EPA-600/2-75-064
November 1975
Environmental Protection Technology Series
SCIENTIFIC IRRIGATION SCHEDULING FOR
SALINITY CONTROL OF
IRRIGATION RETURN FLOWS
Robert S. Kerr Environmental Research Laboratory
Offia&,of Research and Development
U.S. Environmental Protection Agency
Ada, Oklahoma 74820
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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
five series. These five broad categories were established to
facilitate further development and application of environmental
technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in
related fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed
to develop and demonstrate instrumentation, equipment and
methodology to repair or prevent environmental 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 Information Service, Springfield, Virginia 22161.
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EPA-600/2-75-064
November 1975
SCIENTIFIC IRRIGATION SCHEDULING FOR
SALINITY CONTROL OF IRRIGATION RETURN FLOWS
by
Marvin E. Jensen
U.S. Department of Agriculture
Agricultural Research Service
Western Region
Snake River Conservation Research Center
Kimberly, Idaho 83341
Interagency Project No. EPA-IAG-D4-F399
Project Officer
Arthur G. Hornsby
Source Management Branch
Robert S. Kerr Environmental Research Laboratory
Ada, Oklahoma 74820
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
ROBERT S. KERR ENVIRONMENTAL RESEARCH LABORATORY
ADA, OKLAHOMA 74820
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DISCLAIMER
This report has been reviewed by the Robert S. Kerr 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.
ii
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ABSTRACT
A comprehensive review is presented of irrigation water management
principles, factors to be considered in improving irrigation water
management, leaching requirements, climatological approaches to irri-
gation scheduling, scope of irrigation scheduling services in 1974,
basic concepts of scheduling services, and probable effects of scien-
tific irrigation scheduling on salinity of return flows. A definition
of irrigation water management efficiency is presented to evaluate the
annual volume of irrigation water used relative to the optimum amount
needed for maximum annual crop production or income. The term con-
siders the minimum but essential water needed for both consumptive and
nonconsumptive uses. The lack of significant changes in irrigation ef-
ficiency during the past several decades is discussed and attributed to
problems associated with the management of a complex soil-crop-environ-
ment system, a lack of economic incentives to make improvements, and
ineffective traditional approaches to improve irrigation water manage-
ment. New proposed minimal leaching practices are discussed. The
author concludes that substantial improvements in irrigation efficien-
cies can be made before the potential minimal LF is reached on most
western irrigated projects.
This report was submitted in fulfillment of Interagency Project Number
EPA-IAG-D4-F399 by the U. S. Department of Agriculture, Agricultural
Research Service, under the partial sponsorship of the Environmental
Protection Agency. Work was completed as of June 1975.
iii
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CONTENTS
Sections
I Conclusions
II Recommendations
III Introduction
IV Improving Irrigation Water Management
V Development of Scientific Irrigation Scheduling
VI Basic Concepts of Irrigation Scheduling and
Scheduling Services
VII Scope of Irrigation Scheduling Service in 1974
VIII Effects of Irrigation Scheduling on Salinity of
Return Flows
IX References
X Appendix
Page
1
3
5
8
22
27
50
64
80
89
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FIGURES
No. Page
1 Typical example of daily evapotranspiration (potential, 29
dashed line) from a row crop in an arid area (Wright,
1975)
2 Simulated soil water status, irrigation and rainfall, 30
and drainage
3 Alternative soil water management goals for shallow- 33
rooted crops in southern Idaho
4 Irrigation management processes 35
5 Typical example of gravity irrigation 40
vi
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TABLES
No. Page
1 Characteristics of Groups Providing Irrigation Scheduling 52
Services in 1974 in Western USA
2 Reasons Customers Gave for Continuing or Discontinuing 57
Irrigation Scheduling
3 Adequacy of Climatic Data for Scheduling Purposes 58
4 Principal Mode of Communications Used, Percentage of 58
All Groups
Al Commercial Irrigation Scheduling Services Provided for a 90
Fee in 1974 in Western USA
A2 Irrigation Scheduling Services Provided by State (S), 91
Federal Agency (U), Company (C), or Project (P) in
1974 in Western USA
vii
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SECTION I
CONCLUSIONS
1. Efficient irrigation water management requires basic decision-
making data that generally are not available to most farm manager/
operators of irrigated farms.
2. Few economic incentives to improve irrigation efficiency have
existed during the past several decades.
3. Increasing labor costs have decreased inputs for operating surface
irrigation systems, and unless offset by significantly improved
irrigation facilities, decreased labor inputs have contributed to
a lack of significant improvements in irrigation efficiencies.
4. Rapid expansion of commercial and agency irrigation scheduling
services, which include weekly field monitoring by trained tech-
nicians, during the past 5 years represents the beginning of a new
era in irrigation water management.
5. Substantial improvements in irrigation water management efficien-
cies could be made before potential minimal leaching fractions for
maintaining salt balance in soils are reached on most western USA
irrigated projects.
6. About a 10 percentage point improvement in average farm irrigation
efficiencies, which now averages about 40%, could be expected dur-
ing the next decade without significantly increasing energy re-
quirements. Part of this improvement could be expected with im-
proved irrigation scheduling technology, but some adoption of more
efficient gravity irrigation facilities and practices will be need-
ed. This change is not expected to significantly influence salin-
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ity in return flows except where salt pickup is a major factor.
7. Major improvements in gravity or low pressure surface irrigation
systems and practices, along with changes in water delivery policies
controlled by institutions and state organizations regulating water
rights, will be needed to achieve sufficient increases in irrigation
water management efficiencies to significantly reduce salt loads in
irrigation return flows without large energy inputs. Scientific
irrigation scheduling can significantly reduce the salt load in
return flows with irrigation systems that enable uniform applica-
tions of known amounts of irrigation water. Potential efficiencies
of new irrigation systems and potential reductions in salt loads
probably could not be achieved without scientific irrigation sche-
duling. Scientific irrigation scheduling is economically feasible
with most existing irrigation systems, but will be more effective
with new and better irrigation systems. Major benefits to the farm
manager/operator result from improved crop yields and quality, and
general improvement of irrigated farm management.
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SECTION II
RECOMMENDATIONS
1. Significant improvements in irrigation water management efficiency
will require improved irrigation scheduling and facilities to uni-
formly apply a known volume of water. Decision-making data must
be made available to farm manager/operators if they are to utilize
modern irrigation management science and technology because they
have limited available time to independently obtain these data.
2. Greater emphasis should be placed on new innovative approaches to
improving irrigation management rather than continuing traditional
approaches that have been relatively ineffective over the past 3
decades.
3. A major effort should be directed toward developing an urgently
needed portable and rapid technique so irrigation management ser-
vice groups can accurately measure the soil water content with
depth, or the integrated water content, without first inserting
access tubes or drilling holes. Microwave or combination micro-
wave nuclear techniques should be considered.
4. Increased effort is needed to complete the development of a second
generation computer program for irrigation water management, which
incorporates recent improvements in simulation models for energy
balance, evaporation and transpiration, and plant growth.
5. As techniques for irrigation scheduling services are improved,
workshops are needed to rapidly acquaint professional staffs of
irrigation scheduling service groups with the latest practical
technology and operating procedures, and assist new service groups
to initiate similar services in new areas.
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6. Similar increased efforts to hasten the development of gravity
irrigation or low pressure irrigation facilities and systems and
workshops should be planned on this subject.
7. As major irrigation scheduling services are initiated to improve
irrigation water management efficiency and reduce salt pickup in
selected areas, a data base should be established to document the
direct benefits that can be attributed to more intensive applica-
tion of irrigation science and technology in irrigation water
management.
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SECTION III
INTRODUCTION
IRRIGATION WATER MANAGEMENT
Irrigation is the application of water to soil, supplementing natural
precipitation, to provide water essential for plant growth. Water
plays a vital role in transporting mineral nutrients and translocating
materials in solution throughout the plant. There is a liquid phase
continuity from the water in the soil through the plant to the liquid-
gas interface at evaporation sites in the leaves of all actively growing
plants (Slatyer, 1967). The root system provides an extensive absorb-
ing surface through which virtually all the water and mineral nutrients
utilized by plants pass. In a crop-soil-climate system, maintaining
the soil water level within an optimum range is essential to avoid ad-
verse effects on plant growth and crop production.
Managing the soil water reservoir is not easy. The manager/operator of
an irrigated farm must regulate a reservoir which has a level that is
neither visible nor uniform throughout the field. There is essentially
no lateral flow to equalize the reservoir, and the farm manager/operator
cannot control the outflow rate. In addition, since water transpired by
plants and evaporated from the soil surface is salt free, sustained crop
production also requires controlling the concentration of soluble salts
in the soil solution. Leaching, in which a fraction of the water pene-
trating the soil surface passes through and leaves the root zone at a
higher salt concentration, is the only practical way to control soluble
salts and specific toxic ion concentrations in the root zone. Control
of soluble salts and sodium is probably one of the oldest problems en-
countered by irrigated agriculture.
It is difficult to efficiently manage the soil water reservoir component
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of a complex crop-soil-climate system. But the farm manager/operator,
who has a limited understanding of the complicated mechanisms that con-
trol the system, is expected to use water very efficiently. Many as-
sume he will automatically achieve full benefits of irrigation water
if it is delivered to some high point of his land. Irrigation water
management, with the objective of maintaining the soil water reservoir
within an optimum range, requires daily or weekly decisions. A single
overirrigation during the growing season can drastically lower the sea-
sonal irrigation efficiency. Opportunities for mismanagement are less
with some systems. For example, excessive water applications are not
as frequent with sprinkler irrigation systems because water is applied
at a relatively low rate which is controlled by the system and not the
soil. Thus the amount of water applied can more easily be prescribed
and controlled, but the time of application is still very flexible. In
contrast, most surface or gravity irrigation systems allow little oppor-
tunity to control the amount of water applied. Sufficient water must
be applied so it flows from the upper to the lower end of each field.
The rate water is added to the soil reservoir is controlled by the soil
surface and the water must be allowed to run long enough to permit suf-
ficient intake at the lower ends of the fields. Many older systems do
not have water measurement devices, adjustable control structures, and
lined channels or enclosed distribution systems. Under these condi-
tions, opportunities for mismanagement are greater and generally irri-
gation efficiencies are lower.
An important fact often overlooked is that irrigation scientists, engi-
neers, and technicians do not make the required daily or weekly irriga-
tion decisions. Irrigation management decisions generally are made by
very busy people with limited technical background and training in the
management of a complex crop-soil-climate system. In addition, the de-
sired decision-making data seldom are available to those who make these
decisions on a day-to-day basis. What are the desired decision-making
data? Efficient irrigation water management requires knowing: (1) the
current level and expected change in available soil water for each field
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during the next 5 to 10 days.; (2) the expected latest date of the next
irrigation for each field to avoid detrimental plant water stress, and
the earliest date to permit efficient irrigation and avoid overirriga-
tion; (3) the amount of water to be applied on each field if the irri-
gator is able to determine and control the amount; and (4) some indica-
tion of the adverse effects of irrigating too early, too late, or per-
haps terminating irrigations. These data are required for efficient
operation of existing systems, and as irrigation systems are improved
these data become even more important if the farm manager/operator is
to realize full benefits of a modern irrigation system. An exception
would be a fully automatic system controlled by soil water sensors.
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SECTION IV
IMPROVING IRRIGATION WATER MANAGEMENT
FACTORS INVOLVED
There are many factors that affect the management of an irrigation sys-
tem and the soil water reservoir. The most obvious is the quantity of
water available for irrigation. When the average annual water supply
is significantly less than the net consumptive water requirement for
maximum yields, many assume that the primary factor limiting crop pro-
duction is the water supply. What is not obvious is that because of
obsolete facilities and poor management practices, a limited water sup-
ply may be used very inefficiently. Why would limited water supplies
be used inefficiently? Institutional policy, water rights, and limited
storage facilities may cause water to be delivered only at preset time
intervals regardless of the rate of water use by the crop. Under these
conditions the farm manager/operator, not knowing the soil water level
or the depletion rate by evapotranspiration, applies water when avail-
able regardless of the amount that can be stored in the soil. Why
would more water be applied than be stored? Because the manager/opera-
tor cannot risk delaying the irrigation until his next turn and he can-
not apply a light irrigation with most surface irrigation systems. For
example, basin irrigation may require the application of at least 100
to 150 mm of water to cover all high spots in the basin, but a shallow-
rooted crop may have depleted only 30 to 50 mm since the previous irri-
gation or rain. The excess water applied drains through the profile
carrying accumulated soluble salts and some plant nutrients, like nitro-
gen. Some leaching may be needed, but the localized areas with above
average salt concentration usually are in the high spots, not the low
areas. Under these conditions, it is very difficult to improve irriga-
tion efficiencies or significantly change the quality of irrigation re-
turn flow from deep percolation.
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In some countries, and to some extent in some projects of the USA,
water delivery is controlled by people technically trained in the hy-
draulics and not in the agricultural aspects of irrigation. Their goal
is to operate the system efficiently from a hydraulic viewpoint. Under
these circumstances, significant improvement in irrigation water manage-
ment will be difficult to achieve, even if all desired irrigation de-
cision-making data were available, because the control of the main irri-
gation distribution system does not rest with the farm manager/
operators.
When adequate water supplies are available, irrigations often are delay-
ed which may reduce crop yield and quality, followed by excessive irri-
gations with their adverse effects. Studies of irrigation practices
have shown this to be a common practice, along with irrigating too soon
and generally applying more water than the soil will hold.
The soils also significantly influence irrigation water management.
Areas with extremely low or high infiltration rates, like those with
high silt or sandy soils, are extremely difficult to manage efficiently
with gravity irrigation systems. Nonuniformity of soils within a field
influences the uniformity of water application with surface systems.
Sodium problems and restricted natural subsurface drainage further com-
plicate irrigation water management.
IRRIGATION EFFICIENCY CONCEPTS
The effectiveness of irrigation water management and the unavoidable
losses of water must be known to plan, operate, and improve irrigation
projects. The term "irrigation efficiency" is used to describe the ef-
fectiveness of one or more irrigation operations. This term is common-
ly used to describe the application efficiency, or ratio of water stored
in the soil during an irrigation relative to the volume of water de-
livered to the field or other unit area (E ). If water is delivered
3.
uniformly to the unit area at a rate equal to evapotranspiration, or if
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just enough water is delivered to the area to replace the soil water
that has been depleted since the previous irrigation and uniformly ap-
plied, the irrigation efficiency will be 100%. The return flow from
the unit area under these conditions will be (1 E ) = 0. But a high
3.
application efficiency does not necessarily result in good irrigation
water management for crop production. A high application efficiency for
a single irrigation can easily be achieved if only half as much water is
applied as can be stored. Under these conditions, plant growth and crop
production may be adversely affected in portions of the field if the
next irrigation is not applied early enough to avoid excessive depletion
of soil water.
The common general definition of irrigation efficiency is the ratio of
water used in evaporation and transpiration by crops on an irrigated
field, farm, or project, to the water pumped or diverted from a river or
other natural source for this purpose. A variation in the definition of
irrigation efficiency includes the ratio in the volume of water required
for other beneficial uses. The main terms in the numerator are the
water used in evapotranspiration plus the amount necessary for leaching.
Israelsen (1950) defined irrigation efficiency as "the ratio of the
water consumed by the crops of an irrigation farm or project to the
water diverted from a river or other natural water source into the farm
or project canal or canals."
