layers or compartments. With
this geometry established and the initial conditions defined, the approach is
to compute the movements from one compartment to the next based on the
gradient between layers, and diffusion and dispersion functions.
In the heat flow submodel, the heat flow rate equals the temperature
gradient between compartments multiplied by the thermal conductivity of the
soil. The heat energy in a layer equals the temperature multiplied by the
volumetric heat capacity multiplied by the compartment size. In order to
calculate the heat flow with time, a time distributed surface temperature is
defined along with the initial thermal characteristics of each compartment.
The dynamic nature of the system does not allow for a direct equilibrium
solution describing the heat status at each level in the soil. The procedure
utilized is a centralized, forward integration procedure (explicit). At any
instant in time the heat contents of all compartments are initially given.
From this information the flow rates into and out of each compartment are
computed and the volumetric heat contents are iteratively reviewed. The
analysis can be for layered soils if their thermal coefficients can be
defined.
The salt transport submodel operates in the same manner as the heat flow
submodel except that flow is input at the surface (rate) with salt movement
between compartments based on concentration differences (diffusion) and mass
transport with the flow. Mass balance between compartments is maintained by
recomputing salt accumulations. Dispersion due to tortuosity is also related
with a coefficient to the concentration gradients.
Ionic diffusion in the absence of moisture flow is also based on the
preceding approach with gradients based on ionic concentrations. The ionic
transport submodel includes not only ionic diffusion, dispersion, and mass
flow but cation exchange as well. Again, the ionic composition of each
compartment is iteratively determined in conjunction with the net inflow or
outflow as was described for the heat flow submodel. The added complexity of
the monovalent-divalent ionic adsorption processes is handled by relating the
adsorbed ions proportionally to the moisture content and exchange capacity
and then distributing the relative composition of the adsorbed ion in an
equilibrium constant approach.
in the heat and solute movement submodels, the conductivity and
diffusion-dispersion coefficients were not dependent on the relative
concentration of the parameter being simulated. In the case of infiltration,
however, diffusivity and hydraulic conductivity are moisture dependent. To
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simulate the moisture movement between compartments, an "average" moisture
content is determined for the gradient information. In this submodel the
average is actually a weighted average to take into account the nonlinear
diffusivity-hydraulic conductivity versus moisture content relationships.
The flow between layers is thus the average diffusivity times the moisture
content gradient plus the average hydraulic conductivity. The overall
procedure is to first establish the initial and boundary (inflow) conditions.
Then diffusivity and conductivity are calculated for each layer yielding the
interlayer moisture movement. Net moisture content change and the input
water are routed through the compartments by successively computing-adjusting
until a stable mass balance is achieved or until changes from iteration to
iteration are small.
GENERAL:
Although the model is detailed in the reference, a potential user is
still left with .a considerable effort to integrate the parts into a useable
program. Fortran users will be able to develop such models with about as
much effort.
MODEL; A-4
REFERENCES:
Frissel, M. J. and P. Reiniger. 1974. Simulation of accumulation and
leaching in soils. Centre for Agricultural Publishing and Documentation,
Wageningen, The Netherlands, pp. 70-84.
MODEL SCOPE;
Simulation is of diffusion, dispersion, adsorption-desorption, mass
transport, and volatilization of herbicides in a homogeneous soil profile.
Output includes the time and spatial distribution of the herbicides in the
soil system.
INPUT REQUIREMENTS;
Two options are available: (1) instantaneous equilibrium (no
adsorption); and (2) noninstantaneous equilibrium (adsorption-desorption).
Both model options require volumetric moisture contents, bulk density, gas
and liquid phase fractions, distribution ratios (soil/water, water/gas), soil
profile depth, diffusion coefficient for gaseous phase, surface flux and
concentration, and first-order decomposition rate coefficients for the
herbicides. The noninstantaneous option also requires adsorption-desorption
rates and water system diffusion rates.
SPATIAL AND TIME SCALES:
Simulation is of one-dimensional analysis of a one meter unsaturated
soil profile. Calculations are presented at intervals specified by the user
in hours or days.
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COMPUTER CODE STRUCTURE;
CSMP
BASIC MATHEMATICAL APPROACH;
The flux of the herbicides due to water movement is described basically
by the compartment modeling approach noted in other CSMP models. For the
instantaneous equilibrium model, herbicides migrate in both the water and
gaseous phases (only diffusion flux for the gaseous phase). Decomposition of
the herbicide is a first-order reaction based on a decay rate. The analysis
describes the convection and diffusion flux in the water phase and the
diffusion flux in the gaseous phase. In the noninstantaneous equilibrium
model, a linear adsorption reaction is assumed and added to the instantaneous
equilibrium approach.
GENERAL:
The mathematics for the herbicide models are well defined in the
reference. The models are listed in the reference. Field verification is
not reported.
MODEL: A-5
REFERENCES:
Frissel, M. J. and P. Reiniger. 1974. Simulation of accumulation and
leaching in soils. Centre for Agricultural Publishing and Documentation,
Wageningen, The Netherlands, pp. 54-69.
MODEL SCOPE:
Simulation of diffusion, dispersion, and mass transport of Na , K ,
Ca++, and Hg++ in soils. Model considers nonlinear adsorption-desorption.
Output includes the time and spatial distribution of the salinity ^ cations in
the soil solute, adsorbed phase, and in deep percolation. The soil is
considered homogeneous.
INPUT REQUIREMENTS;
input consists of depth distributed and exchangeable solution Na+, K+,
Mg++, Ca++, moisture content, Na+A+, Ca++/Mg++, and adsorbed cation fraction/
cation exchange capacity ratios, inflow rates and concentrations, fzxed K
and fixation and release rates.
SPATIAL AND TIME SCALES;
Spatial scale is a one-dimensional analysis of a one meter or so depth
of unsaturated soil. Calculations are reported in hours.
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COMPUTER CODE STRUCTURE:
CSMP
BASIC MATHEMATICAL APPROACH;
The diffusion, dispersion, and mass transport are simulated by the CSMP
compartmental model described previously. In addition, the cation exchange
and potassium fixation are included to adjust the transport results. The
exchange reaction is assumed to occur instantaneously. A Gapon equation,
along with nonlinear functions (Vanselow), are incorporated, but the principal
approach is one developed by the authors. Potassium fixation is a first-
order reaction depending on relative concentration in solution and on the
soil.
GENERAL:
More detail concerning the exchange processes is left to the interested
reader. The input can be time distributed, but it is not clear how the
moisture is redistributed since moisture content-conductivity relationships
are not included. Program is listed in the cited reference.
MODEL: A-6
REFERENCES:
Frissel, M. J. and P. Reiniger. 1974. Simulation of accumulation and
leaching in soils. Centre for Agricultural Publishing and Documentation,
Wageningen, The Netherlands, pp. 12-20.
MODEL SCOPE:
Simulation of diffusion, dispersion, and mass transport of completely
soluble compounds in the soil solute. Output includes the time and spatial
distribution of the solute concentration and deep percolation. The soil is
considered homogeneous.
INPUT REQUIREMENTS;
Input to the model includes depth of soil profile, number of compartmenta
soil layers, surface flux and inflow concentration, diffusion constant, anion
exclusion ratio, initial moisture content, tortuosity and dispersion coeffi-
cients with soil depth, output frequency, and initial distribution of solutes.
SPATIAL AND TIME SCALE:
The spatial scale is an approximately one meter, one-dimensional view of
an unsaturated soil profile. Calculation times are on the order of minutes
with output printed at selected aggregate intervals of hours.
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COMPUTER CODE STRUCTURE:
CSMP
BASIC MATHEMATICAL APPROACH;
The soil profile is divided into 40 compartments or layers with initial
conditions defined in each layer. The diffusion coefficient for the boundary
between two layers is computed using mean values of moisture content, tor-
tuosity, and dispersion values. Diffusion rates are based on Pick's Law
which calculates an average concentration gradient between layers and multi-
plies it by the diffusion coefficient to arrive at the flow. Mass flow
is also based on average concentration and moisture flow rates. In this
model, water flux is based on a steady-state infiltration.
The basic mathematical approach involves first calculating the movement
rates between compartments. The rate of change in compartmental solute
concentration is then computed and integrated using a standard CSMP semi-
parallel method such as a fourth order Runge-Kutta method. The rates are
recomputed and the iteration repeated until the compartment concentrations
have stabilized.
GENERAL:
The program is set up for tritiated water. For other anions (Cl ,
, the rates are multiplied by the exclusion ratio. Code is presented in
the cited reference. Verification is not reported.