Ei = 100 ^ [I]
r
where W = the irrigation water consumed by the crops on an irriga-
tion farm or project during their growth period, W = the water di-
verted from a river or other natural source into the farm or project
canals during the same period of time. He also defined two other terms,
W , the water delivered to the farms of a project during a given period
of time, and W , the water stored in the soil root zone. He also il-
s
lustrated with an example how the overall irrigation efficiency is a
10
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product of the conveyance and delivery efficiency and the farm irriga-
tion efficiency. He considered as losses surface runoff, deep percola-
tion and evaporation from the soil. Thus, basically Israelsen proposed
irrigation efficiency to represent the ratio of the water used in trans-
piration to that diverted from a river or other natural water source in-
to the farm or project canal or canals.
Israelsen also defined water application efficiency as
E = 100 ^S. [2]
a Wf
and described the sources of losses of irrigation water from the farm
as surface runoff, R , and deep percolation below the root zone, Df.
Then, neglecting evaporation during the time of application,
[3]
E
a
W = V
= 100
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ed. This concept, however, has resulted in many misleading statements
about irrigated agriculture because as the concentration of soluble
salts in the water increases, the maximum attainable irrigation effi-
ciency decreases regardless of how well the irrigation system was de-
signed, constructed, and operated.
Since leaching is the only practical way to maintain a favorable salt
balance in the crop root zone, it therefore is one of several beneficial
uses of water. Other beneficial uses are for germination, and for frost
protection which has become very common in orchards and vineyards. When
irrigating for frost protection, sprinklers are operated during freez-
ing periods to prevent bud damage. The quantity of water evaporated is
relatively small as compared with the quantity necessary to achieve
frost control.
Future redefinitions of the irrigation efficiency can be expected as
practical techniques for controlling evaporation from the soil surface
are developed. Evaporation control will reduce the consumptive water
requirement for a given crop in a specific climate. Also, as competi-
tion for water increases, greater emphasis will be placed on agronomic-
economic assessment of water use, like crop production per unit volume
of water diverted from a natural source for irrigation purposes.
Engineers and planners also use the term "irrigation efficiency" to
describe the performance of existing systems, or expected performance of
new methods and systems for distributing water. The American Society of
Civil Engineers (ASCE) Committee on Irrigation Water Requirements recent-
ly described several components of irrigation efficiency (Jensen, 1974).
The components are very similar to those described by Israelsen, except
they included the efficiency of storing water in a reservoir specifical-
ly for irrigation and described the unit irrigation efficiency as the
ratio of the water required for beneficial use in a unit area to the
volume of water delivered for that purpose. These terms are:
12
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"Reservoir storage efficiency, E , is the ratio of the volume
s
of water available from the reservoir for irrigation to the
volume of water delivered to the storage reservoir surface or
underground for irrigation.
"Water conveyance efficiency, E , is the ratio of the volume
c
of water delivered to the point of use by an open or closed
conveyance system to the volume of water introduced into the
conveyance system at the supply source or sources.
"Unit irrigation efficiency, E , is the ratio of the volume
of irrigation water required for beneficial use in the speci-
fied irrigated area to the volume of water delivered to this
area.
"The efficiencies of components of an irrigation system are
defined so that the product of the component efficiency terms,
expressed as ratios, gives the overall irrigation efficiency.
E E E
E =
i 100 100 0
"The component efficiency terms may be applied to any project
or segment thereof for any specified period of time. For
clarity and comparative purposes, all efficiency values report-
ed should be identified as to the size of the unit, the period
of time or number of irrigations involved, the adequacy of ir-
rigations, and the computational procedure used in obtaining
the efficiency values."
Traditionally, in arid areas the main irrigation water management objec-
tive has been to obtain near maximum crop yields. The objective of most
yield vs evapotranspiration experiments that have been conducted has
been to determine the optimum level of evapotranspiration needed to pro-
duce near maximum yields . In areas that have more precipitation and
13
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where irrigation water supplies have been limited, many studies have
been conducted to determine the maximum production or income per unit of
annual irrigation water. These management objectives may be the same
for many farm crops in some areas. For example, Jensen and Sletten
(1965) reported the maximum grain sorghum production per unit of irri-
gation water applied annually in the Southern High Plains during a 4-
year period occurred on the same treatment that produced the maximum
average yield per unit area. Musick, et al. (1971), working in the same
area, found that a preplanting irrigation, which increased the total ir-
rigation water applied by 25%, decreased the 3-year average production
per unit of annual irrigation water as compared with seasonal irriga-
tions without the preplant irrigation.
Since the major emphasis in this report concerns the effects of irriga-
tion water management on return flow, particularly irrigation schedul-
ing, the effectiveness of irrigation water management must be consider-
ed. Irrigation water-use efficiency (defined as the increase in crop
yield over nonirrigated yields per unit of irrigation water applied
annually) has been used to evaluate the effectiveness of irrigation
practices (Jensen and Sletten, 1965). Shmueli (1973) describes optimum
irrigation efficiency as the minimum seasonal water application neces-
sary to raise crop yields and reduce the amount of irrigation water
applied below the evapotranspiration level (maximum Y/W ). Also,
3
since irrigation is complementary to precipitation stored in the soil,
these resources should cover the water requirement for maximum produc-
tion or income.
With this background, and assuming that the annual change in available
water in the root zone is negligible,
WT + P = W + W [6]
I cu u
where WT = the total volume of water diverted annually from a river or
from some other natural source for irrigation (irrigation water), P *
14
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annual precipitation in the irrigated area, W = the volume of water
consumptively used annually in the irrigated area, and W = the volume
of water beneficially used annually within the same area but not consum-
ed or evaporated. W represents the volume of water that is potential-
ly available for return flow from an agricultural area (W = W_ + P
W ). It normally equals return flow if water is not diverted from the
area or consumed or evaporated in other nonagricultural processes. The
optimum amount of irrigation water required per unit area is
(WT) - CW + W - P) . [7]
I opt cu u min
where (W + W P) = the minimum amount of water for maximum sus-
cu u min
tained crop production or income.
The overall efficiency of beneficial irrigation water use within an ir-
rigation project, or -irrigation water management efficiency, E. , can
now be defined as
(WT>nnt-
E,_ - 100 ^r-, I.PPt /TT N r [8]
where (W ) = the optimum annual irrigation water required per unit
area, if uniformly applied without surface runoff and without deep per-
colation, to obtain maximum sustained annual crop production or income.
(W ) includes the minimum, but essential water needed for other non-
I opt
consumptive agricultural uses, like frost protection, leaching, hydrat-
ing a crop like potatoes before harvest to control bruising, etc., and
the minimum amount of water required to germinate a crop if it will not
germinate without irrigation. The minimum water required for leaching
is based on the latest available practical technology. It may neces-
sitate leaching during the nongrowing season, like irrigating after
winter precipitation, or in some cases before winter precipitation.
The minimum amount of water required for germination is that amount
needed using the latest available practical technology. The use of the
absolute quantity, (Wj. - (Wj) tI, in the denominator of equation 8
15
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provides for the case where less irrigation water is applied than the
optimum for maximum sustained crop production or income.
EARLY EFFORTS TO IMPROVE IRRIGATION WATER MANAGEMENT
As irrigation expanded in the USA in the late 1800s, numerous studies
were conducted to improve irrigation water management. Studies of crop
water requirements were initiated in the late 1880s and early 1890s to
evaluate the quantity of water being used, crop returns, and water loss-
es by evaporation and deep percolation. Overirrigation was cited as the
first and most serious mistake made by early settlers (Buffum, 1892).
Israelsen, et al. (1944) conducted nearly 150 field tests in the late
1930s and 1940s to evaluate water application efficiencies. They found
that the dominant factors resulting in low water application efficien-
cies were excessive applications, uneven water distribution over the
land, and high soil water contents before irrigation. Excessive water
applications and irrigating too soon are interrelated and are important
factors in scheduling irrigations.
Irrigation scheduling involves determining the optimum time to irrigate
and the optimum amount of water to be applied with existing irrigation
systems. As irrigation systems are improved, the optimum values may
change. Numerous procedures have been proposed and advocated for dec-
ades to assist the farm manager/operator in scheduling irrigations.
Probably the earliest and most common procedure proposed is to note the
appearance of the soil in the root zone using a probe or shovel. But,
typically, authors of irrigation publications indicate that irrigators
are reluctant to expend this effort to assess irrigation needs. Plant
appearance is also used to schedule irrigations, but generally by the
time the plant shows symptoms that soil water is limiting growth, crop
yield and/or quality have already been irreversibly affected. Using
tensiometers to determine when to irrigate has been advocated for many
years and they are commonly used for this purpose in citrus groves.
16
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However, tensiometers have not been routinely used by farm manager/
operators to determine when to irrigate most farm crops. Why? There
are several key reasons. They require the farm manager/operator's time
for installing and removing instruments and recording observations
several times each week. Tensiometers may interfere with cultivation,
and may be a nuisance to service. Interpreting the values recorded
(especially for those crops that do not need to be irrigated frequently
to keep the soil water level in the tensiometer range), can be compli-
cated, and erroneous readings caused by poor contact with the soil,
leaks in the unit, etc. are confusing.
Similarly, electrical resistance or soil moisture blocks also have been
proposed for many decades to determine when to irrigate. Generally,
tensiometers and soil moisture blocks can be used effectively by train-
ed irrigation technicians and irrigation scientists in research, or by
trained technicians monitoring farm fields, but they are not used exten-
sively by farm manager/operators. Thousands of tensiometers and soil
moisture blocks have been sold to farm manager/operators, but they are
rarely used by the farm manager/operator to determine when to irrigate
most farm crops. Similarly, evaporation devices like the U.S. Weather
Service Class A pan, and various other special evaporation devices have
been advocated for many decades to assist in predicting when to irri-
gate. For example, use of evaporation data to schedule irrigations has
been promoted for several decades in the State of Washington (Hagood,
1964; Jensen and Middleton, 1970; and Pruitt, 1956). Coefficients for
use with a standard pan have been determined experimentally for most
farm crops after full crop cover has been established. Daily pan evap-
oration is broadcast or published in newspapers. Irrigation scheduling
boards have also been developed for different crops based on pan evapo-
ration as an index of the climatic effect on evapotranspiration. In
Canada, a black, porous plate supplied with water has been used exten-
sively for irrigation scheduling in orchards, as well as for some other
farm crops (Wilcox and Brownlee, 1969).
17
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Suggested general dates and amounts for irrigations can effectively im-
prove irrigation water management efficiency on soils with deep-rooted
crops not highly sensitive to water stress like cotton, and for climate
that does not vary widely from year to year. Calendar schedules work
best where rainfall does not significantly affect soil water during the
main part of the growing season. The recommended dates are usually
derived from irrigation experiments in which the treatments included
both the time and amount of irrigations.
Significant improvements in irrigation water management efficiency have
been achieved during the past few decades in some areas, especially
where water is expensive or scarce. A significant portion of these im-
provements can be attributed to the development and availability of new
irrigation facilities. Basically, the traditional approach to encour-
aging improved irrigation scheduling generally has not been very effec-
tive. The traditional approach requires the farm manager/operator to
use some device like an evaporation pan to indicate the climatic effect
on the rate of evapotranspiration, or an instrument to indirectly eval-
uate the soil water status. Most traditional approaches basically re-
quire that the farm manager/operator first understand the processes in-
volved and the factors governing soil moisture depletion. In addition,
if a tool or an instrument is required, he also must understand how it
functions and its direct relationship to the soil water status or its
indirect relationship to the soil water depletion rate. Actually, the
traditional approach to improving irrigation scheduling may have handi-
capped progress over the past several decades because alternative pro-
cedures have not been developed and evaluated. For example, would we
consider advising fanners to first study chemistry and associated tech-
niques to obtain representative soil samples, make the necessary anal-
yses, and then independently determine the amount of fertilizer needed?
No. We recognized the complexity of soil and plant analyses, the inter-
pretation of these analyses, and the training and time required. In
lieu of the usual approach, we provided or encouraged them to use ser-
vice laboratories, both private and commercial, to supply this informa-
18
-------
tion. But why have we insisted that the farm manager/operator first
understand the principles of soil water management, atmospheric physics
that control the evaporation rate, and hydraulics that control water
distribution over his field to apply these principles and improve his
irrigation water management efficiency?
I firmly believe, perhaps from hindsight, that we have overemphasized a
single academic approach to improve irrigation water management. We
have not adequately considered alternative procedures to provide the
vital decision-making data needed to improve irrigation water manage-
ment. But we have recognized that if the time and amount of water ap-
plied can be controlled by the irrigation system, irrigations can be
programmed more easily or automated with appropriate soil water sensors
to achieve efficient irrigation water management. This approach does
not require that the farm manager/operator firsj^ acquire technical know-
ledge and training before he can apply irrigation science and technology.
Instead, with an automated system he learns how the complete system
responds to changes in climate, precipitation, and crop growth stage by
observing the irrigation frequency and observing the quantity of water
used.
Efficient irrigation water management with most existing irrigation sys-
tems requires daily or weekly decisions and judgments. Irrigation sche-
duling with most irrigation systems is a decision-making process that
requires current information involving trends, projections, and effects
of alternative actions similar to that required by managers of large in-
dustries.
"The modern farm manager needs and wants a continuing service
that gives the present soil water status on each of his fields,
predicts irrigation dates, and specifies the amounts of water
to apply on each field. He could also use predictions of ad-
verse effects, such as the effects of delaying an irrigation
for several days or perhaps terminating irrigations, on the
19
-------
yield of marketable products" (Jensen, 1972).
Based on my experience during the past two decades, I feel that the
traditional approach to improving irrigation water management will not
result in further significant changes in irrigation water management,
unless the cost of water increases substantially. When the cost of ap-
plying irrigation water is low, as it is in many areas using natural
reservoirs, unlined open channels, and surface irrigation systems, im-
proved irrigation water management will require improving both irriga-
tion scheduling and irrigation facilities.
Irrigation facilities will be improved more rapidly if the farm manager/
operator realizes that he is unable to achieve the desired control of
irrigation water to efficiently maintain the soil water reservoir with-
in the optimum range. Irrigation scheduling services can stimulate the
desire to improve the irrigation system. Improvements in irrigation
scheduling with most existing systems will require the availability of
irrigation scheduling services for the busy farm manager/operator. Ir-
rigation scheduling services can be defined as follows:
Irrigation Scheduling Services (ISS) - A modern service, based
on the latest irrigation science and technology, which provides
up-to-date information on the status of available water in in-
dividual fields; projected date of next irrigation, if another
will be needed, or daily rate water should be applied with high
frequency irrigation systems to maintain the desired soil water
level in each field. The service recommends the allocation and
time of water application when the water supply or its applica-
tion cost is the primary variable input that will control the
net return from the farming enterprise during the current crop-
ping season. The service recommends the allocation of water
and timing to optimize net returns when another variable like
fertilizer, will limit net returns, and provides related
recommendations concerning the operation of the farm irriga-
20
-------
tion system to improve the uniformity of water distribution,
reduce water losses, maintain a favorable salt balance in the
soil, etc. so as to increase the managerial skills of the
operator/manager and his net returns from the farming enter-
prise.
21
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SECTION V
DEVELOPMENT OF SCIENTIFIC IRRIGATION SCHEDULING
FACTORS AFFECTING IMPROVEMENTS IN SCHEDULING
Improvements in irrigation scheduling, utilizing recent advancements in
irrigation science and technology, progressed slowly during the past two
decades while significant advancements were made in irrigation science
and technology and related sciences, like agricultural meteorology.
During the same period, irrigation water management efficiency, E, ,
iin
on some projects improved where water was scarce or expensive, but
changed little on many projects.
Improving E. on most existing projects requires both improved irriga-
tion scheduling techniques and improved irrigation facilities for better
control of irrigation water. There are several reasons why little prog-
ress has been made in controlling irrigation water use. First, water
measurement or volumetric water deliveries to each field requires spe-
cial control structures with surface irrigation systems. Control struc-
tures on many irrigation projects and farms have not changed appreciably
during the past 20 to 50 years. Some older projects do not even have
water measuring structures at farm turnouts, and most irrigated farms
do not have water measurement facilities. Second, the cost of water for
many older projects is very low because the original development costs
have been repaid and there have been few apparent incentives to upgrade
systems. Third, water rights, which are usually limited to beneficial
use, have not been enforced as irrigation technology has improved and
allowed for more efficient use of irrigation water. Basically, there
has been little need to improve E .
Where good water control structures and measurement devices are avail-
able, the water delivered to farms can easily be measured with suffi-
22
-------
cient accuracy, but the magnitude of losses as seepage in unlined on-
farm distribution systems, deep percolation at the upper ends of the
fields, and surface runoff generally are not known to the farm manager/
operator. Also, he is not interested in measuring the amount of water
applied or lost as runoff or deep percolation from each field if he does
not receive direct economic benefits. Direct benefits from improved
water management are difficult to document, especially where direct
water costs are low. The farm manager/operator is not interested and
cannot justify expending additional funds, time, and encountering vari-
ous inconveniences involved in measuring water just to collect data.
Large increases in labor costs during the past two decades have also been
a major factor in the lack of change in irrigation water management ef-
ficiency. A high proportion of the farm irrigation systems in use today
cannot be operated at a high efficiency unless labor input is relatively
high. As labor costs increased, or as labor became scarce, irrigation
water management efficiency either remained relatively unchanged, or
even decreased as labor input decreased. In some cases, reduced labor
input was offset by improvements in the irrigation system.
Improved irrigation scheduling can result in direct and indirect econom-
ic benefits, even when used on most existing systems, but new techniques
are needed. As mentioned in the previous section, numerous devices like
atmometers, evaporation pans, tensiometers, and soil moisture blocks are
tools that have been available to the manager/operator for many years to
determine when to irrigate. A summary of numerous methods of evaluating
soil water is presented by Raise and Hagan (1967). The extent to which
these instruments are used by farm manager/operators is extremely low,
even though many of these devices have been available for 30 years.
Scientists and irrigation technologists continue to improve these instru-
ments and frequently state that "a farmer could easily decide when to
start irrigating and the amount of water required by reading these in-
struments in a field." After 20 to 30 years of advocating using these
instruments without much success, it is time to recognize that perhaps
23
-------
these instruments are not acceptable for practical use by the farm
manager/operator. The lack of change in irrigation scheduling practices
by the farm manager/operator also strongly implies that procedures for
providing irrigation scheduling information have not been very effective.
Irrigation water scheduling information must be current, economical, and
any irrigation scheduling information, whether directly used by the fann-
er or by a service group, only supplements, but does not replace, irriga-
tion experience.
Some research instruments for measuring soil water have been greatly im-
proved, like the neutron probe. The neutron probe is now a very reli-
able, standard instrument for soil water measurement, but its use is
largely restricted to research and investigations conducted by experienc-
ed and trained personnel. New and better instruments for measuring soil
water could improve irrigation scheduling, but experience has shown that
instruments in themselves do not automatically result in improved irri-
gation scheduling.
Scientific irrigation scheduling can be based on gravimetric soil samp-
ling, on instruments that measure soil water directly, or instruments
that indirectly indicate the soil water level, and evaporation devices.
The extent to which trained personnel provide irrigation scheduling
services with these instruments is very limited in the USA. Scientific
irrigation scheduling also can be accomplished utilizing climatological,
soil, and crop data with sophisticated electronic computers or electron-
ic desk calculators to perform the tedious computations. Most of the
recent advanced techniques for scheduling irrigations rely heavily on a
climatological approach, coupled with simple plant growth and soil
water models that simulate daily changes in soil water.
CLIMATOLOGICAL APPROACHES
Scheduling irrigations using current climatological data seems to be
the most attractive, promising technique for improving irrigation sched-
24
-------
uling where the farm manager/operator controls the irrigation system.
The concept of scheduling irrigations using climatic data is not new.
Das (1936) suggested using climatic data to control irrigations in the
1930s. The concept received more attention after the publications of
Penman (1948, 1952) and Thornthwaite (1948). In 1954, Baver stated:
"The meteorological approach to irrigation has the advantage
of simplicity of operation when compared with methods based
upon measurement of soil moisture changes. If it is proved
satisfactory, the costs of using this system would be rela-
tively small. Undoubtedly, new techniques will be developed
that will give an integrated measure of daily temperature,
sunshine, and solar energy. When such methods are available,
meteorological data can be correlated better with evapotrans-
piration."
Many others have since discussed this approach (Baler, 1957, 1969;
Pierce, 1958, 1960; Pruitt and Jensen, 1955; Rickard, 1957; van Bavel,
1960; van Bavel and Wilson, 1952). However, before 1965 this method had
not been adapted for general practical use or tested extensively in the
USA. Since 1969, several procedures that utilize computer technology
and current climatic data in planning irrigation schedules and in pro-
viding irrigation scheduling services have been described (Buras, et al.,
1973; Jensen, 1969; Lord and Jensen, 1975; and Corey and Franzoy, 1974).
DEVELOPMENT OF SCIENTIFIC IRRIGATION SCHEDULING
Probably the most widely used general procedure for providing scientific
irrigation scheduling services is the USDA-ARS Computer Program for Ir-
rigation Scheduling, released in 1970 and modified slightly in 1971
(Jensen, et al., 1971). (Copies of the computer program with sample in-
put-out data and general operating guides are available from the author.)
Many service groups modified the program to suit their special needs.
This program was developed cooperatively with farm managers and service
25
-------
groups, thus enabling the incorporation of farm and service manager re-
actions during formative stages. The computer program is only a tool
for use by technical service groups to provide manager/operators of ir-
rigated farms with current estimates of the soil water status by indi-
vidual fields, and predictions of future irrigations. The computer pro-
gram was purposely based on simple mathematical models and equations so
that limited input data could be used. Also, program operators must
clearly understand how to manipulate or make changes in the input data
to compensate or adjust for irregular conditions. As components of the
original program are improved, they will be replaced with more accurate
subroutines.
The irrigation scheduling program, operated on a manual basis, was eval-
uated in southern Idaho in 1966 and 1967. The computer program and
management services were evaluated in cooperation with farm manager/
operators, the Idaho Agricultural Extension Service, and the Salt River
Project in Arizona in 1968 and 1969. About 50 fields in Idaho, and a
similar number in Arizona were scheduled during this period. A workshop
on operational procedures for the computer program was held at the Snake
River Conservation Research Center in Kimberly, Idaho, in March 1971
(Jensen, 1972; Jensen, et al., 1971; Lord and Jensen, 1975).
26
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SECTION VI
BASIC CONCEPTS OF IRRIGATION SCHEDULING AND SCHEDULING SERVICES
OBJECTIVES
The main objective of irrigation water management for food and fiber
production is to maintain the soil water level within a range that does
not significantly limit plant growth and crop production when adequate
irrigation water supplies are readily available and irrigation water
costs are small. When water supplies are limited, and maximum crop pro-
duction is needed for seed crops, food, and fiber, the main objective
may shift to optimizing production per unit of irrigation water. When
irrigation water costs are high and represent a major part of production
costs, the main objective may shift to maximizing net income per unit of
irrigation water. Net income may be affected by crop quality, its quan-
tity, or both.
IRRIGATION WATER MANAGEMENT
As previously stated, efficient irrigation water management requires
daily and weekly applications of agricultural and irrigation technology.
It requires frequent decisions throughout the irrigation season, except
with fully automated systems, and it requires uniform distribution of
o
5,000 to 15,000 m of water to each ha of irrigated land each year.
With fully automated systems and water available on demand, the time of
application and the quantity of water applied can be controlled by soil
water or salinity sensors and related control valves and structures
(Humpherys and Stacey, 1975). Most soil water sensors respond to soil
water pressure changes. Salinity sensors respond to the concentration
of soluble salts in the soil solution or drainage water. For efficient
automatic, high-frequency irrigation and salinity control, which re-
27
-------
quires a small amount of continual drainage, sensors must respond to the
drainage from the bottom of the root zone, or respond to the salt con-
centration (Rawlins, 1973; Rawlins and Raats, 1975; van Schilfgaarde,
et al., 1974). Fully automated systems usually require a larger capital
outlay per unit area and a higher level of on-site technical skills as
compared with most surface irrigation systems.
The complexity of efficiently managing a soil-crop-climate system can be
described with an example. Figures 1 and 2 illustrate a typical example
of irrigation water management for a shallow-rooted crop in southern
Idaho. Figure 1 shows typical variations in evapotranspiration rates in
an arid area. The dashed line connects daily estimates of "potential
evapotranspiration" or evapotranspiration for a well watered reference
crop like alfalfa. These estimates were made with the Penman combina-
tion equation. The solid line connects daily evapotranspiration as mea-
sured with a lysimeter (Wright, 1975). Typically, the evaporation rate
from the soil increases immediately after irrigating a field before a
crop, like snap beans, emerges. The evaporation rate decreases rapidly
during the first few days after an irrigation as the soil surface dries.
The evaporation rate remains very low, less than 1 mm/day, until the
soil is again wetted by an irrigation, or precipitation such as on June
5, until the bean plants begin to emerge about June 10. When leaf area
is very sparse, the rate of evapotranspiration increases, like on June
14 and June 21, after each irrigation, which wets the soil. After the
bean plants emerge, the evapotranspiration rate with a dry soil surface
increases relative to potential evapotranspiration until the leaf area
index (LAI) approaches 3 (near July 1). When a complete actively grow-
ing crop cover exists, LAI > 3, the rate of evapotranspiration is es-
sentially equal to that of the reference crop or potential evapotrans-
piration (July 1 to August 12). As the crop begins to mature, the
evapotranspiration rate decreases relative to the reference crop.
28
-------
SNAP BEANS, KiMBERLY, IDAHO
FIG 1. TYPICAL EXAMPLE OF DAILY EVAPOTRANSPIRATION
(POTENTIAL, DASHED LINE) FROM A ROW CROP IN
AN ARID AREA. (WRIGHT, 1975)
29
-------
"
-4O
-20
O
20
0.0
ww 40
60
80
8O
I60
Effective field capacity
Maximum
expected, (I + P)
. O J " \
D0(t)
-
*»
c
o
IO 20
MAY
n ,
(
10 20
JUNE
^
n
10 20
JULY
o"
I
-------
ESTIMATING SOIL WATER DEPLETION
The previous example illustrates the complexity of managing a soil water
reservoir when considering only the factors affecting evapotranspiration
rates. Modern irrigation scheduling considers the evapotranspiration
rates and simulates the soil water status to estimate the soil water
status and forecast irrigation dates. The soil water status in this
example is simulated in Figure 2, using estimates of potential evapo-
transpiration and evapotranspiration measured from the shallow-rooted
bean crop. In this example, a drainage rate of 0.1 mm/day was consider-
ed negligible. The rate of unsaturated drainage from the soil profile
is strongly dependent on the soil water content above field capacity,
dW/dt - 0.1 mm/day. Ogata and Richards (1957) showed that the water
content for a soil that is draining can be expressed by
W
cW t
o
-m
where W is the water content; W the water content when t
is a constant for a soil; c is a dimensional constant for
tm;
[9]
1; m
and
t is the time after irrigation has stopped. The drainage rate is
1
£---
W
cW
[10]
When using daily time increments, the cumulative drainage can be approx-
imated using the following equation, which is similar to that proposed
by Wilcox (1960)
W_
I mlW,
- (E ) ]
i-1
Vi
W
[11]
where W,,
the cumulative drainage; i is the number of days after
irrigation; and E is the evapotranspiration for the day. The values
of the empirical coefficients in equations 9 and 10, as used in Figure
2 are: m = 0.043, and W 214 mm for Portneuf silt loam (0 to 0.6 m
depth). Miller and Aarstad (1974) reported m values for several other
soils that range from 0.1 to 0.15.
31
-------
Under conditions in southern Idaho in 1974 there was very little rain-
fall. Without irrigation on this field, the maximum depletion of soil
water expected with this crop is represented by the dashed line in the
upper part of Figure 2. For this illustration, root growth was assumed
to reach the maximum depth of rooting before the soil water content be-
gan to affect the evapotranspiration rate. As the available soil water
was depleted, the evapotranspiration rate was assumed to be proportional
to the ratio, In (AW + 1) /J_n_ 101, where AW represents the percentage
of available water in the root zone on a given day. Extensive experi-
mental data indicate that the soil water could be safely depleted
(D (t)) to some progressively increasing fraction of the maximum avail-
able water for many crops before an irrigation is needed as illustrated
by the curve D (t) (Figure 2). For most farm crops, the optimum de-
pletion generally can approach the maximum value near harvest.
The simulated irrigations (solid bars) and the 1974 precipitation (open
bars) are shown on the lower part of Figure 2, along with the cumulative
drainage, W . If the crop involved is not sensitive to soil water
stress and is deep-rooted, less water can be applied than that required
to refill the soil profile so that drainage during the irrigation season
is negligible. In contrast, when high soil water levels must be main-
tained for crops sensitive to soil water stress and with a shallow root
system (as in this case), it is very difficult to control irrigations
with present surface irrigation systems to avoid significant drainage
after an irrigation or appreciable unexpected precipitation.
There are alternative approaches to managing the soil water reservoir
(as illustrated in Figure 3) for a crop like snap beans. For example,
after the preplant irrigation, available soil water may be depleted
about 30%, at which time an automatic irrigation system could be activ-
vated to apply essentially the same amount of water that has been evapo-
transpired (I + P = E ). After initiating such a practice, the soil
water content remains constant (as illustrated by the solid horizontal
line in Figure 3). The maximum depletion, illustrated by the dashed
32
-------
IOO
LO
u>
80
(E
£60
0
-J
O
V)
Uj 40
_j
m
§
20
X
<
10 2O
MAY
(I+P)-ET after I June
Deficit irrig.
P)
-------
line, represents the expected depletion of soil water with no irrigation
or precipitation (I + P = 0) . Under southern Idaho conditions, and this
shallow-rooted crop, no seed beans would be produced using this practice.
In contrast, in some areas, like the Southern High Plains, a fair crop
can be produced with only a preplant irrigation for a crop like grain
sorghum (Musick, et al., 1971).
SCHEDULING IRRIGATIONS
Some of the factors affecting the decision-making process that must be
considered by the farm manager/operator in irrigation water management
on a unit area are schematically illustrated in Figure 4. The daily and
weekly decisions involve: The status of available soil water for each
crop and field; critical soil water levels, or levels that significantly
affect plant growth or quality under current climatic conditions; the
projected or estimated time when the soil water level will reach the
critical level if an irrigation is not applied; the time required to
complete the irrigation of a field; the expected soil water change dur-
ing irrigation since the entire field can seldom be irrigated at once;
the expected precipitation and its influence on soil water after an ir-
rigation, and future irrigations; the need for cultural practices, like
cultivation or spraying, which must be completed before an irrigation
can be applied; and the economic effects that may result from irrigating
too soon or too late, or from terminating irrigations.