MODEL; A~7
REFERENCES ;
Gupta, S.K., K.K. Tanji, D.R. Nielsen, J.W. Biggar, S.C. Simmons, and
J.L. Maclntyre. 1977. Field simulation of soil-water movement with water
extractions. Water Science and Engineering Paper 4013. University of
California, Davis, California. May. 95 pp.
MODEL SCOPE;
Simulation of "infiltration, vertical seepage, and uptake by plants as
related to the hydraulic properties of the soil, soil leering, ^ ^
supplied by disk or cards.
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INPUT REQUIREMENTS:
The program requires a number of control parameters to direct the
calculations through the alternatives for various steps. First, boundary
conditions when not calculated directly must be specified for field and crop
conditions. The problem size and decomposition structure is defined next,
i.e., distribution of nodes and material numbers above and below each node.
Dates for planting and harvesting are also read in; then a series of flags
are specified. Leaf-area-index options include direct input of LAI versus
time, or a user specified distribution. Partitioning transpiration and
evaporation from inputed values of potential evapotranspiration can be either
computed with techniques using LAI related energy balance at the soil surface,
or an option supplied by the user. Stress effects on transpiration include
options for on-off, logarithmic decrease, linear decrease, a combination on-
off and linear or a user supplied function. Soil profiles may be either
homogeneous, nonhomogeneous, or layered, each of which requires the hydraulic
conductivity-moisture content relationship (expressed as polynomials). Root
growth options involving a negative exponential relationship, distributed
density functions, or a user supplied system is specified and associated data
read into the model. Moisture extracted by the rooting system is evaluated
in a sink term added to the moisture flow equation. The form of the sink may
be a macroscopic Nimah-Hanks type, related to soil suction, or supplied by
the user. Associated data would then be necessary to define the pertinent
coefficients. Finally, surface boundary conditions describing static,
quasi-dynamic, dynamic, or semi-infinite depth must be known.
TIME AND SPATIAL SCALES:
During an irrigation, time increments may be as small as 0.1 to 1.0
hour with aggregated values presented daily. Spatial resolution is a one-
dimensional analysis of the unsaturated region below an irrigated surface.
COMPUTER CODE STRUCTURE:
A main program-subroutine system with common and call information
transfer constitutes the essential code structure. Code is written in
Fortran IV. Execution time and core requirements are not specified in the
reference, but neither are expected to be a problem at most computer
facilities.
BASIC MATHEMATICAL APPROACH:
Water entering the soil via irrigation or precipitation either
redistributes in the root-zone or evaporates from the soil surface. Knowing
the potential evapotranspiration rate, surface evaporation is delineated by a
leaf area index related radiant energy balance computation at the soil
surface. The moisture not evaporating from the soil surface moves through
the soil profile and/or is extracted by the rooting system.
Soil moisture movement in the root-zone is simulated with the single
phase, time dependent solution to the Darcy equation. A sink term is added
to the basic finite difference solutions to accomplish plant uptake. The
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of tL K 6 ~* an interesti"9 W*Y' An iterative solution
* ' suon
, ls.undertake* until a mass balance between internodal flux
and total transpiration is accomplished. A similar mass balance relating the
moisture remaining after plant uptake with the distribution of moisture in
the profile is utilized to iteratively change time resolutions until both
estimates are congruent. Thus, the model automatically insures a mass
balance by forcing the detached solutions to the flow equations to be in
agreement.
GENERAL;
This model is the first phase of a comprehensive study aimed at
simulating nitrogen behavior in soils. Verification has been made against
very detailed field data, although only at one location. The code reflects a
fair state-of-the-art description of the physical processes in the root zone
even though more refined treatments of some segments are possible. However,
the sophistication among these segments appears quite homogeneous. Although
classified primarily as a research tool, the approach allows application to a
number of field situations. Code is available from the authors cited above.
MODEL: A-8
REFERENCES :
Hill, R.W., E.K. Israelsen, and J.P. Riley, 1973. Computer simulation of the
hydrology and salinity systems within the Bear River Basin. PRWG 104-1.
Utah Water Research Laboratory, Utah State University, Logan, Utah. 122 pp.
Huber, A.L., E.K. Israelsen, R.W. Hill, and J.P. Riley, 1976. Basin
simulation assessment model documentation and users manual. PRWG 201-1.
Utah Water Research Laboratory, Utah State University, Logan, Utah. August.
30 pp.
Narasimhan, V.A., and E.K. Israelsen, 1975. A water- land use management
model for the Sevier River Basin, phases I and II. Information Bulletin 24.
Utah Water Research Laboratory, Utah State University, Logan, Utah.
September. 44 pp.
MODEL SCOPE;
Simulation involves the basic hydrologic relationships in river basins
and/or watersheds. The model incorporates relationships describing hydrologic
processes which are linked together by the conservation of mass principle.
Salinity is added to simulation by assuming the hydrologic processes change
the chemical concentration in the system by storing, concentrating, diluting
and/or picking up salts. Salinity related processes are specifically
developed for irrigated soils and small storage reservoirs.
The model concept is one of inflow-outflow = change in storage. Inflows
simulated include stream inflows, tributary inflow, precipitation, s^face
inflow, and imports. Outputs are surface and subsurface flows, evaporation
and evapotranspiration, and exports. Internal system simulations consider
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canal and groundwater diversions, surface and subsurface irrigation return
flow, and soil moisture groundwater systems. Reservoir operations are
provided with a specific submodel.
Use of this model falls under two categories. First, given the
necessary input data, the simulation of various hydro-salinity flows is
compared with actual measured values in order to calibrate the model. A
pattern-search optimization technique is utilized to systematically optimize
the calibration. The second model use which is based on proper calibration
is to test management alternatives (changes in cropping patterns, alternative
reservoir operations, and improved irrigation practices) on the water quality
of the system outflows.
INPUT DATA;
Control parameters allow user to select one of four input options:
(1) read run data for reduction and handling internally; (2) read data
directly to simulation system; (3) read data from computer memory; and
(4) read parameter bounds for calibration process. Other control parameters
set boundaries for calibration process. Input of a hydrologic nature which
must be supplied includes monthly percentage of daylight hours, crop and
phreatophyte consumptive use coefficients, canal diversion, land use acreages,
precipitation, soil moisture characteristics, temperature, river inflow,
tributary inflow, subsurface inflow, surface outflow, and reservoir operations.
Salinity data are also necessary for the above noted water flows.
SPATIAL AND TIME SCALES;
Simulation is of single or multiple river system subbasins. Analysis is
partially dynamic in nature although steady state is assumed within a time
frame of 1 month. Time resolutions are monthly with multiple year analyses
inherent. Model is two-dimensional in nature.
COMPUTER CODE STRUCTURE;
Internal program linkage is accomplished by subroutines with call and
common data transfer. The program in its initial versions was written and
utilized on a digital-electric analog hybrid computer at Utah State University.
Hydrology is currently available in a digital format. The digital segments
should be adaptable to other computer systems without much difficulty.
Language is Fortran IV.
Execution time and memory requirements are not given. General
examination indicates core requirements on the order of 50-70 k bytes and
execution times on the order of 3-5 minutes for major calibration-simulation
runs.
BASIC MATHEMATICAL APPROACH;
The basic mathematical relationship in the model is the conservation of
mass within a river basin subarea:
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(PPT + QSI + QGI) - (GET -h QSO + QG0) = AS
where, PPT = precipitation
QSI = total surface inflow
QGI = total groundwater inflow
GET = evapotranspiration
QSO = total surface outflow
QGO = total groundwater outflow
AS = change in system storage in snow, soil moisture, surface
reservoirs, and groundwater.
The principal model variable is QSO. As a generally known quantity the
model simulations of QSO are compared with measuLd values. Calibration'
involves adjusting various time lag and simulation coefficients until the
has bein SrJT it ***** °f QS° iS minimal- The «^<<^ation approach
has been termed a pattern-search technique which is essentially a process of
iteratively varying the parameters to achieve minimum variance!
GENERAL:
This model is one of the most recent in a long-term program of
hydrologic modeling at Utah State University. Many of the earlier models or
versions thereof have been incorporated or omitted from this model based on
applications in a number of field situations. A listing is included in the
reference by Huber, et al. (1976).
MODEL; A-9
REFERENCES;
Hillel, D. 1977. Computer Simulation of Soil-Water Dynamics. International
Development Research Centre, Ottawa, Canada, pp. 155-198.
MODEL SCOPE:
Simulation is of the hydrology of a sloping agricultural field.
Processes modeled include precipitation, infiltration, runoff, evaporation,
redistribution, deep percolation, capillary rise from the water table, and
groundwater drainage. Plant uptake and transpiration are not included.
Output depicts the time and spatial distribution of water in the system.
INPUT REQUIREMENTS;
The geometry of the system is defined by stating the number of columns,
number of layers per column, width of each column, and vertical dimensioning.
Initial conditions such as saturated hydraulic conductivity and initial
moisture distributions are detailed. The runoff hydraulic parameters are
input, including roughness factors for the Manning's equation and surface
slope. Boundary conditions are defined to depict the potential evaporation
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rate, water table elevation at its outflow to the drain and rainfall inputs.
As in other unsaturated soil models, the relationships between water content,
hydraulic conductivity, and suction must be known.
TIME AND SPATIAL SCALES;
The scale is a two-dimensional evaluation of a field sized system
extending vertically to an impervious layer. The principal event is the
period immediately preceding and shortly after a rainfall (until equilibrium
conditions occur).
COMPUTER CODE STRUCTURE:
CSMP
BASIC MATHEMATICAL APPROACH;
Water added to the soil surface is divided into infiltration, surface
storage and runoff. Infiltration is calculated by dividing the vertical
unsaturated soil profile into layers and then routing flows through it using
an iterative computation of contents, gradients, rates, and new contents.
Runoff is handled by dividing the field into a number of vertical columns and
then routing the surface water down slope with a successive kinematic wave
equation. At the bottom of the slope the time distributed runoff is yielded
by the last computations. In each column, the infiltration crossing the last
compartment in the unsaturated zone is the inflow to the groundwater basin.
A water table height increase is distributed through each column and then the
saturated soil properties along with hydraulic gradients are used to
calculate a lateral groundwater flow for the bottom of the slope.
GENERAL:
No field verification is presented. Code is listed in cited reference.
MODEL: A-10
REFERENCES:
Makkink, G. F. and H. D. J. Van Heemst. 1975. Simulation of the water
balance of arable land and pastures. Centre for Agricultural Publishing and
Documentation, Wageningen, The Netherlands. 79 pp.
MODEL SCOPE:
Simulation is of water movement and storage in a cropped field and the
saturated-unsaturated soil beneath. The processes modeled include snow
precipitation and melt, canopy interception and evaporation, soil surface
evaporation and infiltration, unsaturated and saturated flow, evapotranspira-
tion, micellar flow and plant withdrawal and hydration-dehydration. Output
involves the time and spatial distribution of water in the system.
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INPUT REQUIREMENTS;
The data listed as required input include the date of the last balance
period and the current date, global and extraterrestrial solar radiation, a
"place factor", air and dew point temperatures, relative humidity, wind
speed, crop height, and depth of root zone, all for each day of the analysis.
However, a number of data not listed as input are required nevertheless. For
instance, the conductivity and moisture versus suction relationship, initial
conditions, infiltration, and soil and crop characteristics are internal to
the program as DATA statements.
SPATIAL AND TIME SCALES;
The analysis is of a large crop area, but the evaluation is made at one
vertical "average" location. Time increments of 0.2 days are used in the
calculation with output based on daily intervals. Model is one-dimensional
and steady-state.
COMPUTER CODE STRUCTURE:
Code is written in Fortran IV for the CDC series computers. A main
program with common linked subroutines and functions comprise the basic code
structure. Although neither core nor time requirements are discussed, they
are obviously small enough for most computers.
BASIC MATHEMATICAL APPROACH:
The plant-soil system is divided into "compartments" representing the
various forms of moisture storage in the system. The computational system
involves an iterative evaluation of compartment storage, the rate of moisture
movement between compartments, and the calculation of contents (CONTENT at
time t = CONTENT at time t-1 + RATE times Delta Time). The storage compart-
ments include snow (existing as solid precipitation or heavy frost and
flowing in the system by evaporation and melting), adhering water which stems
from canopy interception (this moisture evaporates, drops into pools, or
drops onto and infiltrates unsaturated soil). Pools filled with precipitation
at rates greater than the saturated hydraulic conductivity of the soil
(released from pools by evaporation and infiltration) , the unsaturated soil
profile (transmitting water to the saturated zone below or to the atmosphere
above through transpiration and evaporation), the saturated zone (venting to
transpiration, groundwater flow, water table storage changes, and flow to the
unsaturated region through capillary use) and micellar water taken up by and
stored in the clay structure. Subsets of the mass balance in the unsaturated
zone are developed for the upper soil profile contributing to evaporation and
the region of plant uptake. There are also storage balances for the »°"ture
stored above and below 16 atm and field capacity, respectively. The basic
rate processes are evapotranspiration and soil moisture redistribution.
Elation and transpiration are computed with modified forms of the Penman
equation. UnsaturateS flow occurs when the soil moisture level exceeds field
capacity.
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GENERAL:
Although this model has many aspects of a general nature, its existing
format is entirely site specific to the case studies it was validated against.
To apply this model to other situations, much more data.are needed. The code
is presented in the reference cited above.
MODEL; A-ll
REFERENCES;
Margheim, G.A., 1967. Predicting the quality of irrigation return flows.
Unpublished M.S. Thesis, Civil Engineering Department, Colorado State
University, Fort Collins, Colorado, December. 57 pp.
MODEL SCOPE:
Simulation is of the salinity constituents in flows entering the
groundwater basin under irrigated croplands. Parameters include the concen-
trations of Ca , Mg , Na , and SO^ in the flows entering the water table
from deep percolation. The computer code does not contain an input dump or
a printing of intermediate results, but does contain partial error flags for
iteration limitations.
INPUT PARAMETERS;
No control parameters are utilized. Input data_include the irrigation
water concentrations of Ca , Mg , Na , Cl , and 804, and the concentrations
of exchangeableNa , Ca , and Mg in the soil. Input also requires concen-
trations of soluble Mg , Na , Ca , and SO^ in the soil, solubility product
of gypsum, and cation exchange constants for homovalent and monovalent-
divalent exchange equations.
SPATIAL AND TIME SCALES:
Analyses are at a single or representative site in an irrigated field
(accomplished by assuming uniform irrigation water applications). Steady-
state moisture flux is assumed so time scale is set by user. However, an
irrigation interval is generally selected.
COMPUTER CODE STRUCTURE;
Internal program linkage is facilitated by subroutines with data and
results transferred through call statements. Program can be easily adapted
to other computer systems since tape and disc functions are not performed.
Core requirements are less than 40 k bytes while execution times are not
known (probably 10-15 seconds per run). Model is programmed for CDC 6400
using Fortran IV language.
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BASIC MATHEMATICAL APPROACH;
Under assumptions that gypsum is the only slightly soluble salt present
in the soil system, the model first computes the ion concentrations in water
percolating below the root zone and then the concentration of water inter-
cepted by a drainage system. Effects of evapotranspiration are not directly
considered in the soil chemistry phase. Computations in the soil chemistry
phase involve the following iterative procedures controlled by iteration to
iteration changes in calcium concentrations. A volume of irrigation water
equal to the stored soil moisture volume in equilibrated with the soil-CaSO4
system using the Gapon equation for exchange processes, and the
Debye-Huckel gypsum solubility relations for solution equilibrium. The
iterative procedure is repeated once the successive approximation procedure
has converged for other volumes of irrigation water until the desired leaching
is accomplished. Then, using a drainage equation developed by R.E. Glover,
the volume of return flow is computed. The model assumes groundwater quality
is poorer than deep percolation and mixing does not occur. An interface is
thereby formulated from which the relative displacement is determined to
arrive at the outflow mixture.
GENERAL;
Code is available in reference cited above. However, code is not
documented nor verified against field data.
MODEL; A-12
REFERENCES;
Childs, S. W., and R. S. Hanks. 1975. Model of soil salinity effects on
crop growth. Soil Sci. Soc. Amer. Proc., 39:617-622.
Melamed, J. D., R. J. Hanks, and L. S. Willardson. 1977. Model of salt flow
in soil with a source-sink term. Soil Sci. Soc. Amer. Proc., 41:29-33.
Nimah, M. D., and R. J. Hanks. 1973. Model for estimating soil water,
plant, and atmospheric interrelations: I. Description and Sensitivity, and
II. Field test of model. Soil Sci. Soc. Amer. Proc., 37:522-527 and
37:528-532.
MODEL SCOPE:
The model described here is actually the result of several individual
models. Nimah and Hanks (1973) added a plant root extraction system to an
earlier one-dimensional moisture flow model. Childs and Hanks (1975)
modified the Nimah and Hanks (1973) model for crop growth, salt flow, and
irrigation uniformity. Then, the "Dutt Model" described herein by Dutt et
al (1972) and a simple source-sink model to account for chemical reactions
were added. The simpler salt system was added to the water model by Melamed
et al. (1977). Because the more complex Dutt model is described elsewhere,
the version will not be detailed here.