Irrigation water management concerns the soil water extraction rate by
the crop relative to potential evapotranspiration and the absolute quan-
tity of soil water. Irrigation scheduling groups estimate the soil
water depletion after a thorough irrigation or general precipitation
with the following equation:
n
34
-------
Figure 4. IRRIGATION WATER MANAGEMENT PROCESSES
IRRIG. SCHEDULING
SERVICES
Estimated or meas-
ured soil water &
previous precip.
Expected precip.
Recommended irrig.
dates & amounts
Observed soil water
crop growth, etc.
IRRIG. WATER MGMT.
Current soil water
status?
Critical levels (CL)?
Time to reach CL?
Time to complete?
Expected rainfall?
Cultural practices
needed?
Economic effects of
irrig..delaying,etc.
IRRIG. SCIENCE
and
TECHNOLOGY
IRRIGATION
GUIDES
Ext. Serv.,SCS,
etc.
CROP,GROWTH = F
(Soil water, CM
mate, nutrients
etc.)
PRECIPITATION
(Stochastic)
» F(Location,
Time, etc.)
SOIL WATER RESERVOIR
THROUGHOUT EACH FIELD
=F(ET,W,P ,1 and R)
SUBSURFACE DRAINAGE
F (Soil water, ET,
Precip., Irrig. Amt
and Uniformity
EVAPOTRANSPIRATION
= F(Climate, Crop,
Growth stage, Soil
water)
SURFACE
RUNOFF (R)
RETURN
FLOW
-Observations, direct and indirect
Automatic control
-Flow of information, mass, and coupling of related processes
35
-------
where D is soil water depletion (after a thorough irrigation D = 0) ;
P is effective daily precipitation (excluding runoff) ; I is the
daily irrigation applied; W is the daily drainage from the root zone
or upward movement for a saturated zone; and i = 1 for the first day
after a thorough irrigation when D = 0.
The expected date of the next irrigation is predicted by considering
the difference between the present depletion and the soil water that
can be safely depleted, D , and the current or expected average deple-
tion rate before the next irrigation:
N
EldD/dt]
N = 0, for D - D
o
where N = the estimated days to the next irrigation; D = the cur-
rent optimum depletion of soil water; D = the estimated depletion to
date; and E[dD/dt] is the expected mean rate of soil water depletion
until the next irrigation is needed. The expected mean E and P are
usually modified for the first 5 days by the current weather forecast,
after which long-term means are used. The contribution from the satu-
rated zone is based on AW, depth of roots, depth to the water table,
and soil characteristics (Jensen, 1972). Most computer programs evalu-
ate (D D) on a daily basis until D - D .
o o
Under some conditions, the soil water-holding characteristics have
little effect on the optimum depletion level that will determine the
date of the next irrigation. For example, when the amount of water that
can be applied during an irrigation is limited by either the irrigation
system or the soil, D is the net irrigation depth if the soil water
content is maintained above a constant level, (D D ) = a constant.
Most current estimates of daily evapotranspiration for each crop are
based on a crop coefficient, K , and either the evapotranspiration
36
-------
for a well watered reference crop like alfalfa with more than 10 cm of
top growth, or an estimate of potential evapotranspiration, E . Most
current estimates of E are based on either the combination equation
developed by Penman (1948), or a two-parameter empirical equation that
uses only solar radiation and air temperature (Jensen and Raise, 1963).
Et ' KcEtP '14]
where K is a dimensionless coefficient similar to that proposed by
van Wijk and deVries (1954), and Makkink and van Hermst (1956). Most
crop coefficients are derived experimentally and represent the combined
relative effect of the resistance of water movement from the soil to the
various evaporating surfaces and the resistance to diffusion of water
vapor from the surface to the atmosphere, and the relative net radiation
as compared with the reference crop (Jensen, 1968). New procedures
separate estimates of evaporation and transpiration with (T -I- P) - E .
The daily crop coefficient, K is adjusted for the wetness of the soil
surface and for decreasing soil moisture as follows:
K = K K + K [15]
c co a s
where K is the expected crop coefficient based on experimental data
where soil water is not limiting and normal plant densities are used;
and K is a relative coefficient related to available soil water. The
a
USDA-ARS computer program assumes K to be proportional to the loga-
3.
rithm of the percentage of remaining available soil water (AW):
K = In (AW + l)/ln 101 [16]
a ~~~"
K is the increase in the crop coefficient when the soil surface has
S
been wetted by irrigation or rainfall. The maximum K value normally
will not exceed 1.0 for most crops, except when short grass is used as
the reference crop. Values for K for the first, second, and third
s
day after a rain or irrigation are estimated in the USDA-ARS computer
program as fo]
respectively.
the reference crop. Values for K for the first, second, and third
s
i £
program as follows: (0.9 - K )0.8; (0.9 - K )0.5; and (0.9 -K )0.3,
0 CO CO CO
37
-------
The USDA-ARS computer scheduling program has also been modified for
scheduling irrigations with center-pivot sprinkler irrigation systems and
evaluated in the eastern Colorado area (Heermann, et al., 1973).
There are much more complicated models that have been proposed for
managing water resources, which use linear and dynamic programming.
Several extremely important assumptions must be carefully considered if
models that incorporate modern irrigation science and technology are to
be used to improve irrigation water management. The individual or ser-
vice group utilizing the models must thoroughly understand how the
models operate, and service groups also must have access to facilities
and input data that may be required by complex models. Neglect of these
basic assumptions is probably the major reason why very few complex
models have been utilized to date in practice. Individuals involved in
developing models for management purposes should first determine who
will be using the models and the probable input data that will be availa-
able. Also, the degree of refinement relative to the ability to control
the variables in the field, in addition to the degree of complexity that
a service group or the farm manager/operator will accept, must be con-
sidered.
The general interrelations involved in irrigation water management are
illustrated in Figure 4. Currently, most new developments in irrigation
science and technology are assimilated by groups like the Extension Ser-
vice and the Soil Conservation Service. These groups develop and pro-
vide general irrigation guides to farm manager/operators. As irrigation
science and technology continue to advance, it becomes more and more
difficult to provide comprehensive irrigation guides that are readily
understood by busy farm manager/operators to achieve greater control and
improved management of irrigation water. As labor costs increase and
becomes less available, the available time they have to pursue modern
technology may decrease. Thus, commercial or agency irrigation schedul-
ing services will be playing a more significant role in applying modern
irrigation science and technology in the day-to-day irrigation water
38
-------
management activities.
The professional staff of irrigation scheduling service groups utilize
currently available science and technology, and periodically measure the
soil water status and sometimes record precipitation on each farm that
is served. Expected precipitation is estimated and irrigation dates and
amounts are recommended to the farm manager/operator. These recommenda-
tions are updated at least once weekly and the fields are inspected
about once weekly during the summer growing season and once every 2
weeks during the winter season. Some irrigation scheduling groups pro-
vide professional guidance or irrigation system operating or redesigning
recommendations so that the farm manager/operators can achieve better
irrigation water control. Service groups must be technically competent,
provide economical service, maintain basic farm data and records related
to current and previous irrigation practices, maintain active communica-
tions with farm manager/operators, and periodically verify the estimated
soil water status of the fields utilizing trained and experienced tech-
nicians .
NEED FOR IMPROVED IRRIGATION SCHEDULING
Figure 5 illustrates a typical example of recent irrigation practices on
a single field in southern Idaho which resulted in a low seasonal irri-
gation efficiency. This example is not too different from current prac-
tices on most irrigated projects. The first two irrigations were ap-
plied because young seedlings needed water, but only a small amount of
water could be retained in the root zone. Less water was applied in the
third irrigation than the soil would hold, which was followed by an ex-
cessive irrigation, apparently in an attempt to assure refilling the
soil profile. The fourth irrigation resulted in a large amount of deep
percolation and a water application efficiency of only 46%. If improved
irrigation water management is to influence return flows, the quantity
of water applied must be measured or controlled so that the amount of
water applied is limited to or less than that which has been depleted
39
-------
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42
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APRIL MAY JUNE JULY AUG.
FIG 5. TYPICAL EXAMPLE OF GRAVITY IRRIGATION
SEPT
-------
by evapotranspiration. If less water is applied than has been depleted
at each irrigation, sufficient available soil water must make up the
balance of water needed for crop growth during the growing season. The
entire root zone normally must then be filled some time before the next
crop by off-season precipitation, preplant irrigations, or both.
OTHER RECENT PROPOSALS TO IMPROVE IRRIGATION SCHEDULING
Buras, et al. (1973) developed a computer program similar to that des-
cribed above for planning irrigation schedules before the irrigation
season and for updating schedules within the irrigation season. This
program considers the hydraulics of the distribution system, the irriga-
tion methods, climate, farming practices, and general farming conditions.
The planning operations are generally based on monthly data.
Woodruff (1968) and Woodruff, et al. (1972) described irrigation schedul-
ing procedures recommended for irrigating corn which were developed for
Missouri, an area with significant summer precipitation. Woodruff, et
al., observed that the highest corn yield since 1888 was obtained in
1965. They analyzed the precipitation distribution and soil water dur-
ing that year and found that the surface soil water was always within
the tensiometer range, but soil water at lower depths was progressively
depleted. The average depletion rate was 4.6 mm/day (0.18 in/day).
They observed that annual precipitation in Missouri varied about 34%.
while pan evaporation varied only 12%, thus precipitation was the pri-
mary variable involved in their procedures. Basically, the recommended
amount of water to be applied is less than that which could be held by
the soil. This procedure, which can be considered "deficit irrigation,"
has also been evaluated under arid conditions (Miller and Aarstad, 1975).
It is similar to irrigating in alternate furrows and not completely fill-
ing the soil to field capacity at each irrigation, or irrigating with
center-pivot sprinklers whose average application rate is less than the
average evapotranspiration rate. The Missouri procedure is started when
the corn is 30 to 45 cm high and the soil water deficit is about 50 mm.
41
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Wilcox and Brownlee (1969) summarized many years of experimental work in
developing irrigation scheduling procedures in British Columbia. The
idea of scheduling irrigations, utilizing some measure of climatological
data, began in the early 1950s, after publications like those of Penman
(1948) and Thornthwaite (1948). Because of the availability of evapora-
tion data, although they considered the Class A pan to be too cumbersome,
they elected to use Bellani plates as a measure of climatic effects
(Korven and Wilcox, 1965). A balance sheet procedure was developed
similar to that proposed by others for recording water use, rainfall,
and critical soil water depletion after the first irrigation. The evapo-
transpiration rate relative to measured evaporation must be calibrated
for each of the various crops. Wilcox and Brownlee emphasized that many
growers wait too long and they apply too much water, and that scheduling
does not compensate for inadequacies in the irrigation system. One tech-
nique for communicating information on evapotranspiration and precipita-
tion to growers involved using blackboards along the main thoroughfare
where farmers could stop and record rates of evapotranspiration and
daily precipitation. A detailed description of these procedures, in-
cluding estimates of net irrigation water requirements, precipitation
lost by deep percolation or runoff, or both, and risk analyses, can be
found in a recent technical bulletin (Wilcox and Sly, 1974).
Hagood (1964) described similar procedures in use in the Columbia Basin
of Washington based on Class A pan evaporation. Many years of experi-
mental work have been invested in developing crop coefficients after
full crop cover for use with the Class A pan in Washington.
Brosz and Wiersma (1970) developed an accounting procedure for eastern
South Dakota to assist farmers to determine when and how much water to
apply. This procedure was based on average expected evapotranspiration
rates for corn and alfalfa. Busch. (1971) developed a computer program
that uses weather forecasts for scheduling irrigations on cotton. Hiler
and Clark (1971), and Hiler, et al. (1974) developed procedures for sche-
duling irrigation utilizing a stress-day index. The index was based on
42
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a coefficient related to the crop's susceptibility to stress and a
parameter characterizing the magnitude of daily stress. The magnitude
of daily stress was derived either from direct plant measurements or
from one of several methods of estimating potential evapotranspiration.
Allen and Lambert (1971a) described concepts for a general model to be
used for deciding whether to irrigate or postpone an irrigation based
on irrigation costs and probable yield losses. The objective was to
minimize seasonal losses and costs, and calculate the corresponding
risk. They tested this program, using data for three growing seasons
and found that the new criterion resulted in less total cost and better
utilization of water than the 50% depletion criterion (Allen and
Lambert, 1971b).
Corey and Franzoy (1974), who operate a commercial irrigation scheduling
service company in the Central Great Plains and the Arizona-New Mexico
area, use their computer program to assist their technicians in making
field decisions. After several years of experience, their mode of
operation involves periodic visits to each farm serviced to evaluate
field conditions. At each visit, a recommendation sheet requires the
experienced technician to write in the date of the last irrigation, to
estimate the present depletion of soil water, and enter the estimated
next irrigation date for each field from weekly updated estimates of
evapotranspiration and projected irrigation dates for various crops.
Instead of giving the quantity of water to be applied, they give the
hours of set for either a furrow or sprinkler system and make other gen-
eral recommendations.
There are many other proposals for scheduling irrigations and proposals
for optimizing the use of water from surface reservoirs, like that des-
cribed by Huang, et al. (1975). But, to my knowledge, water use opti-
mization programs are not being used in day-to-day water management
practices in the western USA.
43
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Many of the current procedures used by irrigation scheduling service
groups utilize general procedures described by Jensen (1969). The com-
puter program, described by Jensen, et al. (1970, 1971), was released
in 1970 and updated in 1971, and has formed the basis for many of the
computer programs used by service groups.
SIMULATION MODELS FOR IRRIGATION SCHEDULING
The preceding discussion illustrates the trend toward using models that
simulate the soil water status. A digital simulation model of the soil
water reservoir enables a service group to estimate daily changes in
soil water since the last field inspection and to project future changes
based on expected climatic conditions, growth stage, and direct changes
in soil water due to precipitation. A simulation model that includes
all aspects of water management on a farm becomes very complex since
water supplies, cultural practices, alternative costs, soil type, crop
responses to soil moisture and climate, plant density, plant nutrient
levels, etc. all become involved. The development of a simulation model
requires the consideration of the developmental costs, operational costs,
the costs of obtaining the necessary current input data, and the avail-
ability of computer facilities on which to run the simulation model.
Many service groups operate on a relatively small scale and cannot jus-
tify using complex simulation models. Experience by irrigation schedul-
ing groups also has shown that it is difficult to prove direct economic
benefits from using their services as compared with current scheduling
practices where water supplies are plentiful and perhaps overirrigation
is the rule rather than the exception. Many farm manager/operators,
however, believe the guidance provided and field monitoring is worth a
significant part of the fee charged.
The simulation model is mainly needed to estimate the change in soil
water, since the field was last irrigated or inspected, to forecast the
next irrigation date. If general recommendations are made before plant-
ing specific crops, then the simulation model could utilize historical
44
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data to evaluate alternative management practices for planning purposes.
But, once seasonal decisions have been made and crops planted, the simu-
lation model is used mainly between irrigation intervals. Where water
supplies are very limited, decisions to irrigate perhaps only one to
three times during the season, or not at all, may be made early in the
growing season. Under these conditions, estimates may need to be up-
dated only once or twice during the growing season. These procedures
are used where rainfall is significant and where more frequent irriga-
tion may not be profitable.