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The model simulates the one-dimensional flow of water in the transient,
unsaturated soil profile including the spatial distribution of root uptake in
response to surface evapotranspiration demand. Salts added to the system in
the irrigation water or that exist in the solid form in the soil structure
are transported through the system to the groundwater below. Dissolution and
precipitation of salts in the root zone are simulated as a source-sink
process rather than by chemical equilibrium. Ionic exchange and biological
transformations are neglected. Output describes the spatial and time
distributed water-salt system including evapotranspiration.
INPUT REQUIREMENTS;
The primary data necessary for the soil moisture flux are the hydraulic
conductivity-water content and pressure head-water content relations, air dry
and saturated moisture contents, root-water potential below which wilting
occurs, depth distributed root distribution, initial moisture distribution,
potential evapotranspiration at the surface and water inputs (irrigation or
rainfall), osmotic potential of the irrigation water and of soil (depth
distributed), the presence of a lower boundary water table, and crop cover.
The salinity simulation requires the electrical conductivity of the irriga-
tion water and solid soil salts, diffusion and dispersion coefficients, and
initial solute distribution (electrical conductivity).
TIME AND SPATIAL SCALES:
The model is a one-dimensional analysis of the root-zone. Time scales
vary in length from an irrigation interval to several years.
COMPUTER CODE STRUCTURE;
Programs are written in Fortran IV with common and call data transfer to
subroutines. Execution time depends on the length of the real time simulation
Core requirements appear moderate (50-70 k bytes).
BASICJ4ATHEMATICAL APPROACH;
The model involves a moisture flow simulation, a salt transport system,
and a simulation of the dissolution-precipitation processes (source-sink
term). The moisture flow equation is the moisture content form of the
unsteady flow equation developed by writing Darcy's Law and mass conservation
equations in one dimension. This primary flow equation is modified by the
inclusion of a root extraction term and solved with a implicit finite
difference technique. The essential features of the root extraction term are
described by Nimah and Hanks (1973). (See also the model of Feddes and
Zaraday, 1977.) Flow to the roots is based on the difference between the
root potential plus root resistance and the combined soil-water and osmotic
potentials. This value is then multiplied by the hydraulic conductivity and
a depth distributed rooting pattern. The root potential is determined by
maximizing transpiration as was the case in the model by Feddes and Zaraday
(1977) (which was based on this model).
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f *nsp°r* relationshiPs are based on the equation of Bresler
mcludes diffusion and hydrodynamic dispersion.
matrix
The source-sink term accounting for precipitation and dissolution of
salts is an on-off linear equation based on the difference the salinity
concentration of the soil solution and a value at which precipitation will
occur. The equilibrium concentration must be evaluated in the field
GENERAL;
The model which has been calibrated against field data shows good
predictive agreement with observed field data once calibrated. The code
should be requested from its authors.
MODEL; A-13
REFERENCES;
Oster, J. D., and B. L. McNeal. 1971. Computation of soil solution
composition variation with water content for-desaturated soils. Soil Sci.
Soc. Amer. Proc., 35:436-442.
Oster, J. D., and J. D. Rhoades. 1975. Calculated drainage water compositions
and salt burdens resulting from irrigation with river waters in the western
United States. J. Environ. Qual., 4:73-79.
MODEL SCOPE:
This simulation involves the salinity composition of root-zone drainage
flows. Parameters include dissociated ionic species, gypsum and lime dissolu-
tion/precipitation, SAR, C0_ partial pressure at bottom of root-zone (Pco ),
pH, EC, TDS, ionic strength, activities, and activity coefficients. Output
also includes CO^ and HCO^ ion pairing, and 1st and 2nd dissociation constants
for carbonic acid. The computer code features an input dump, error flags for
iterative sections, but no intermediate results.
INPUT REQUIREMENTS^
Control parameters are provided so a user may include magnesite (not
fully developed) and forced saturation of soil lime. Up to 100 individual
analyses can be run without dimension and common modification. Data include
an identification of the test, Pco2 at lower root-zone boundary, pH, Na ,
Mg++, Ca++, K+, HC03, Cl~, and SO^ in applied irrigation water, and the
estimated leaching fraction.
SPATIAL AND TIME SCALES;
Simulation is of a single or representative site in an irrigated field.
Analysis is steady-state and encompasses the drainage period of the irriga-
tion interval. Model is one dimensional.
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COMPUTER CODE STRUCTURE;
Internal program linkage is accomplished by subroutine and functions
with common data transfer. The program can be easily adapted, all or in
part, to other computer systems or programs. Approximate core and execution
time requirements are 70 k bytes and 10-15 central processor seconds,
respectively. Programmed originally for an IBM 360 computer using a Fortran
IV language.
BASIC MATHEMATICAL APPROACH:
The basic modeling approach along with important assumptions, supportive
literature, and data requirements is well described in the previously cited
references. The interested user is therefore referred directly to these
sources for more detailed information. In general terms, the computations
begin by pre-concentrating the ions in the irrigation water by dividing each
by the leaching fraction. Then an iterative procedure (governed by con-
vergence tolerances on ionic charge balance, changes in ionic concentrations
for Ca and CC>3, and dissociation constants) is used to compute the salinity
constituents in the drainage. The extended form of the Debye-Huckel equation
is utilized to calculate activity coefficients in the equilibria analyses.
Gypsum and lime precipitation/dissolution is used to achieve convergence.
Leachate pH is governed by the soil lime equilibria and cation exchange is
not included.
GENERAL:
The computer code is available from the authors cited above.
Documentation within the code is excellent and substantial verification
trials are reported to demonstrate the utility and limitations of the model.
MODEL; A-14
REFERENCES;
Rai, D., and W.T. Franklin, 1973. Program for computing equilibrium solution
composition in CaCO3 and CaSO4 systems from irrigation water compositions.
Water Management Technical Report No. 29. Colorado State University,
Fort Collins, Colorado. October. 42 pp.
MODEL SCOPE;
Simulation is of the salinity composition of root-zone drainage.
Parameters include dissociated ionic species, gypsum and lime dissolution -
precipitation, SAR, pH, EC, TDS, ionic strength,_activity, and activity
coefficients. Output includes 0)3, HCO^, and 804 ion pairing with Ca+ ,
Mg , K+, and Na+. The computer code features an input dump, error flags for
iterative sections, but no intermediate results.
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INPUT REQUIREMENTS;
The program does not include any control options, input data are sample
identification, total concentrations of Ca++, Mg++, Na+, K+, HCO~ S07 and
Cl in the irrigation water, CO partial pressure, and the leaching fraction.
Units are in equivalents per liter except for CO (atm) and leaching fraction
(expressed as a fraction). Program operates on one sample during each run.
SPATIAL AND TIME SCALES;
The simulation involves a single or average condition in an irrigated
field on a one-dimensional basis and steady-state system is assumed.
COMPUTER CODE STRUCTURE;
The 'code consists of only a main program in the latest version, although
the program in reference cited above contained a subroutine for solution of
cubic roots. The program can be easily modified for use on computers other
than the CDC 6400 at Colorado State University. Core requirements are
approximately 30 k bytes. Execution time per run is 15-70 central processor
seconds. Code language is Fortran IV.
BASIC MATHEMATICAL APPROACH;
The basic modeling approach, along with pertinent assumptions and
simplifications, is well documented in Rai and Franklin (1973) . Program is
now revised to include nesquehonite. Computations begin by preconcentrating
ionic concentrations in the irrigation water by dividing each by the leaching
fraction. Ionic strength and activity coefficients are initially calculated
by assuming no pairing. Then, a three-dimensional successive approximation
loop is entered in which calcium equilibrium is used to arrive at leaching
water chemistry. The primary loop flow involves (1) computation of free
anions; (2) calculation of free cations; (3) calculation of charged ion
pairs; (4) calculation of ionic strength; and (5) calculation of activity
coefficients. Then, lime or gypsum equilibrium is checked. If solution is
in equilibrium, the program goes to next calcium system or concludes. If
nonequilibrium exists, approximations resume at beginning of loop. The
Debye-Huckel equation is used for activity coefficients and literature
references for equilibria constants are used for the calcium system. Cation
exchange is not included.
GENERAL;
The code is available from the authors cited above. Code is moderately
documented itself whereas the reference is extensive. Model has been verified
against literature and field data. This program is also available for an
HP 9825 programmable calculator from the writer.
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MODEL; A-15
REFERENCES:
Saxton, K.E., G.E. Schuman, and R.E. Burwell. 1977. Modeling nitrate
movement and dissipation in fertilized soils. Soil Sci. Soc. Amer. Proc.,
41:265-271.
MODEL SCOPE;
Simulation is of the nitrate-nitrogen-occurrence, movement, and
dissipation in 1-2 meters of the unsaturated zone under fertilized agricul-
tural lands. Processes included in the model are solute transport (infil-
tration, redistribution, and percolation), crop uptake, disposition of added
fertilizer, rainfall additions, mineralization and nitrification. Denitri-
fication, fixation, and runoff losses were assumed negligible. Other nitrogen
processes were considered negligible. The moisture flow requirements must be
input to the model and must include soil evaporation, plant adsorption from
each 15-cm layer, infiltration, soil-moisture redistribution between layers,
and soil-moisture in each layer.
INPUT REQUIREMENTS;
Water flow data for this model comes from the model described by Saxton,
et al. (1974). Output from that model or similar daily values for uptake by
soil segment, infiltration, soil moisture storage volumes, and deep percola-
tion must be incorporated. Then, the time distributed data describing
nitrogen additions to the system, total and depth distributed plant uptake
of nitrogen, and the initial nitrogen profile in the soil are read.
TIME AND SPATIAL SCALES:
Daily calculations are made of a one year event in a one-dimensional,
transient, unsaturated zone.
COMPUTER CODE STRUCTURE:
Using a main program to read tape output from the soil-moisture model,
the general nutrient calculations are carried out in subroutines using common
and call statement data transfer. Execution times for the Fortran Code are
on the order of 10-20 seconds. Core requirements should not exceed 50-60 k
bytes.
BASIC MATHEMATICAL APPROACH:
Nitrogen transport is based on the assumption that within each 15-cm
soil segment, the concentrations of nitrate are uniformly distributed. Thus,
nitrates leaving or entering a layer are calculated as the product of the
moisture movement and the existing nitrate concentrations. New concentra-
tions are computed for each hour and soil layer to aggregate the transient
system on a daily basis. The withdrawal of nitrates from the soil layers is
assumed proportional to water absorption. An annual uptake figure is
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distributed on a daily basis assuming a proportionality with dry matter
production and then distributed in the soil profile according to root
distributions. Nitrogen added to the upper soil layers in precipitation or
fertilizer moves as indicated above, but ammonium fertilizers are assumed to
be either adsorbed or converted to nitrate. A first-order temperature
dependent decay function was used to describe the ammonium conversion to
nitrate. A 10% loss due to volatilization was assumed on initial application.
Mineralization is fixed at 52 kg/ha per year within the upper soil layer and
is distributed with time proportionally to the temperature in the soil
between May and September.
The calculation proceeded in the following sequence: fertilizer
addition, mineralization addition, uptake, infiltration addition and
transport, and transport by redistribution and percolation. A revised
nitrate profile is calculated after each process and printed at the day's
end.
GENERAL;
The model has been verified (with good agreement) with field data from
two watersheds. This is a readily usable model only if the moisture flow is
supplied by the earlier model. Thus, modifications should be made to operate
this one independently for widespread application. Computer codes must be
requested from the authors of the cited references.
MODEL: A-16
REFERENCES ;
Tanji, K. K. , L. D. Doneen, G. V. Ferry, and R. S. Ayars. 1972. Computer
simulation analysis on reclamation of salt-affected soils in San Joaquin
Valley, California. Soil Sci. Soc. Amer. Proc. 36:127-133.
MODEL SCOPE;
Simulation is of the interaction of soluble salts (Ca , Mg , Na , K ,
SO*, Cl~, HCOo, and NO^) and Boron in a soil profile. The model was developed
to describe leaching processes so the output includes drainage water quality,
soil profile chemistry (changes in soluble, and adsorbed salts). It also
predicts the depth of leaching water required to achieve specific values of
soluble salts, boron, and exchangeable sodium concentrations in the soil
profile. Output includes ionic strengths, activity coefficients, ion pairs,
stoichiometric .solute concentrations (dissociated and undissociated) , total
soluble cations, SAR, and ESP.
INPUT REQUIREMENTS:
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TIME AND SPATIAL SCALES;
The principal time frame is the period covered by an application of
irrigation water and the subsequent leaching. The model is one-dimensional
and encompasses the unsaturated zone of depth specified by the user.
COMPUTER CODE STRUCTURE:
The program is a Fortran code without subroutines or user supplied
functions. Core and execution times are not given but appear small (40-60 k
bytes, and about 1-2 minutes).
BASIC MATHEMATICAL APPROACH;
The soil profile is divided into layers with moisture movement between
layers when field capacity is exceeded. A chromatographic approach to salt
transport between layers is assumed. Solute from one layer moves into a
lower layer, mixes with the resident solution (at equilibrium) and
equilibrates.
The calculation of the equilibrium chemistry for the salts utilize the
Guggenheim-Davis equation for activity coefficients and experimental
solubility and dissociation coefficients. Boron adsorption-desorption is
simulated with a Langmeier isotherm.
GENERAL:
Model was verified with field data and has served as a starting point
for several steady-state models developed to predict the chemistry of
leaching water. Code must be requested from authors cited above.
MODEL; A-17
REFERENCES;
Dutt, S.R., M.S. Shaffer, and W.S. Moore, 1972. Computer simulation model of
dynamic bio-physicochemical processes in soils. Technical Bulletin 196,
Agricultural Experiment Station, University of Arizona, Tucson, Arizona.
October. 101 pp.
Shaffer, M.S., R.w. Ribbens, and C.W. Huntley. 1977. Prediction of mineral
quality of irrigation return flow, Volume V. Detailed return flow salinity
and nutrient simulation model. EPA-600/2-77-179e. Robert S. Kerr Environ-
mental Research Laboratory, Office of Research and Development, U.S. Environ-
mental Protection Agency, Ada, Oklahoma. August. 228 pp.
MODEL SCOPE;
This model simulates chemical and physical processes associated with
agricultural lands drained by subsurface drainage systems. Basic input data
involves the field application of irrigation or precipitation with associated
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salts and nitrogen nutrients. Model output is the prediction of drainage
quality with evaluations at intermediate points in the plant-soil-aquifer
system. Program includes intermediate output and various error flags.
INPUT DATA;
This is a very large computer model well documented by Shaffer, et al.
(1977). As a consequence, the input requirements are elaborate. The
following input data summary is given by Shaffer, et al., (1977):
(1) Drainage parameters: drain spacing and depth, depth to impermeable
barrier, envelope size, saturated hydraulic conductivity, porosity,
and specific yield or storage coefficients;
(2) Unsaturated flow parameters: the hydraulic conductivity versus
moisture content function, and pressure head versus moisture
content relations;
(3) Soil chemistry data: meq/1 of Ca , Mg , Na , 003, HCO^, Cl~,
804, NH4, NO-j, and urea in soil extract, pH, cation exchange
capacity, gypsum content, presence of lime, bulk density, organic
nitrogen, carbon-nitrogen ratio, and the nitrifier population-salt
response relationship;
(4) Crop information: rooting depth and distributions,
evapotranspiration rates as read in or calculated using an
irrigation scheduling program, and plant uptake of nitrogen;
(5) Water application data: irrigation schedules and amounts (or
precipitation amounts and timing) and an effective precipitation
relationship;
(6) Fertilizer data: fertilization schedules, amounts of NO3, NH4,
Urea, Ca++, 304, and 003, application depth and organic nitrogen
plowed in with corresponding C-N ratio, and application ratio;
(7) Irrigation water analyses: concentrations of NH4, NO3, Ca++, Na ,
Mg++, HCOg, 003, Cl~, and 304; and
(8) Miscellaneous soils data: soil temperature and coefficients
relating to nitrification and denitrification.
SPATIAL AND TIME SCALE:
The spatial scale is a two-dimensional evaluation of the vertical region
from the soil surface to the drainage system. Time scales are mixed depending
on the calculation being made. Time varies from fractions of a day to an
irrigation interval and even up to an irrigation season. The model is
transient in nature.
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COMPUTER CODE STRUCTURE:
The primary code structure is in a main program-subroutine format
utilizing common, call, and tape file data transfer. The size of this model
requires extensive use of overlay systems if the entire package is used.
Consequently, core requirements are only 140-150 k bytes at any instant.