The main use of a simulation model under semihumid conditions may be to
estimate when irrigations must begin after a general rain to complete
the next irrigation on all fields before soil water deficits become
severe and crop yields or quality decrease. Typically; farm manager/
operators wait too long before they begin to irrigate after a general
rain. This problem is very common in areas where summer precipitation
provides much of the water required by crops. Lembke and Jones (1972)
used a simulation model to evaluate when to begin irrigating in humid
areas.
Irrigation scheduling utilizing simulation models has also enabled sche-
duling center-pivot sprinkler systems so that they operate only during
off-peak electrical load periods. Stetson, et al. (1975) have shown
that the peak power demand by an irrigation district can be reduced if
center-pivots are scheduled to operate when industrial, home, and air
conditioning loads are below normal. Simulation models have also been
used to determine design capacities for sprinkler systems where rainfall
is significant (Heermann, et al., 1974; Stegman and Shah, 1971).
Numerous related simulation models have been developed during the past
5 years. Yaron and Strateener (1973) developed a very detailed simu-
lation model to estimate yield functions and determine optimal irriga-
tion policies under stochastic rainfall conditions. This model was
developed utilizing 4 years of experimental data in Israel and would be
45
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very site-specific. Its main benefit would be for use in years of low
rainfall. Mapp, et al. (1975) developed a very detailed, but site-
specific, simulation model of soil water and atmospheric stress-crop
yield relationships for economic analyses in the central basin of the
Ogalala formation of the Southern High Plains of Texas, the Oklahoma
panhandle, and southwestern Kansas. The model was developed for a spe-
cific soil, but is based on many years of experimental data for grain
sorghum, winter wheat, and corn collected in the general area. This
simulation model was tested, utilizing 20 years of data to evaluate
alternative irrigation strategies like the combination of dryland and
irrigated farming and the effects of reduced pumping on crop yields.
Empirically derived rainfall probabilities for 2-week periods are used
along with an estimate of the initial soil moisture at the beginning of
the growing season. The crop yield model is a function of the soil
moisture, the soil moisture depletion during various stages of growth,
and a measure of atmospheric stress based on pan evaporation relative to
critical pan evaporation. This model could be very effective in deter-
mining optimum management decisions for nonirrigated and irrigated farm-
ing conditions in the area.
Dudley, et al. (1971) used a two-state dynamic programming model to eval-
uate optimal intraseasonal irrigation water allocations. The decision
variable involved the available soil water depletion level. Evapotrans-
piration was estimated as the function of pan evaporation. This particu-
lar model lacked verification with experimental data, and some of the
biological assumptions may not have been based on extensive available
experimental data. A very comprehensive computer simulation model of
cotton growth was developed by Stapleton, et al. (1973). The developers
of this model indicated that by using this model a knowledgable manager
will broaden his information base and complement his experience in manag-
ing the crop for profit.
Simulation models utilize many different techniques for arriving at the
needed estimates. Similarly, operators of commercial firms providing
-------
ISS utilize many different techniques for modifying simulation models
for scheduling purposes. Further discussion of simulation models for
soil water and crop production can be found in Agricultural Meteorology,
Vol. 14, pages 229-320, 1974.
The demand for climatological data to provide irrigation scheduling ser-
vices using simulation models also has provided new challenges for mete-
orologists of the U. S. Weather Service. For example, a 2-year evalua-
tion of the accuracy in forecasting climatological data versus the accu-
racy of forecasting a change in evapotranspiration at Kimberly, Idaho,
indicated that evapotranspiration estimates calculated with forecasted
climatological parameters were more accurate than direct estimates by
the meteorologist of above or below normal potential evapotranspiration.
The direct estimates were less accurate because the meteorologist esti-
mated expected changes in radiation, windspeed, temperature, and humidity
more accurately because of his many years of experience in meteorology
than estimates of the composite effects on potential evapotranspiration.
Simulations used in irrigation scheduling also may involve very simple
relationships. For example, one company that has provided irrigation
scheduling services for many years in the Columbia Basin routinely takes
gravimetric soil samples, plots these values on a master chart along
with direct measurements of water applied with center-pivot sprinkler
systems. Periodically, the master chart of soil water is updated and a
copy mailed to the farm manager/operator with recommendations for in-
creasing or decreasing the daily rate of water application. In addition,
recommendations for plant nutrients and other aspects of the system
management also are made on the chart.
EXPECTED IMPROVEMENTS IN OPERATIONAL SIMULATION MODELS
Improved simulation models will be adopted by service companies if models
are very general. A company must have coefficients for plant growth rate,
crop cover development, maturation, etc. for 10 to 15 crops before it can
47
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justify a major revision of an existing operating computer program. The
actual cost of running the simulation model is a minor part of the cost
of providing irrigation scheduling services. The bulk of the cost is
in the professional staff, trained technicians, and travel costs associ-
ated with weekly monitoring of each field served.
There will be many minor improvements in the energy balance-evapotrans-
piration model, particularly improvements in estimates of net radiation,
vapor pressure deficit, and the wind function for use with the combina-
tion equation. In addition, estimates of soil temperature, or actual
measurements of soil temperature can be expected in the near future.
Another significant change to be expected is to separate daytime wind-
speed and vapor pressure from nighttime values, which should signifi-
cantly improve the accuracy of estimating potential evapotranspiration.
Equations have been developed to offset the contributions to evapotrans-
piration from the saturated zone where a shallow water table exists.
Some of these equations have been adapted to computerized scheduling
programs, but the field technicians have not yet gained full confidence
in adjusting the proper coefficients to match conditions in the field.
In addition, in the near future crop production functions probably will
be incorporated into most computer programs along with programs for esti-
mating the plant nutrient status and predicting nutrient deficiencies.
The first major improvement in the scheduling programs will be to sepa-
rate evaporation from transpiration, which will greatly improve the ac-
curacy of existing models from the time of planting to the development
of full crop cover. This is the principal area where the current simu-
lation models seem to overestimate evapotranspiration and where exist-
ing models do not properly respond to adverse climatic conditions which
may significantly delay germination, emergence, and development of leaf
area.
Ritchie (1972) presented a model for separating evaporation from trans-
48
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piration. In this model, evaporation from the soil surface during the
first stage is controlled by climatic conditions. After a given amount
of water has evaporated from a soil type, the cumulative evaporation
increases proportionally with the square root of time. Net radiation
that reaches the soil surface is estimated by considering the increase
in leaf area and the absorption of solar radiation by the foliage. Sci-
entific literature during the recent past 5 years contains numerous mea-
surements of leaf area for crops like soybeans, grain, sorghum, corn,
and alfalfa. With the data available in the literature, generalized
models for evaporation and transpiration are being developed which can
be substituted into existing computer scheduling programs in place of
the experimentally derived crop coefficients. Improvements in the square
root of time evaporation equation coefficient during the transition
period are expected as the result of a paper recently 'prepared by Jackson,
et al. (1975). The square root of time evaporation coefficient also
seems to be related to soil temperature-vapor pressure relationships.
Other techniques for estimating evaporation that will be considered are
those of Gardner (1973), and Staple (1974). Similarly, Hanks (1974)
developed a model to predict plant yield as influenced by water use,
which separates evaporation and transpiration and relates plant growth
to transpiration. This concept will form the basis of significant im-
provements in irrigation scheduling models. Separating evaporation from
transpiration and including the effect of soil salinity like that pro-
posed in the model developed by Childs and Hanks (1975) also will be
considered.
49
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SECTION VII
SCOPE OF IRRIGATION SCHEDULING SERVICE IN 1974
SURVEY CONDUCTED
A survey of irrigation scheduling services (ISS), which were described in
Section IV, was conducted to determine the scope and nature of services
provided in 1974 following the release of the USDA-ARS computer program in
1970. The survey was restricted to those agencies and commercial firms
providing day-to-day and week-to-week decision-making information to farm
manager/operators. Many service groups also provide engineering services
and general recommendations to improve irrigation supplies, improve facil-
ities to deliver and distribute water over the fields, and other services
like soil and plant analyses, pest management, etc. Agencies or commercial
service groups that contacted farm manager/operators or visited individual
fields less than three or four times per season were not considered to be
providing irrigation scheduling services as defined. Only those groups
that provided current information weekly and visited farms or fields one
to three times per week during the summer growing season were included in
this summary.
The scope and nature of ISS provided in 1974 are summarized under two cate-
gories: (1) professional or commercial services; and (2) agency, produce
company, project or district services. The principal differences between
the two categories is that the first group represents independent, private
entrepreneurs who provide this service for a fee. They could also be con-
sidered as consultants to farm manager/operators. Commercial groups are
competitive and must remain competitive to remain in business and operate
at a profit, which stimulates new techniques and personalized service. The
consultants essentially work for the farmers' best interests. As consul-
tants, they normally should not sell products or receive commissions on
products being sold, since this represents a basic conflict of interest.
50
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Several commercial groups seem to have some conflict of interest.
Groups in the second category provide similar services, but farm manager/
operators receiving ISS may be required to either pay only a portion of
the costs, or all of the costs are borne by the service group. This
category, however, represents an arrangement whereby a group of small
farmers can obtain ISS since commercial firms prefer to select mainly
the large farm manager/operators.
The survey was believed to have reached a high percentage of the larger
commercial firms, but not all of the smaller independent firms operating
in the California area. Most of the questionnaires sent to groups in the
western USA were returned, including several sent to Alberta, Canada.
General characteristics of these groups are summarized in Table 1. Addi-
tional details are presented in Appendix Tables Al and A2.
PROFESSIONAL IRRIGATION SCHEDULING SERVICES (COMMERCIAL)
The commercial ISS groups contacted in the western USA had 1 to 19 years
of experience. Of the 10, seven had 5 years or less. ISS were provided
for a fee to about 4,450 fields involving over 100,000 ha (> 250,000 ac)
of summer crops in 1974 in eight western states. All 10 commercial firms
also provide plant nutrition services, seven provide pest management ser-
vices, six provide engineering services to either improve the farm irri-
gation system or its operation, or improve the system so that it could be
operated to provide uniform distribution of the desired amount of water.
Gravimetric soil samples (either by weighing and drying in an oven, or
by the "speedy" method which involves the addition of calcium carbide
to weighed sample of the soil placed in an enclosed cylinder which pro-
duces acetylene gas in proportion to the soil water content) were used
to monitor the soil water level or schedule irrigation on about one-half
of the area. Estimated evapotranspiration, based on current climate and
crop data, was used to estimate changes in soil water on about 90% of
51
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Table 1. CHARACTERISTICS OF GROUPS PROVIDING IRRIGATION SCHEDULING
SERVICES IN 1974 IN WESTERN USA
Parameter, total or Commercial service Agency service
weighted averages for a "fee" without direct fees
Experience, years
Size of operation:
No. of fields
Total area (summer crops) ha
Services provided:
Irrigation scheduling, %
System improvements, %
Plant nutrition, %
Pest management, %
Scheduling method used:
Gravimetric samples, %
Tensiometers or blocks, %
Pan evaporation, %
Climatic estimates of Et, %
Monitoring method used:
Gravimetric samples, %
Tensiometers, %
Probe and "feel," %
Plant symptoms, %
Area p*ir field technician, ha
Average daily round trip , km
Field visits per week
5.6
A, 477
102,100
100
60
100
56
48
3
34
90
27
37
83
12
2,350
190
1.5
5.5
3,465
53,640
100
14
23
18
19
7
42
100
26
15
100
2,370
110
1.1
the area served. Evaporation from a USWS Class A pan was used on about
33% of the area, and; tensiometers alone were used to schedule irriga-
tions on about 5% of the area. Since most firms use more than one
method, these percentages total more than 100%.
The area served by an irrigation technician, or the professional when
he worked alone, averaged 2,350 ha (5,800 ac), and his average daily
round trip to monitor fields was 190 km (120 mi). All fields served
52
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were visited an average of 1.5 times per week.
Some of the main reasons customers gave for continuing to use commercial
ISS were: (1) improved water management; (2) increased yield and/or
quality; (3) lower production costs; and (4) general satisfaction with
the service. Some of the main reasons customers gave for discontinuing
commercial ISS were: (1) unable to see a direct benefit or value; and
(2) poor service and communications, or because they learned nothing new.
Major problems encountered by commercial ISS groups in providing their
services were: (1) lack of farm managers' confidence the first year;
(2) soil variability; (3) difficulty of arranging discussions with cus-
tomers; (4) lag time in getting the results of soil and plant tissue
analyses from state laboratories; and (5) difficulties with getting
gravimetric soil samples and finding reliable employees to obtain the
samples. Additional information on commercial groups can be found in
Table Al.
AGENCY, CANAL COMPANY, PROJECT OR DISTRICT IRRIGATION SCHEDULING SERVICES
Many groups have initiated ISS on a developmental basis and several dis-
tricts have added ISS as a routine service to their members or the water
users they serve. Tax supported agencies providing ISS on a develop-
mental basis assume that costs of the services will eventually be borne
by the irrigation districts involved, or commercial firms will become es-
tablished in the area, or a joint agency/commercial arrangement will be
established to satisfy the needs of the water user/farm manager.
Two state agencies, one federal, the U. S. Bureau of Reclamation (USER)
operating at 16 locations, three produce companies, and one irrigation
project had 1 to 10 years of experience. Of the 22, 21 had 5 years or
less. ISS were provided without direct costs to the farmer on about
3,500 fields involving over 54,000 ha (133,000 ac) of summer crops in
1974 in 12 of the 17 western states. About 25% of these groups also
53
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provide plant nutrition services, 20% provide pest management services,
and about 15% provide engineering services .
Gravimetric soil samples were used to monitor and schedule irrigations
on about 25% of the area, but evapotranspiration estimates based on cur-
rent climate, crop, and soil data were used on 100% of the area served.
Pan evaporation and crop coefficients were used along with climate-based
estimates on about 40%, and tensiometers were used on about 7% of the
area. The average area served per technician was about the same as for
the commercial groups, but the average daily round trip to monitor fields
was less, about 110 km (70 mi) because specific areas were selected for
these developmental efforts. All fields were visited an average of 1.1
times per week.
Some of the customers' main reasons for continuing to use the free ISS
were: (1) improved water management; (2) increased yield and/or quality;
and (3) lower production costs. Some of their main reasons for discon-
tinuing ISS were: (1) scheduling did not fit operations; (2) they could
do as well without it, or did not have time; and (3) service and communi-
cations were poor, or they learned nothing new.
Major problems encountered by agency ISS groups in providing their ser-
vices were: (1) communications; (2) lack of farm managers' confidence
the first year; (3) unknown amount cf water applied or stored in the
soil during an irrigation; (4) unavailability of trained personnel and/
or temporary summer employees who lacked the desired incentive; (5)
getting farm managers to modify current practices; and (6) obtaining
water when needed. Additional information on agency ISS groups can be
found in Table A2.
GENERAL INFORMATION ABOUT ALL USA ISS GROUPS
Field-by-field ISS were provided to 7,900 fields and over 155,000 ha
(> 385,000 ac) of summer crops in 1974. In addition to field-by-field
54
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services, the USER has been trying two other types of services. One
provides weekly estimates for each major crop on each farm and are based
on early, medium, and late planting dates, and the general soil type in-
volved. This service was not accompanied by regular scheduled visits by
irrigation technicians. General irrigation guides for major crops in an
area which are not accompanied by regular farm visits were also provided.
The USER provided current weekly farm and general irrigation guides for
about 10,000 fields involving about 94,000 ha (233,000 ac) in 1974.
Estimates of evapotranspiration from which the current soil water status
is estimated, and the projected next irrigation date were based on some
measure or index of the current potential evapotranspiration rate (E )
tp
and crop coefficients. Crop coefficients relate the evapotranspiration
rate for a given crop to the potential rate. These coefficients vary
with the stage of crop growth and most computer programs adjust the co-
efficients for the wetness of the surface soil after a rain or an irriga-
tion. E estimates for 57% of the area were based on the Jensen-Raise
tp
equation, 37% on the Penman equation, 7% on long-term E means for the
region, and about 3% on evaporation from the USWS Class A pan. Since
groups alternated between methods, these total more than 100%. The major
improvement requested by ISS groups was more accurate crop coefficients
or curves, especially early in the season. Better techniques for auto-
matically adjusting these curves, as the crop develops, are also needed.
Currently a time-based method is used by most groups to adjust the crop
coefficient.
AGENCY ISS IN CANADA
Similar agency ISS were provided by the Alberta Department of Agriculture
in Canada. About 140 fields and 4,500 ha (11,000 ac) were scheduled in
1974 on a field-by-field basis and guides were provided to about 100
fields totalling about 3,200 ha (8,000 ac). Evapotranspiration was esti-
mated, using atmometers and crop coefficients. Plant nutrition and pest
management services were not provided, but services were provided for im-
55
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proving the operating efficiency of irrigation systems. Reasons for
continuing or discontinuing the services and problems encountered were
similar to those reported in the USA.
ACCEPTING OR REJECTING ISS
Table 2 provides additional information on the reasons for continuing or
discontinuing ISS. Reasons listed are basically the same for both com-
mercial and agency service groups. Some exceptions are reasons for dis-
continuing services (items 4 and 5).
CLIMATIC DATA
Table 3 summarizes comments on the adequacy of climatic data. These
comments indicate that improved procedures are needed to facilitate
access to good climatic data.
COMMUNICATIONS
Table 4 summarizes the principal mode of communications used between
various personnel in the ISS groups. Telephone, oral, and written pro-
cedures are used extensively. Two-way radios for technical communica-
tions and data transmission by telephone probably will be used more ex-
tensively in the future.
MAJOR PROBLEMS ENCOUNTERED
Many problems were listed, but these should not be considered as a deter-
rent to future growth, nor should they be considered as roadblocks to ISS.
Instead, they represent actual conditions that any relatively new service
group probably will encounter to some degree. Also, many problems listed
were essentially the same as those which farm manager/operators encounter
daily and have learned to cope with to some degree. For example, soil
variability is very real, and if a field is treated as a single homogene-
56
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Table 2. REASONS CUSTOMERS GAVE FOR CONTINUING OR DISCONTINUING IRRIGA-
TION SCHEDULING SERVICES
Frequency as listed by service groups
Commercial service Agency service
Reasons for for a "fee" without direct fees
Continuing scheduling service:
1. Improved water management 4 11
2. Increased yield and/or quality 7 5
3. Lower production costs including 4 3
water
4. Satisfied, good service, etc. 3
5. Consultants available for other 2
problems
6. Educational 1 1
7. Miscellaneous - curious, peace of 1 5
mind, no charge, etc.
8. None 1
Discontinuing scheduling service:
1. Believe no direct benefit or value, 4 2
fee too high, or not reducing
operating costs
2. Learned nothing new, confusing, poor 3 3
service, or poor communications
3. Does not fit operations, water on 1 4
turn basis, etc.
4. None requested to be discontinued 5
yet
5. Can do as well without service, or 4
do not have time to follow
6. Sold farm or lost lease 1 1
7. No longer in high-value crops 1 1
8. Poor yield due to other factors, 1 1
like disease, etc.
9. Works only with sprinklers 1
10. Documenting water use for possible 1
adverse action
57
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Table 3. ADEQUACY OF CLIMATIC DATA FOR SCHEDULING PURPOSES.
Frequency as listed by service groups
Responses to "How can better Commercial service Agency service
climatic data be provided for you?" for a "fee" without direct costs
Comments:
1. No comment 3
2. Need more readily accessible data, 3
perhaps on call, regularly mailed,
in newspaper daily, or computer
readout
3. Adequate 1
4. Need better solar radiation data
5. Need more, and alternative or 1
localized stations
6. Need more comprehensive reports 1
7. Should purchase equipment and read 1
own instruments
8. Expect local station to lose solar
radiation site - concerned
9. Have station provide net radiation
10. Need more rain gauges
11. Need better forecasts related to
irrigation needs.
9
3
3
2
3
1
1
1
1
1
Table 4. PRINCIPAL MODE OF COMMUNICATIONS USED, PERCENTAGE OF ALL
GROUPS.
Nature of
communication
To field
technicians
To fanners
From farmers
To field offices
Oral
81
74
81
81
Written
42
68
23
56
Telephone
.
1 peicenc
48
61
68
81
Two-way Via computer
radio terminals
13
6
3
13
10
19
58
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oua unit, irrigation scheduling recommendations must represent the best
technical judgment for the situation. In some cases, the irrigation
system might be operated so as to use only the upper increment of the
available soil water storage capacity if large differences exist within
a field. Many times the allowable depletion of soil water between irri-
gations is determined by the irrigation system's capacity to apply water,
and the soil's water-holding characteristics have little to do with ir-
rigation scheduling.
IMPROVEMENTS OR NEW DEVELOPMENTS NEEDED
Needed improved crop curves were listed as a high priority. Studies are
now underway to adapt plant growth models and procedures that separate
evaporation and transpiration in estimating evapotranspiration which
will alleviate many of the problems associated with crop curves.
The need is urgent for an improved, rapid technique for assessing soil
moisture content throughout the soil profile. Obviously, improved irri-
gation systems that apply known amounts of water also would help im-
mensely to alleviate this problem.
FIELD MONITORING
None of the service groups currently use remote sensing or make visual
observations of fields from aircraft. Because of the large travel re-
quirement some commercial groups will probably begin using helicopters
to rapidly scan the farm fields, initially perhaps only to make a visual
overall appraisal of crop growth, uniformity, etc. The airplane or
helicopter pilot, or an observer, could then either radio his observa-
tions to an irrigation technician operating from a land vehicle in the
area to check portions of those fields that need immediate attention.
If a helicopter is used, it could land and the pilot could make a first-
hand appraisal of the situation. Eventually, remote sensor/recorders
probably will be used in aircraft to evaluate soil surface and plant
59
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temperatures.
ATTITUDES TOWARD ADOPTING ISS
A comprehensive study of attitudes toward new water use practices also
was conducted in 1974 by the University of Idaho with particular empha-
sis on the adoption of irrigation scheduling (Carlson, 1975). This re-
search study was supported by the U. S. Bureau of Reclamation. A trained
graduate student administered a questionnaire in an interview format to
187 farmers, or about 50% of the farmers in the A & B Irrigation Dis-
trict owning 20 ha (50 ac) or more. Attitudes toward the USBR's three
levels of scheduling services (field-by-field, and farm and irrigation
guide methods) were evaluated considering two groupings, farm and guide
users. One result of this study that is of particular concern to ISS
groups is the farmers source of new information or the best place for
him to obtain irrigation advice. As their best source of new informa-
tion, about 44% placed a high premium on peers (neighbors and friends),
about 22% indicated the irrigation district, 15% the Extension Service,
14% the Agricultural Experiment Station, and only about 2% listed pri-
vate consultants. The principal difference between the two types of
service offered was that the guide user would likely tell a new farmer
to see his neighbor and friends, wuile more farm users would tell a new
farmer to see the irrigation district or the Agricultural Experiment
Station. Most farmers (65%) preferred periodic workshops as a means of
obtaining new farm information, with farm demonstrations listed next
(18%), followed by trained personnel on the farm (11%).
A key element in accepting a technical service is in recognizing the
need for such services. The survey showed that farmers who recognize
their irrigation problems center around the amount and timing of water
application are more likely to be more involved in an ISS program.
"Thus, perception of the problem precedes adoption of tech-
niques to alleviate the problem" (Carlson, 1975).
60
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Another important issue is the ability to control water application and
the adoption of a scheduling service. ISS seems to be more easily
adapted to sprinkler irrigation since substantially more farm users have
sprinkler systems than do guide users. This may have other significant
implications, however. Service groups providing ISS will need to monitor
the soil water level more closely on surface irrigated farms because it
is more difficult to estimate the amount and uniformity of water applied
with surface systems. Carlson (1975), in citing Crouch (1972), stressed
another key element concerning the adoption of new practices. Many pro-
posed new practices are not adopted because Research and Extension fail
to recognize farmers' specific needs. Technology for ISS was developed
because decades of experience had shown that the traditional recommenda-
tions for timing and scheduling irrigations were inadequate for the
farmers' needs. Service groups must not lose sight of the fact that
unless they continue to provide farmers' needs and improve their product
(service), demand for their product will decline.
GENERAL COMMENTS
The more experienced ISS groups still encounter problems explaining the
function, purpose, and need for an irrigation management service, espe-
cially in new areas. Some groups feel they must first convince employees
of agricultural agencies that irrigation scheduling problems exist and
that an ISS program is needed. Agency ISS groups particularly are con-
cerned about documenting that ISS can result in increased net returns to
the farmer; whereas, the increasing demand for ISS should be sufficient
to gage the value of ISS to farm manager/operators. In some areas,
action agency and Extension Service groups have taken the lead in pro-
moting new technology and have readily recognized potential benefits.
In other areas, individuals perhaps do not recognize the need and prob-
ably are not equipped to provide these services themselves with the in-
tensity needed. Many individuals have too many other responsibilities,
do not have the necessary training or support staff, and may be reluc-
tant to take on the demands and responsibilities of making actual de-
61
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cisions and recommendations throughout the irrigation season. Commer-
cial ISS groups must be "available" or on call at all times. In addi-
tion, the Extension Service in most states probably could never obtain
adequate funding to secure the necessary staff to provide the needed
ISS, but they can play a major role in aiding commercial and agency
groups in becoming established and improving their services because they
have access to experimental data, and soils and crop information, etc.
A very common problem encountered by many ISS groups is in finding and
training qualified employees. As expected with a relatively new service,
many potentially valuable employees do not have prior experience, and
many current employees need more experience. Probably one of the most
frustrating problems encountered, especially for the newcomer, is coping
with nonuniform soils and water applications. Also, communicating essen-
tial technical information to the farmers each week is difficult. Better
communication techniques are needed.
The more experienced groups believe that irrigation recommendations
should be made while visiting the farmer, but only "after" thoroughly
checking the fields that are serviced. This is especially important when
the amount of water applied is unknown. Experienced personnel use esti-
mates of past and expected evapotranspiration and irrigation intervals,
along with observations of soil moisture in each field. This procedure
limits the area that an employee can serve.
As previously stated, better crop curves, especially early in the season,
are urgently needed. Also, these curves must reflect current growing
conditions and not be based solely on time. Presently, several investi-
gators are working on evaporation-transpiration models which greatly im-
prove evaporation estimates, and changes in the transpiration component
will be dependent on the current crop growth rate. These models should
reduce or eliminate the early season crop curve problem. A closely re-
lated problem involves guidelines for determining or prescribing optimum
soil moisture depletion levels at all growth stages and for various soil
62
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types. An ISS should emphasize increased net returns, not maximum yields.
A commercial ISS group is primarily working for the farm manager/operator,
and increased net returns is the primary reason he is seeking ISS.
Finally, implementing ISS for some 16 million irrigated ha (40 million
ac) in western USA is a tremendous undertaking. Developing the technology
plays a significant role, but it does not assure a viable ISS operation.
Organizations with qualified and trained personnel will be playing a vital
role in implementing better irrigation water management. The current
capacity of existing ISS organizations could serve about 242,000 ha
(600,000 ac) on a field-by-field basis in 1975. This represents a tre-
mendous increase during the past 5 years, and represents the beginning of
a new era in irrigation water management. Feedback from ISS groups also
will greatly aid in improving ISS technology during the next 5 years.
63
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SECTION VIII
EFFECTS OF IRRIGATION SCHEDULING ON SALINITY OF RETURN FLOWS
IRRIGATION WATER MANAGEMENT AND RETURN FLOW QUALITY
The average volume of irrigation water delivered annually to many irri-
gated farms on western USA irrigation projects, W , plus the average
annual precipitation, P, or (W + P) is greater than the annual
volume of water used consumptively, W . The prime justification water
users and project management give for continuing this practice of (Wf
+ P) » W is to maintain a salt balance in the soil. A favorable
cu
balance of the more soluble salts is necessary (except for carbonate and
gypsum minerals) for sustained high crop production on irrigated lands.
However, sustained crop production probably can be maintained with much
less (W + P) as compared with present practices (van Schilfgaarde, et
al., 1974).
A current popular assumption is that since Wf is much greater than
necessary to maintain a salt balance, the salt load of return flows could
be reduced substantially with more efficient irrigation. This assumption
also implies that increased long-term national economic benefits will
accrue by reducing W to many existing projects on river systems where
salinity problems now exist.
The leaching fraction (LF), or the water used for leaching, W , must
L
be - W , or the water needed to meet the "leaching requirement" (LR)
LK
to maintain a favorable salt balance in the soil. Recent lysimeter stud-
ies (Bernstein and Francois, 1973; and Bernstein, et al. 1975) suggest
that the LF could be much less than previously recommended without an
appreciable yield reduction. Achieving a very low LF, 0.05 to 0.1,
will require sophisticated irrigation systems that assure very accurate
control of the amount of water applied and very uniform water applications,
64
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Most studies of low LF haye concerned the salt and ion balance in the
soil, the salt and ion concentration in the drainage water, and related
plant response. Little attention to date has been directed toward prac-
tical methods of achieving a low LF, the increased costs, including the
capital investment, operation and maintenance costs required, and the
technical skills necessary to manage more complex systems to achieve a
low LF. These requirements must be considered relative to economic
benefits that may result from reducing water deliveries, or increasing
irrigation management efficiency (E. ). There will be obvious benefits
on some projects which require costly drainage facilities to remove
several times more drainage water, W , than necessary to maintain a
salt balance, and all other water that is not used consumptively. Pump-
ing costs on some gravity irrigated projects could be reduced substan-
tially by reducing W .
Return flows from irrigation projects normally are composites of water
from deep percolation or drainage through the soil profile, seepage from
canals, surface runoff, and direct spills of irrigation water into the
drainage system which bypass the soil. Usually, the higher the proportion
of steeply sloping and rolling land in a gravity irrigated project, the
greater the proportion of surface runoff in the return flow. In some pro-
jects with level land and basin irrigation, little surface runoff occurs.
Also, where the soils on sloping lands are sandy and have very high intake
rates, there may be little surface runoff. Reducing the total water di-
verted into irrigation projects, WT, which would increase the bypass flow
in the river, may have little effect on the salt load in downstream waters
depending on the sources of return flows from a project, the physical
features, particularly the underlying strata, of the project and river
system, and the relative flow rates from the project and in the river.