Time for execution varies from one to two minutes up to several minutes
depending on the scope of the job. Although the basic program language is
Fortran IV, this model is not easily adapted in its entirety. Except for
some planning use by the Bureau of Reclamation, this is primarily a research
tool and not applicable by most planning groups.
BASIC MATHEMATICAL APPROACH;
This model is composed of individual models' which extend across several
disciplines. Consequently, there is more likely to be more interest in the
various submodels rather than the total package. The program is linked
together with detailed overlay systems and numerous by-pass options provide
substantial flexibility. The interested user can find excellent explanatory
comments in the references noted above. However, to identify the model's
basic characteristics, this section will be divided by major submodels.
Irrigation scheduling. Unlike the irrigation scheduling program
available to schedule and update irrigations, this model predicts irrigation
schedules and amounts using historical data exclusively. Based on geograph-
ical data, solar radiation, and temperature, the program first computes
potential evapotranspiration for alfalfa using the Jensen-Haise equation.
Adjustments are made for seven other crops using standard growth stage coeffi-
cients. Then, a simple mass balance of the root zone is developed. Soil
moisture is allowed to be depleted to an "allowable moisture depletiqn"
before an irrigation is scheduled. Coefficients for various irrigation
efficiency and losses produce deep percolation and surface runoff estimates.
The output from the model is recorded on cards punched for subsequent use in
the unsaturated moisture-chemistry models (infiltration) and the drainage
calculation (deep percolation).
Soil moisture movement. The unsaturated flow submodel is a finite
difference simulation of the transient moisture flow equation in one dimen-
sion. Hydraulic conductivity is taken as a function of moisture content as
is pressure head. Simulation includes infiltration, redistribution,
drainage, and plant uptake. Plant uptake is determined from information of
total uptake which is distributed throughout the soil profile in proportion
to an average root distribution.
Unsaturated soil chemistry. The complex soil-water system is simulated
from a chemical-biological standpoint by a group of subroutines designated as
the unsaturated chemistry program. The program has two primary components:
(1) simulation of the soil nitrogen system; and (2) the soil salinity series.
This model is the basic programming package reported by Dutt, et al. (1972).
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f MH^M nitr°gen Phfse °f the Pr°9ram considers urea hydrolysis, nitrification
of NH4-N, net mineralization-immobilization of both organic-N and NH+-N
immobilization of NO3-N, plant uptake of NH^-N and NO^-N, and denitrifi-
cation. in addition, the NH^-N reaction in the cation exchange system is
considered. With the exception of the cation exchange process, the nitrogen
simulation assumes a first-order kinetic approach using regression functions
for the transformation rates. Denitrification is evaluated using zero-order
kinetics. Nitrogen uptake may be evaluated by either of two methods. First,
the user may specify total N uptake and root distribution which yield the
plant N uptake in each soil segment per unit of time. In this case, the user
specifies the fractions of N03, and NH| uptake. The second assumes uptake is
proportional to water uptake. Output from these segments of the model
predicts mass distribution of the various nitrogen components.
The salinity phase of the models evaluates ion exchange, solution-
precipitation of gypsum and lime, formation of undissociated ion pairs, and
transport of the soluble species (Ca++, Mg++, Cl~, Na+, etc.). Because these
various reactions occur quickly, solution equilibria are used to describe the
segments of the salinity system. The ion exchange and solution-dissolution
of the slightly soluble salts are computed using standard approaches such as
the Gibbs phase rule, Debye-Huckel theory, and Gapon equations. An iterative
successive approximation procedure is used to determine the chemistry within
the soil profile and in the deep percolation. The procedure involves
(1) calculation of lime solubility; (2) calculation of gypsum solubility;
(3) undissociated CaSC>4 and MgSO4 ion pair reaction,- (4) Na+-Ca+ , Mg++-Ca ,
and Na+-NH4 ion exchange; and (5) evaluation of CaCO3 dissociation.
The movement and distribution of water is either read in or produced by
the unsaturated flow program. For the soluble ions, a mixing cell concept is
used to simulate solute dispersion and movement. This assumes complete
mixing occurs in each soil-time increment and that molecular diffusion is
negligible.
Drainage Model (Dynamic Equilibrium). Flow moving below the crop root-
zone is routed to surface receiving waters via this drainage program. The
model was designed to predict the response of a subsurface drainage system
consisting of parallel, equally spaced tile drains, to deep percolation
inputs. Principal outputs are the mid-space water table elevation and drain
discharge. Deep percolation may be developed for the program through card
read input or directly from the unsaturated flow model. The mathematical
approach is the general Bureau of Reclamation drainage equations.
The water table shape is approximated as a fourth degree parabola which
uses specific yield to compute instantaneous rises and declines in water
table portions. Hooghoudfs equivalent depth is used to correct for near-
drain convergence effects not treated by the Dupuit-Forchheimer assumptions.
The Moody solutions are used to approximate these systems.
Drainage Model (steady-state). The general modeling system also
Drainage MOO.e v * model based on potential theory to
3" ^average deep percolation rate is computed from the
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values determined by the unsaturated flow program. A system of nodes is
defined at which the infinite series mathematical solutions are computed.
Under this approach, the flow velocities and travel times are estimated
thereby yielding an estimate of the time required for a water volume to reach
the drainage system. These data are determined for use in the saturated
chemistry and drainage effluent chemistry parts of the program.
Saturated Chemistry Program. This program predicts the two-dimensional
distribution of saturated flow chemistry below an irrigated field. The
stream tube flows predicted by the steady-state drainage model along with
chemical soil analysis serve as model input. Each stream tube is analyzed
separately and is divided into segments of equal volumes. Flow is accumulated
until it reaches the pore volume of a segment and then is moved by piston
displacement. After each displacement, the solution is equilibrated with the
solid and exchangeable phases. Lateral dispersion and diffusion are neglected
since lateral and longitudinal mixing among stream tubes is not considered.
Chemical transformations are identical to those in the unsaturated chemistry
model except denitrification is simulated in the transition zone between
unsaturated and saturated regions. The denitrification rate is assumed
temperature dependent but not affected by substrate concentration. Above a
"saturation level" the rates are assumed to be zero-order and below this
level to be first-order. The aquifer profile is assumed to be homogeneous
until the heterogeneous nature is computed through consideration of
individual stream tubes.
Drain Effluent Prediction. This program takes the flow rates from the
steady-state drainage analysis and its corresponding chemistry and aggregates
each stream tube into a monthly drainage discharge. The salt load from the
drainage system is thus determined by mixing and routing the flows in each
stream tube.
GENERAL:
This modeling system is well documented, verified in field conditions,
and available for broad use. However, the system itself is far larger than
required by most research groups and far too complex for use by planning
groups. Many possible uses can be made using individual segments listed
above. Computer and data requirements are substantial. The modeling system
is probably the largest, most exhaustive available as an integrated package.
Code listings are available from the Bureau of Reclamation.
MODEL; A~18
REFERENCES^
Walker, W.R. 1970. Hydro-salinity model of the Grand Valley. M.S. Thesis,
Civil Engineering Department, CET-71WEW8.' Colorado State University, Fort
Collins, Colorado. August. 94 pp.
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MODEL SCOPE;
vaT^f10^13 limited tO ** h*drol°9ic "* salinity systems in an
valley. Diversions from reservoirs, rivers, tributary inflows or
groundwater are routed through the agricultural system. SurfacI as well'as
subsurface flows and flow qualities are predicted along with consumptive use.
INPUT REQUIREMENTS;
_ Input data consist of mean monthly values for temperature, percent
daylight hours, Blaney-Criddle climatic coefficients, areas of each land use,
Blaney-Criddle crop growth stage coefficients, river inflows, tributary
inflows, river outflows, lateral diversions, surface drainage outflows,
drainage base flows, soil moisture storage capacities, root- zone diversions,
exports, imports, water table fluctuations, precipitation, and equilibrium
salinity concentrations of each of these flows. A series of groundwater data
are also required. These include the number of strata at the groundwater
basin's return flow point, and the number of normal locations where the
gradient in each of these strata is defined. At the outflow point or close
by, an initial value of hydraulic conductivity and cross-sectional area is
required.
SPATIAL AND TIME SCALES;
The scale of the model is a macroscopic simulation of an irrigated area
and the hydrology immediately surrounding it. A time resolution of one month
is taken. Analysis is steady-state.
COMPUTER CODE STRUCTURE;
The computer code consists of a main program and subroutine served by
both common and call statement data transfer. Written in Fortran IV, it
should be adaptable in most computers. Core requirements are approximately
43 k bytes and time needs are about 25-30 central processor seconds per run.