Another current popular assumption is that long-term national economic
benefits will accrue from improving the "quality" of irrigation return
flows. In its extreme case, there is little basis for attempting to
reduce the salt concentration in river flows entering oceans. No one
65
-------
questions the fact that consumptive use or evapotranspiration concen-
trates the natural dissolved salts that are present when water is di-
verted from a river. But there are questions about the long-term
national economic benefits from attempting to attain very low leaching
fractions and operate farms with soil salinity near critical levels.
Decreasing the quantity of water delivered to the farms, W , normally
will reduce the quantity of return flow to the river, but the concentra-
tion of soluble salts in the drainage water may be greater. When com-
bined with the bypass river flow, the net effect on downstream water
users may not always be beneficial on some rivers since there may be
little net reduction in the salt concentration, and there may be an in-
crease in percentage of sodium and the chloride ions. The sodium hazard
is normally represented by the sodium absorption ratio, SAR, (SAR =
1/2
Na/ICa + Mg)/2] , with concentrations expressed in milliequivalents
per liter). Under special circumstances, decreasing return flows will
result in lower direct costs and thus increase long-term national econom-
ic benefits. For example, an International Treaty with Mexico and Con-
gressional Acts specify that return flows from the We11ton-Mohawk Project
in Arizona must be desalinized to decrease the salt concentration in the
lower Colorado River that flows to Mexico.
There are many facets to consider in arriving at long-term national
economic benefits to be derived from alternatives for improving E. .
For example, where the water diversions to an irrigation project, W ,
are much greater than W , the irrigated project and its underground
aquifers may serve as a recharge area to an aquifer supplying water for
other beneficial uses, or the project may serve as a temporary storage
reservoir which stabilizes downstream flow. This occurs along the South
Platte River in Colorado and Nebraska, and Silver Creek near Hailey,
Idaho. Studies currently are underway in Idaho to evaluate the effects
of converting gravity systems to sprinklers in the gravelly upper reaches
of Silver Creek on the late season and total flow to downstream water
users. There is no surface reservoir on the Big Wood River above Silver
Creek and the aquifer is the principal means of retaining flood flows,
66
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regulating flow to the water users, and preserving a large trout fishing-
based recreational industry. Under these conditions, an economic assess-
ment of all benefits that may accrue from changing E in a recharge area
of a project serving as a temporary storage and stabilizing reservoir must
be considered. Some less apparent benefits have been mentioned, but there
are also some long-term economic costs of erosion caused by excessive
water use, soil salinization caused by poor drainage, larger pumping costs,
higher initial and maintenance costs of larger distribution systems, and
indirect drainage costs. Many water users also believe that if they im-
prove E on their project or farm, the resulting decrease in WL
im f
would be available to them for irrigating additional land in the area.
This was very apparent to me based on questions asked of speakers at the
National Conference on Managing Irrigated Agriculture to Improve Water
Quality, Grand Junction, Colorado, May 16-18, 1972. If farm deliveries
are reduced and the water savings are diverted to new land in the area,
an increase in E on the original project may have detrimental effects
on downstream users because it could significantly increase W , reduce
cu
the river flow below the area, and increase the concentration of soluble
salts because of more evapotranspiration. This apparent attitude of
water users attending the Conference was not in harmony with those pre-
senting papers, nor the theme of the Conference. Estimating long-term
national economic benefits from different water management alternatives
cannot be realistically approached in a simplistic manner because most
systems are complex with many complicated interactions that have phys-
ical, biological, and social implications.
Some of the pros and cons concerning the goals of irrigation water
management for salt control, particularly reducing the LF, can be
found in recent articles by Christiansen (1973), and an article by van
Schilfgaarde, et al. (1974), with discussions by Christiansen (1975),
Olsen (1975), and Willardson (1975). Additional articles closely relat-
ed to this subject are those by Rhoades, et al. (1973 and 1974).
When considering the practical ramifications of irrigation water manage-
67
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ment for salt control, the question of approaching the theoretical mini-
mum leaching fraction may be academic for the next one or two decades
for most western irrigated projects since most apparent leaching frac-
tions now range from 0.3 to 0.6 based on recent use of water studies that
have been conducted. However, large improvements in E. are possible
in most irrigated areas without being too concerned about the final ac-
ceptable minimum LF necessary to maintain a favorable salt balance in
the soil profile over entire fields. Improving E. will provide
direct economic benefits from increased crop yield and quality and
lower drainage costs. Also, reducing deep percolation should lower the
salt load by reducing the soil mineral dissolution rate.
Basically, the minimum leaching fraction will be applicable to only that
area that regularly receives the least water application, which may be
< 10% of each field. The average LF for each field will be dependent
on the actual average E. for each field. Minimizing the salt load
in the return flow will require a very efficient irrigation system to
permit controlling the amount of irrigation water applied to each sub-
area of each field throughout the year so that, annually,
Z(WD).
J
m n m n
j-i [!i(Et>i]j j!i {!i(Et)i]j
where the interval between successive irrigations is represented by the
subscript j; I = the depth of irrigation water applied during the
irrigation interval; P = the precipitation during the irrigation in-
terval that does not run off the unit area; E = the daily evapotrans-
piration for each day, i, during the irrigation interval; W = the
cumulative drainage during the irrigation interval; and LF* = the
required annual minimum leaching fraction.
The main problem that will be encountered in attempting to approach the
LF* over the entire field is the uniformity of water distribution along
68
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with nonuniform soils and nonuniform crop growth. Also, the quantity of
rainfall after an irrigation must be considered as a stochastic process.
In many areas, leaching caused by unexpected rainfall could make a LF
< 0.1 very difficult to achieve. Probably the most difficult management
problem that will be encountered in attempting to maintain a very low
LF will be controlling irrigations needed to germinate small seeds, and
irrigating shallow-rooted crops with gravity irrigation systems. Effi-
cient germination irrigations can be achieved relatively easily with
special sprinkler systems used only for germination purposes. Similarly,
limiting the first irrigation applied to a shallow-rooted row crop when
only a small amount of soil water has been depleted will be very diffi-
cult with irrigation systems that flood the entire soil surface. Under
these conditions, a cropping sequence that schedules a deep-rooted crop
to precede the shallow-rooted crop, and scheduling irrigations to allow
the soil to be depleted before the end of the previous season, will re-
duce deep percolation.
ESTIMATING THE LEACHING REQUIREMENT
When planning for a favorable salt balance in a soil profile under steady-
state conditions, it is commonly assumed that the salt uptake by crops
is negligible, and that mineral dissolution and precipitation are 0, al-
though with recent leaching models these assumptions are not necessary.
The leaching requirement (LR) is the potential minimum leaching frac-
tion (LF*) that will maintain the salinity near the bottom of the root
zone below the maximum level for each specific crop in relation to the
expected concentration of soluble salts in the applied water.
LR = LF*
r
where C is the concentration of salts in the applied water (irriga-
aw
tion and effective precipitation) ; C is the concentration of salts in
the soil solution at the bottom of the root zone that can be tolerated
by the specific crop without a significant yield reduction (<; 5%) when
69
-------
the soil water content is near field capacity, 6 , or when the rate
of drainage is less than 0.1 mm/day; a = C, /C , and C, is the
average salt concentration in the drainage water. The ratio a is
needed under some situations because the average salt concentration in
the drainage water may be significantly less than the concentration in
the soil as the drainage rate approaches 0. For example, immediately
after a heavy irrigation of a shallow soil, the rate of drainage is very
rapid and the salt concentration probably is less than C . Also, even
within the smallest subarea, drainage may not be uniform. In addition,
because of other uncertainties, the maximum value of C probably is
not independent of environmental conditions, especially the evapotrans-
piration rate. Therefore, maximum C values for specific crops deter-
mined with steady-state lysimeters in greenhouses may need to be modified
for field conditions. The acceptable maximum value of C for each
crop also may vary with environmental conditions at different stages of
crop growth. In practice, salt concentrations expressed in equation 18
are determined and reported as electrical conductivity (EC) in mmho/cm.
Since no irrigation system can apply water at 100% uniformity under
ideal conditions, including some of the most sophisticated center-pivot
and moving lateral sprinkler and drip systems, the average LF for a
field, which will be used in designing, operating, and managing irrigat-
ed systems will be much larger than the LF* applied to that portion of
each field that regularly receives the least depth of application. The
average leaching fraction for the field is strongly dependent on the
uniformity at which water can be applied if the entire field is to be
leached with a LF - LF*. The average LF for this condition could be
estimated as follows:
LF = 1 - a. (1 - LF*) [19]
a
where a, = the expected distribution coefficient which is the ratio of
the average depth of water applied to some agreed upon fraction of each
field that receives the least amount of water, to the average applied to
the entire field (Jensen, et al., 1967). The distribution coefficient
70
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as defined is not the same as that commonly used to describe the uni-
formity of water application by sprinkler systems, U . But a, can
c a
be estimated from U as illustrated in the following example:
Example
Assume that the average depth of water applied in the 10% of
a field regularly receiving the least amount of water is (1 +
LF*)W ; the application of water by a sprinkler system is
normally distributed independent of the amount applied, and
the uniformity coefficient, U , can be estimated with equa-
tion 21; the distribution coefficient, ad, at 5% of the area
(see Figure 61-1, Jensen, et al., 1967) represents the average
depth of water applied to 10% of the area that regularly re-
ceives the least amount of water; irrigations are timed exact-
ly so that only LF* W drains through the soil; and W
1W CU
is not affected by the soil salinity level. Under these con-
ditions, the average LF for the field for various LF* and
uniformity coefficients will be:
Average LF with a LF* of:
U
C
%
100
95
90
85
I/
I/
s-'
%
0 1
6.25
12.50
18.75
Estimated
a.
d
.00
.89
.79
.69
from
0.05
0.05
.15
.25
.34
equation 21
0.10
0.10
.20
.29
.38
with x = 100%.
0.15
0.15
.24
.33
.41
The data presented above clearly indicate that even with an irrigation
system that has a uniformity coefficient of 90%, which is excellent with
present equipment, the average LF for a field will be 2.2 LF* with
LF* = 0.15; 2.9 LF* for LF* = 0.10; and 5 LF* for LF* = 0.05.
When considering these data, the apparent fractions on western irrigated
71
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projects with salinity problems that are as high as 0.4 may not be un-
reasonable with present irrigation systems. However, many areas do not
have salinity problems and there is no basis for large apparent leaching
fractions if water could be put to other beneficial uses.
Areas within fields that receive the least water application can be pre-
dicted with gravity irrigation and some sprinkler systems. However, it
is difficult to change the distribution pattern within a field with
gravity or solid set sprinklers during the growing season.
In my opinion, since the apparent LF for many surface irrigated pro-
jects in the western USA is about 0.4 or greater, which may be five to
ten times larger than LF*, substantial reductions can be made in the
LF on many western irrigated projects without adverse salt balance ef-
fects in the soil if irrigations are scheduled (time and amounts), and
water applied much more accurately and uniformly than is now being done.
Improving irrigation water management efficiency will reduce the salt
pickup in some projects and river systems.
ATTAINABLE IRRIGATION EFFICIENCY
The attainable irrigation efficiency must be considered in attempting to
reduce the average LF. It depends on the potential water application
efficiency, E*, for an irrigation system and how the system is managed.
cl
E* values are more predictable when the application rate is controlled
3.
primarily by the system, like with sprinkler systems, and is not influ-
enced by soil characteristics. Even with sprinkler systems, E* will
a
be influenced by operating pressures, wear on the nozzles and heads,
damaged heads, plugged nozzles, broken springs, windspeed and wind
direction. Similar problems, except for wind, affect drip irrigation
systems, but mechanical problems are different (no moving parts in the
emitters). Clogged nozzles, both mechanically and chemically, and pres-
sure variations probably are the principal factors affecting E* for
drip systems. The uniformity of water application is a major factor
72
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affecting E*. For sprinkler systems, it is most commonly expressed by
3.
the Christiansen (1942) uniformity coefficient
U =100 11.0 - ZIX-~XM [20]
c ^ NX J L J
where x = the amount of water applied per unit subarea; x = the mean
for the entire area; and N = the number of subareas. The same uniform-
ity coefficient also can be expressed as a function of the standard de-
viation, s, since most distribution patterns from operating sprinkler
systems are normally distributed if U > 70% (Hart and Reynolds, 1965).
Therefore,
0.8s
U - 100 1 - ^r
c \ x
[21]
E* can be calculated if the standard deviation of the amount applied by
a
the overlapping laterals or from the moving laterals is known. A reason-
able percentage of the area that is to receive the full irrigation must
be considered to estimate E* for sprinkler systems (Jensen, et al.,
3.
1967). For example, it would be uneconomical and impractical to operate
systems until the last 1% of the area received the full desired amount
while 99% was greatly overirrigated. No standard has been established
for this purpose since the economic returns are related to the value and
sensitivity of the crop to underirrigation and salinity. Also, because
of changes in windspeed and direction, the seasonal uniformity coeffi-
cient tends to be greater than that for a single irrigation if the soil
is not filled to the effective field capacity at each irrigation (Jensen
and Erie, 1971). Also, moving laterals tend to apply water more uni-
formly than stationary operated laterals, since each sprinkler becomes a
line source rather than a point source. However, if stationary operated
sprinkler laterals are not set in identical positions at each irrigation,
the areas receiving less than the average amount during one irrigation
may receive an above-average amount the next irrigation. Solid set
sprinklers usually are not moved during the season and the same areas
tend to receive the same distribution all season. The uniformity of
water application by sprinklers is not greatly influenced by the amount
applied, i.e., very light or heavy irrigations have little effect on U
73
-------
and E*.
a
With basin irrigation, E* largely depends on the accuracy to which the
a
basins are leveled. Nonuniform intake rates throughout the basin, the
stream size, and time required for spread of water throughout the basin
also may significantly affect E*. Basin irrigation requires the appli-
3
cation of at least a minimum amount to cover high areas, or assure that
all level furrows receive water from end to end if furrows are used in
the basin. This amount may be more than the soil will hold early in the
season.
Furrow irrigation on sloping fields can produce very uniform application
of water if sufficiently large stream sizes are used. However, if run-
off cannot be recirculated, E* may not exceed 75 to 80%, and often it
3
will be less than 75% because of surface runoff. Surface runoff normally
does not directly influence the magnitude of W and leaching within the
fields, since the runoff may either be returned to the canals, reused on
the farm, or diverted to the drains. Therefore, a low E does not
3
necessarily result in a high LF as is often assumed. Normally, furrow
irrigated fields are uniformly graded, but fields leveled to a concave
shape may improve the uniformity of water distribution and E* (Powell,
et al., 1972).
Graded borders can be very efficient if proper stream sizes are used for
the slope, length of run, type of crop, and intake rates involved. Some
surface runoff is common with borders, but usually runoff is less than
with furrow systems if properly designed and managed. Where intake rates
are high, the ends of low gradient borders are commonly diked; E* for
3
surface irrigation systems can be as high as for sprinkler systems.
In practice, E* is seldom attained or approached as often with gravity
3.
systems as compared with sprinkler systems, because the actual efficiency,
E , is influenced much more by management and other factors . For exam-
a.
pie, E for gravity systems is affected by initial and maintenance
74
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land leveling, the soil surface conditions, stream sizes, and other phy-
sical factors that affect overland or furrow flow, like crop density.
U can easily be evaluated and calculated for drip systems similar to
c
sprinkler systems. It can also be evaluated and calculated for gravity
systems, but not as easily since the quantity of infiltration is more
difficult to evaluate than the quantity being applied. A more detailed
discussion of surface irrigation efficiency can be found in an article
by Willardson and Bishop (1967).
ACTUAL WATER APPLICATION AND IPvRIGATION EFFICIENCIES
0. W. Israelsen was one of several scientists who made some of the first
detailed studies of irrigation practices. He assisted in studies of farm
irrigation practices in California from 1913 to 1915, and conducted very
detailed studies in Utah from 1937 to 1941. A total of 145 individual
irrigations were evaluated in Utah County, and 28 irrigations were eval-
uated in Salt Lake County. The results of the Utah studies showed that
the average water application efficiency for higher lands near the moun-
tains was 38%, for lands of medium elevation 44%, and for low lands 34%.
He found "that low irrigation efficiencies accompany abundant water sup-
plies and that losses occur when irrigation water is applied to soils
that already have plenty of moisture" (Israelsen, 1943). He also stated:
"Every irrigation farmer knows that he cannot put a gallon of
water into a quart cup, but unknowingly, many try to put 4
acre-inches of water into a soil which has capacity for only
1 acre-inch. Unfortunately, the excess 3 acre-inches flow
away by deep percolation."
Willardson (1972) reported that the water application efficiency on a
furrow-irrigated field of potatoes in 1959 averaged 46% for 11 irriga-
tions with a standard deviation(s) of 20%. Tyler, et al. (1964) con-
ducted a 5-yr study of 41 farms in southern Idaho using measured water
deliveries, and estimated seasonal consumptive irrigation requirements
75
-------
made with the Blaney-Criddle procedure. The average seasonal irrigation
efficiency for 203 farm-years was 50% with s = 97,. The U. S. Bureau of
Reclamation (1971) made a detailed study of four farms in 1964 and six
farms from 1965 to 1968 in the same general area and measured water
deliveries, farm system losses, and surface runoff. Deep percolation
was estimated, using spring and fall gravimetric soil samples, measured
water applied to individual fields, and estimated consumptive use during
the growing season. The mean irrigation efficiency was 42%, with s =
6%. The measured and estimated average losses expressed as percentage
of the water delivered were:
Farm system losses 10.5%
Surface runoff 16.4%
Estimated deep percolation 31.8%
The USER (1973) conducted a similar study of four gravity- and five
sprinkler-irrigated farms in the Columbia Basin during 1970-72. Water
was measured to each farm or area with weirs or line meters. Runoff from
the gravity-irrigated areas was measured with Parshall and V-notch trap-
ezoidal flumes. The average seasonal irrigation efficiencies for the
various systems are summarized below:
System Events Average E.
J. S
% ~T~
Furrow or rill 10 35 4
Sprinkler, side roll 8 49 12
and squarematic
Center-pivot 4 58 8
A recent estimate of irrigation efficiencies for the years 1970 to 1972
in the Wellton-Mohawk Project in Arizona indicated the sandy mesa farms
averaged 33% and the valley farms with mainly basin irrigation averaged
65%. The average was 56% for the entire project (Advisory Committee,
1974).
76
-------
The data briefly summarized above clearly show remote prospects during
the next decade of improving irrigation water management efficiency on
80% of the irrigated land that is now gravity-irrigated to such an ex-
tent that the recent theoretical minimum leaching fractions become the
critical issue except on parts of fields that receive the least amount
of water, and areas that have salinity problems. Drip irrigation can
greatly improve efficiencies in some areas, but the total area of drip
irrigation is not expected to be over 0.5% of the total irrigated area
in the USA by 1980.
ROOM FOR IMPROVEMENT
One of the key issues that investigators of irrigation efficiencies have
emphasized for over three decades is that the farm manager/operator is
not aware of the amount of water a soil can hold, and often the total
amount of water that is applied, or the distribution of applied water.
This is the prime purpose of making scientific irrigation scheduling
services available to as many irrigators as possible. Estimates of the
response to suggested irrigation schedules obtained during the 1974 sur-
vey indicated the following expected density distribution:
Degree to which Irrigators who respond to
schedules are followed recommended irrigation dates
%
Irrigate within ± 1 day 45
Irrigate within ± 2-3 days 33
Irrigate within ± 3-5 days 22
Nearly 80% irrigated within ± 3 days of the recommended irrigation
dates. Most of the recommended irrigation dates consider the expected
amount of water that is normally applied with the existing system so as
to avoid very inefficient irrigations. The application of scientific
irrigation scheduling technology, coupled with improved surface irriga-
tion systems that are being developed and other irrigation practices,
77
-------
could result in average increases in irrigation efficiencies from the
current average level of about 40% to about 50% during the next decade.
At first this may not seem to be a large change, but it represents a 25%
improvement in practices that have continued with little change for three
decades. Conversion to sprinklers has been the main factor resulting in
improved irrigation efficiency in many areas. But sprinkler irrigation
requires much more energy than gravity irrigation. Significant improve-
ments in gravity irrigation systems and practices are needed to avoid
improving irrigation efficiencies by conversion to systems that have
very high energy requirements.
POTENTIAL FOR SCIENTIFIC IRRIGATION SCHEDULING TO REDUCE SALINITY OF
RETURN FLOWS
Reducing the average LF in each field can reduce the salinity of return
flows by decreasing dissolution or weathering of soil minerals, and dis-
solution of salt deposits or displacing highly saline waters underlying
irrigation projects. In addition, if the volume of water applied per
unit area is closely controlled so that the LF on the parts of each
field that receive the least amount of water approaches LF*, precipita-
tion of carbonate and gypsum compounds in the soil would further reduce
the salinity in return flows. However, as shown by the previous example,
extremely uniform water applications will be needed along with very
accurate control of the amount applied to a major part of each field to
have a significant effect on return flow quality except where canal
seepage and other easily avoidable water losses are involved. The mater-
ial presented earlier indicates that scientific irrigation scheduling
alone may result in direct net economic benefits to the farm manager/
operator from increased crop yields and better quality. Scientific ir-
rigation scheduling with some improvements in gravity irrigation systems
probably could increase average irrigation efficiencies 10 percentage
points during the next decade. However, this amount probably would have
little effect on return flow quality. Similar conclusions were reached
by Skogerboe, et al. (1974) in evaluating irrigation scheduling for
78
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salinity control in the Grand Valley.
Major improvements in gravity or low pressure surface irrigation systems
and irrigation practices, along with changes in water delivery policies
controlled by institutions and state organizations regulating water
rights, will be needed to achieve sufficient increases in irrigation
water management efficiencies to significantly reduce salt loads in
irrigation return flows without large energy inputs. Scientific irri-
gation scheduling can significantly reduce the salt load in return flows
with irrigation systems that enable uniform applications of known amounts
of irrigation water. Potential efficiencies of new irrigation systems
and potential reductions in salt loads probably could not be achieved
without scientific irrigation scheduling. Scientific irrigation schedul-
ing is economically feasible with most existing irrigation systems, but
will be more effective with new and better irrigation systems. Major
benefits to the farm manager/operator result from improved crop yields
and quality, and general improvement of irrigated farm management.
79
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SECTION IX
REFERENCES
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28. Heermann, D. F., H. H. Shull, and R. H. Mickelson. Center Pivot
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31. Huang, W. Y., T. Liang, and I. P. Wu. Optimizing Water Utilization
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33. Israelsen, 0. W. The Foundation of Permanent Agriculture in Arid
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34. Israelsen, 0. W. Irrigation Principles and Practices, (2nd Ed.),
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Sta. Bull. 311, 1944. 55 p.
36. Jackson, R. D., S. D. Idso, and R. J. Reginato. Calculation of
Evaporation Rates During the Transition from Energy Limiting to
Soil Limiting Phases Using Albedo Data (Manuscript), 1975.
37. Jensen, M. C., and J. E. Middleton. Scheduling Irrigation from
Pan Evaporation. Wash. Agric. Exp. Sta. Circ. 527, 1970.
38. Jensen, M. E. Water Consumption by Agricultural Plants. In:
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39. Jensen, M. E. Scheduling Irrigations with Computers. J. Soil and
Water Conserv. 24:193-195, 1969.
40. Jensen, M. E. Programming Irrigation for Greater Efficiency. In:
Optimizing the Soil Physical Environment Toward Greater Crop Yields,
Hillel, D., (ed.). New York, Academic Press, 1972. p. 133-161.
41. Jensen, M. E. (ed.) Consumptive Water Use and Irrigation Water
Requirements. Rept. of the Irrigation Water Requirements Comm.,
Irrig. and Drain. Div., Am. Soc. Civil Eng., 1974. 215 p.
42. Jensen, Marvin E., and L. J. Erie. Irrigation and Water Manage-
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Practices. Ames, Iowa State Univ. Press, 1971. Chapter 8, p. 189-
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43. Jensen, M. E., and H. R. Raise. Estimating Evapotranspiration from
Solar Radiation. J. Irrig. and Drain Div., Am. Soc. Civil Eng.
89(IRA):15-41, 1963,
44. Jensen, M. E., and W. H. Sletten. Evapotranspiration and Soil
Moisture-Fertilizer Interrelations with Irrigated Grain Sorghum in
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R. C. (ed.). Madison, Am. Soc. Agron. Monograph No. 11, 1967.
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46. Jensen, M. E., D. C. N. Robb, and C. E. Franzoy. Scheduling
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47. Jensen, M. E., J. L. Wright, and B. J. Pratt. Estimating Soil Mois-
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48. Korven, H. C., and J. C. Wilcox. Correlation Between Evaporation
from Bellani Plates and Evapotranspiration from Orchards. Can. J.
Plant Sci. 45:132-138, 1965.
49. Lembke, W. D., and B. A. Jones, Jr. Selecting a Method for Sched-
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50. Lord, J. M., Jr., and M. E. Jensen. Irrigation Scheduling for
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No. T140, 1975. 63 p.
53. Miller, D. E., and J. S. Aarstad. Calculation of the Drainage
Component of Soil Water Depletion. Soil Sci. 118:11-15, 1974.
54. Miller, D. E., and J. S. Aarstad. Effect of Deficit Irrigation on
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55. Musick, J. T., W. H. Sletten, and D. A. Dusek. Preseason Irriga-
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56. Ogata, G., and L. A. Richards. Water Content Changes Following
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87
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to Reference Evapotranspiration, Crop Status, and Environmental
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88
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SECTION X
APPENDIX
89
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Table Al. COMMERCIAL IRRIGATION SCHEDULING SERVICES PROVIDED FOR A FEE IN 1974 IN WESTERN USA
Serv.
Grp
No.
1
2
3
4
5
6
7
8
9
10
Exp.
yrs.
4
1
2
4
2
5
6
5
19
9
Area
Served
C. Ariz.
C. Calif.
Ida.
Ida. ,Colo.
and Wash.
Ida.
Ida.
Ida. , Nev.
Neb. , Kan.
and Colo.
Wash.
Wash
Total or average use
a
b
c
Main Services Provided
Irrig
sched
X
X
X
X
X
X
X
, x
X
X
100%
. Syst.
oper.
X
X
X
X
X
X
60%
Pit. Pest
nutr. mgmt. Other
x a
x
XX
XX
XX
x x b,c
x x b,c
x x a,b
x
XX
100% 56%
Summer Crops
Fields
800
60
35
1,000
44
43
75
2,000
180
240
4,447
Area
10
2
5
28
2
2
3
32
7
8
102
(ha)
,490
,400
,670
,330
,100
,020
,240
,480
,280
,090
,100
Winter Crops
Fields Area
(ha)
300 4,380
100 4,050
25 400
20 800
445 9,630
System design
Cropping
Farm or
practices,
tillage
ranch management for
operations
absentee
, etc.
owners
-------
Table A2.
IRRIGATION SCHEDULING SERVICES PROVIDED BY STATE (S), FEDERAL AGENCY (U) COMPANY (C) OR
PROJECT (P) IN 1974 IN WESTERN USA
Grp . Exp .
No. yrs.
S-l 5
S-2 5
U-l 1
U-2 1
U-3 2
U-4 3
U-5 2
U-6 3
U-7 2
U-8 5
U-9 4
U-10 1
U-ll 1
U-12 2
U-13 3
U-14 2
U-15 1
U-16 4
Sub. Tot.
C-l 1
C-2 1
C-3 1
P-l 10
Total or %
or
Dev.
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
Wyo.
D
D
D
0
Main Services Provided
Area
Served
N.Dak.
Wyo.
Ariz.
Ariz.
Calif.
Calif.
Colo.
Colo.
Ida.
Ida.
Kans.
Nebr.
N.Mex.
N.Mex.
Tex.
Utah
Wash.
Ida.
Ida.
Ida.
Ariz.
involved
Irrig. Syst. Pit.
sched. oper. nutr.
X X
x
X
X
X
XX
x
X
XX
x
X
X
xc
X
X
x
X ^ ^mmr
X
X X
X X
X x
XXX
100% 14% 23%
a Periodic visits and weekly farm guides
b System
improvement
Pest Summer Crops
Winter Crops
mgmt. Other Fields Area Fields
(ha)
x 21 660
50 1,010
82 1,210
57 1,140
108 1,780
133 8,170
1 50
502 2,800
108a 1,010
b 30 320
306 3,600
6 120
102 860
135 1,400
313 1,850
20 80
525 6,070
2,499 32,130
x 36 400
x 50 1,620
x d 26 230
854 19,260
18% 3,465 53,640
c Computer services to
d Regular contracting
14
69
102
26
_
___
10
- .
___
......
___.
5
226
^H^
___
260
486
Area
(ha)
200
1,380
1,710
2,010
_
80
__
___
___
___
20
5,400
.__
5,580
10,980
State University
services
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing}
1. REPORT NO.
EPA-600/2-75-064
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
SCIENTIFIC IRRIGATION SCHEDULING FOR SALINITY
CONTROL OF IRRIGATION RETURN FLOWS
5. REPORT DATE
November 1975 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Marvin E. Jensen
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
USDA-ARS-Western Region
Snake River Conservation Research Center
Route 1, Box 186
Kimberly, Idaho 83341
10. PROGRAM ELEMENT NO.
1HB617
11. CONTRACT/GRANT NO.
EPA-IAG-D4-F399
12. SPONSORING AGENCY NAME AND ADDRESS
Robert S. Kerr Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Ada, Oklahoma 74820
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
A comprehensive review is presented of irrigation water management principles,
factors to be considered in improving irrigation water management, leaching
requirements, climatological approaches to irrigation scheduling, scope of
irrigation scheduling services in 1974, basic concepts of scheduling services,
and probable effects of scientific irrigation scheduling on salinity of return
flows. A definition of irrigation water management efficiency is presented to
evaluate the annual volume of irrigation water used relative to the optimum
amount needed for maximum annual crop production or income. The term considers
the minimum, but essential water needed for both consumptive and nonconsumptive
uses. The lack of significant changes in irrigation efficiency during the past
several decades is discussed and attributed to problems associated with the
management of a complex soil-crop-environment system, a lack of economic incentives
to make improvements, and ineffective traditional approaches to improve irrigation
scheduling. New proposed minimal leaching practices are discussed. The author
concludes that substantial improvements in irrigation efficiencies can be made
before the proposed minimal LF are reached on most western irrigated projects.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
*Irrigation
Irrigated land
Leaching
Salinity
Management
*Scheduling
*Irrigation efficiency
*Irrigation scheduling
Leaching fraction
Return flow
2C
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report}
UNCLASSIFIED
1. NO. OF PAGES
100
20. SECURITY CLASS (This page)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
92
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