BASIC MATHEMATICAL APPROACH;
A mass balance is made of the water flow system and the salt system is
attached by multiplying by the concentrations associated with each segment of
the system. Flows entering the main conveyance are segregated into seepage
by a conveyance efficiency, lateral diversions as input, and operational
wastes by difference. Lateral diversions are likewise delineated into
seepage, root-zone diversions, and tailwater. Finally, root -zone diversions
are allocated to root- zone storage, deep percolation or consumptive use.
Summing the surface and subsurface flows from each breakdown yields an
estimate of the respective return flows. At this point an estimate of the
groundwater return flows is made on the basis of a groundwater model. A
comparison of the mass balance and groundwater model estimates of subsurface
return flows is made in a manner which calculates an adjusted aquifer
hydraulic conductivity by month. The model is then systematically adjusted
until the values of hydraulic conductivity are homogeneous. When the water
system is satisfactorily modeled, the salinity system is added.
223
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GENERAL:
The model is a second or third generation modeling concept initiated
at Utah State University. Consequently, these early models will not be
described; however, this model is an approach that can be applied with
generally available data. The evapotranspiration and root zone analysis
needs updating. Application has been made with good accuracy in the
Grand Valley of western Colorado.
224
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APPENDIX B
REFERENCES REQUIRED FOR IRRIGATION RETURN FLOW STUDIES
This appendix is included to provide a list of references which should
be obtained for all Irrigation Return Flow studies. This list is not
intended to be complete, but will perhaps provide a minimum basic reference
library for persons working in this area.
BASIC REFERENCES
Hagan, R.M., H.R. Haise and T.w. Edminister (eds.), Irrigation of Agricultural
Lands. American Society of Agronomy, Monograph No. 11.
Kovda, V.A., D. Van Den Berg, R.M. Hagan, editors. 1973. Irrigation
Drainage and Salinity. FAO/UNESCO, an International Source Book.
Hutchinson & Co., Ltd., London.
Skogerboe, G.V. and J.P. Law, Jr. 1971. Research Needs for Irrigation
Return Flow Quality Control. 13030-11/71. Water Pollution Control
Research Series. United States Environmental Protection Agency, Office
of Research and Monitoring, Washington, D.C. November.
Utah State University Foundation, 1969. Characteristics and Pollution
Problems of Irrigation Return Flow. Robert S. Kerr Research Center,
Ada, Oklahoma. May. 237 p.
van Schilfgaarde, J. (ed.), 1974. Drainage for Agriculture. Agronomy Monograph
No. 17. American Society of Agronomy. Madison, Wisconsin, pp. 433-
463.
EVAPOTRANSPIRATION
Doorenbos, J. 1976. Agro-meteorological Field Stations. Irrigation and
Drainage Paper No. 27. Food and Agriculture Organization of the United
Nations. Rome. 94 pp.
Doorenbos, J. and W.O. Pruitt. 1977. Crop Water Requirements (Revised
Edition). FAO Irrigation and Drainage Paper 24, Food and Agricultural
Organization of the United Nations, Rome, Italy.
Jensen, M.E. (ed.), 1973. Consumptive Use of Water and Irrigation Water
Requirements. ASCE, New York, New York.
225
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Jensen, M.E. 1975. Scientific Irrigation Scheduling for Salinity Control
of Irrigation Return Flows. EPA-600/2-75-064. U.S. Environmental
Protection Agency, Ada, Oklahoma, November.
Kincaid, D.C., and D.F. Heerman. 1974. Scheduling Irrigations Using a
Programmable Calculator. ARS-NC-12. Agricultural Research Service,
U.S. Department of Agriculture. February.
FLOW MEASUREMENT
Bos, M.G. 1976. Discharge Measurement Structures, Publication No. 20.
International Institute for Land Reclamation and Improvement/lLRl.
P.O. Box 45, Wageningen, The Netherlands.
Skogerboe, G.V., R.S. Bennett, W.R. Walker, 1973. Selection and Installation
of Cutthroat Flumes for Measuring Irrigation and Drainage Water.
Colorado State University Experiment Station. Technical Bulletin 120.
Fort Collins, Colorado. December. 74 p.
Skogerboe, G.V., M.L. Hyatt, R.K. Anderson, and K.O. Eggleston. 1967.
Design and Calibration of Submerged Open Channel Flow Measurement
Structures. Part 3, Cutthroat Flume. Utah Water Research Laboratory,
Report WG31-4. Utah State University, Logan, Utah. April.
Skogerboe, G.V., M.L. Hyatt, and L.H. Austin. 1967. Design and Calibration
of Submerged Open Channel Flow Measurement Structures. Part 4, Weirs.
Utah Water Research Laboratory, Report WG31-5. Utah State University,
Logan, Utah. May.
Skogerboe, G.V., M.L. Hyatt, and K.O. Eggleston. 1967. Design and
Calibration of Submerged Open Channel Flow Measurement Structures. Part
1, Submerged Flow. Utah Water Research Laboratory, Report WG31-2.
Utah State University, Logan, Utah. February.
Skogerboe, G.V., M.L. Hyatt, J.D. England, and J.R. Johnson. 1967. Design
and Calibration of Submerged Open Channel Flow Measurement Structures.
Part 2, Parshall Flumes. Utah Water Research Laboratory, Report WG31-3.
Utah State University, Logan, Utah. March.
USDI, USER. 1974. Water Measurement Manual. Second Edition. Denver,
Colorado.
GENERAL
Skogerboe, G.V., V.T. Sahni, and W.R. Walker. 1972. Selected Irrigation
Return Flow Quality Abstracts 1968-1969. EPA-R2-72-094. U.S. Environ-
mental Protection Agency, Washington, D;C. October.
226
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Skogerboe, G.V-, w.R. Walker, D.J. Meyer, R.s. Bennett. 1973. Selected
Irrigation Return Flow Quality Abstracts 1970-1971. EPA-R2-73-271
U.S. Environmental Protection Agency, Washington, D.C. June.
Skogerboe, G.V., W.R. Walker, R.S. Bennett, and B.J. Zakely. 1974.
Selected Irrigation Return Flow Quality Abstracts 1972-1973. EPA-660/2-
74-049. Office of Research and Development, U.S. Environmental Protec-
tion Agency, Washington, D.C. June.
Skogerboe, G.V-, W.R. Walker, and S.W. Smith. 1976. Selected Irrigation
Return Flow Quality Abstracts, 1974. EPA-600/2-76-019. Office of
Research and Development, U.S. Environmental Protection Agency,
Washington, D.C. March.
Skogerboe, G.V., S.W. Smith, and W.R. Walker. 1977. Selected Irrigation
Return Flow Quality Abstracts, 1975. EPA-600/2-77-094. U.S.
Environmental Protection Agency, Ada, Oklahoma. May.
Skogerboe, G.V., S.W. Smith, and W.R. Walker. 1978. Selected Irrigation
Return Flow Quality Abstracts, 1976. EPA-600/2-78-042. U.S.
Environmental Protection Agency, Ada, Oklahoma.
Stewart, B.A., D.A. Woolhiser, W.H. Wischmeier, J.H. Caro, M.H. Frere. 1975.
Control of Water Pollution from Cropland. Volume I. A Manual for
Guideline Development. U.S. Environmental Protection Agency and USDA,
ARS. Report No. EPA-600/2-75-0262. November.
Stewart, B.A., D.A. Woolhiser, W.H. Wischmeier, J.H. Caro, M.H. Frere. 1976.
Control of Water Pollution from Cropland. Volume II. An Overview.
U.S. Environmental Protection Agency and USDA, ARS. Report EPA-600/2-
76-0266.
IRRIGATION SYSTEM EVALUATION
Evans, R.G., W.R. Walker, G.V. Skogerboe, and S.W. Smith. 1978. Evaluation
of Irrigation Methods for Salinity Control in Grand Valley. EPA-600/2-
78-161. U.S. Environmental Protection Agency, Ada, Oklahoma.
Karmeli, D., L.J. Salazar, and W.R. Walker. 1978. Assessing the Spatial
Variability of Irrigation Water Applications. EPA-600/2-78-041.
U.S. Environmental Protection Agency, Ada, Oklahoma.
Merriam, J.L., J. Keller, J. Alfaro. 1973. Irrigation System Evaluation
a^d improvement. CUSUSWASH Report UMC 35, Utah State Unxversxty,
Logan, Utah, September.
227
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MODELING
Hornsby, A.G. 1973. Prediction Modeling for Salinity Control in Irrigation
Return Flows. EPA-R2-73-168. U.S. Environmental Protection Agency,
Corvallis, Oregon.
Walker, W.R. 1978. Identification and Initial Evaluation of Irrigation
Return Flow Models. EPA-600/2-78-144. U.S. Environmental Protection
Agency, Ada, Oklahoma.
SEDIMENT, BIOCIDES, AND NITRATES
Carter, D.L. and J.A. Bondurant. 1976. Control of Sediments, Nutrients,
and Adsorbed Biocides in Surface Irrigation Return Flows. EPA-600/2-
76-237. U.S'. Environmental Protection Agency, Ada, Oklahoma. 44 p.
U.S. Environmental Protection Agency. 1973. Methods for Identifying and
Evaluating the Nature and Extent of Nonpoint Sources of Pollutants.
EPA-430/9-73-014. Washington, D.C. October. 261 p.
Wendt, C.W., A.B. Onken, O.C. Wilke, R.D. Lacewell, 1978. Effects of
Irrigation Methods on Groundwater Pollution by Nitrates and other
Solutes. EPA-600/2-76-291. U.S. Environmental Protection Agency.
Ada, Oklahoma.
SOIL-PLANT RELATIONSHIPS
Black, C.A. , Soil-Plant Relations. 1968. 2nd ed., John Wiley and Sons, Inc.,
New York, New York.
Black, C.A., D.D. Evans, J.L. White, L.E. Engsminger, and E.E. Clark, editors.
1965. Methods of Soil Analysis, Parts 1 and 2. American Society of
Agronomy, Agronomy Monograph No. 9. Madison, Wisconsin.
Maas, E.V., and G.J. Hoffman. 1977. Crop Salt Tolerance—Current Assessment.
Journal of the Irrigation and Drainage Division, ASCE, Vol. 103, No.
IR2, pp. 115-134.
Richards, L.A., editor. 1954. Diagnosis and Improvement of Saline and
Alkali Soil. Agricultural Handbook, No. 60. U.S. Department of
Agriculture, Agricultural Research Service Salinity Laboratory.
WATER QUALITY
Ayers, R.S. 1976. Quality of Water for Irrigation. Journal of the
Irrigation and Drainage Division, ASCE, Vol. 103, No. IR2, pp. 135-154.
228
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Christiansen, J.E., E.G. Olsen, and L.S. Willardson. 1976. Irrigation
Water Quality Evaluation. Journal of the Irrigation and Drainage
Division, ASCE, Vol. 103, No. IR2. pp. 155-169.
Hem, J.D. 1970. Study and Interpretation of the Chemical Characteristics
of Natural Water, USGS Water Supply Paper 1473. Washington, D.C.
229
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APPENDIX C
CONVERSION FACTORS
Metric Units
English Units
English to
metric (multiply
x English Units)
Length
kilometer, km mile, mi
meter, m yard, yd
meter, m foot, ft
centimeter, cm inches, in
centimeter, cm feet, ft
millimeter, mm (precip. inches, in
and evaporation)
1.609
0.914
0.3048
2.54
30.5
25.4
Area
kilometer2,
kilometer2, km2
hectare, ha
meter2, m2
meter2, m2
mile , mi
acre, ac
acre, ac
feet2, ft2
mile2, mi2
2.590
0.00405
0.4046
0.0929
3.861 E-07
Volume
meter-3, na-
me ter3 , Di-
meter3 , m~
meter3, m~
liter, H
acre-inch, Ac-in
acre-feet, AF
feet3, ft3
bushel (US), bu —
quart (liquid), qt
2/
102.8
1233.6
0.02832
28.38
0.946
_!/ To convert from metric to English units, divide the metric value by the
value in the column.
2/ Bushels as a unit of weight will vary from crop to crop. Metric yields
are expressed in kg/ha. The ASAE 1977 Yearbook has information on some
crop weights per bushel.
230
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Mass
ton (metric)
kilogram, kg
gram, gm
Pressure (Force per
unit area)
bar
bar _
kg (weight)/cm
kilopascal, KPa
atmosphere, atm
millibars, mb
atmosphere, atm
Metric Units
ton (English)
pound, lb
ounce (avdp), oz
Ib/inch , psi
atmosphere, atin
atmosphere, atm
Ib/inch2, psi
Ib/inch2, psi
inches of mercury (20°C)
feet of water (2Q'°)
English Units
0.9072
0.454
28.35
0.06895
1.
1.
6.
0.
.013
.033
.895
.06805
33.86
34.01
English to
metric (multiply
x English Units)
Water Measurement
hectare-meters, ha-m
hectare-meters, ha-m
hectare meters/hectare
meters/sec
liter/sec
liters/sec
liters/sec
Sediment
tons (m)/day
kg/m3
acre-feet, AF
acre-inches, ac-in
acre feet/acre
feet^/sec
gallons/minute
feet^/sec
3/
Colorado Miners Inch —
tons (E)/day
ppm
0.1233
0.01028
0.3047
0.02832
0.0631
28.32
0.74
0.907
depends on
density
Temperature
Celsius
Fahrenheit
5/9(°F-32)-/
3/ Miners inches are established by State legislation and will vary from
state to state.
4/ For Celsius to Fahrenheit use (9/5°C) + 32.
231
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Miscellaneous conversions
one gallon (US gal) gallon/minute (gpm)
= 231 in3 = 0.00223 cfs
= 0.13368 FT = 0.00442 AF/DAY
one cubic foot (FT ) cubic feet/sec (cfs)
= 1,728 in3 = 448.8 gpm
= 7.481 gallons = 1.984 AF/day
= 62.4 Ib (mass)
one acre foot (AF) acre foot/day
= 43560 FT3 = 0.504 cfs
= 325,851 gallons = 226.3 gpm
- 1357 tons
Chemical Quality to Tons of Salt
(PPM) x (AF) x (0.00136) = TONS (ENGLISH) TO SALT/UNIT TIME
(PPM) x (m3/sec) x (0.0864) = TONS (METRIC)/DAY OF SALT
232
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.REPORT NO
EPA-600/2-79-062
TECHNICAL REPORT DATA
Please read Instructions on the reverse beforfcompktin
\
4. TITLE AND SUBTITLE
ENVIRONMENTAL PLANNING MANUAL FOR SALINITY MANAGEMENT
IN IRRIGATED AGRICULTURE
'. AUTHOR(S)
Gaylord V. Skogerboe, Wynn R. Walker, and Robert G.
Evans
9. PERFORMING ORGANIZATION NAME AND ADDRESS ~
Agricultural and Chemical Engineering Department
Colorado State University
Fort Collins, CO 80523
12. SPONSORING AGENCY NAME AND-ADDRESS~ ~~~
Robert S. Kerr Environmental Research Lab. - Ada, OK
Office of Research and Development
U.S. Environmental Protection Agency
Ada, OK 74820
3. RECIPIENT'S ACCESSIOf*NO.
5. REPORT DATE
March 1979
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO
10. PROGRAM ELEMENT NO.
1BB770
11. CONTRACT/GRANT NO.
Grant No. R-804672
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/15
15. SUPPLEMENTARY NOTES
248 pages, 58 figures, 11 tables, 259 references
16. ABSTRACT ~
An Environmental Planning Manual for Salinity Management in Irrigated Agriculture has
been prepared. The primary focus of this manual is a delineation of the combinations
of technological and institutional solutions, the various levels of planning effort,
use of existing data and necessary field investigations which are required for the
different planning levels, methods of data analysis, technological and socio-
economic considerations in implementing a salinity control program, and finally,
recommendations for formulating an action program. It is intended that the primary
audience for this manual would be environmental planners such as EPA Regional
Offices, state water pollution control agencies, regional councils of governments,
and 208 (Section 208 of PL 92-500) planning groups. In addition, it is intended to
serve as a guide to be used and tailored at the discretion and guidance of the
supervisory personnel to persons without prior training or experience in assessing
the nonpoint source pollution problems of irrigation return flows due to salinity.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Irrigation, Salinity, Water quality,
Irrigation efficiency
3. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
b.IDENTIFIERS/OPEN ENDED TERMS
Salinity control, Irri-
gation management. Irri-
gation return flows,
Nonpoint sources, Agri-
cultural pollution ,
Water pollution sources,
Irrigation effects
19. SECURITY CLASS (This Report)
UNCLASSIFIED_
20. SECURITY CLASS (This page)
UNCLASSIFIED
COSATi Field/Group
43F, 48B
68D, 98C
21. NO. OF PAGES
249
22. PRICE
EPA Form 2220-1 (9-73)
233
U. S. GOVERNMENT PRINTING OFFICE: ',"•) — 6?7->;0/l-
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