EPA-660/2-74-052
June 1974 Environmental Protection Technology Series
Evaluation of Irrigation Scheduling
For Salinity Control In Grand Valley
Office of Research and Development
U.S. Environmental Protection Agency
Washington, D.C. 20460
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EPA-660/2-74-052
June 1974
EVALUATION OF IRRIGATION SCHEDULING
FOR SALINITY CONTROL
IN GRAND VALLEY
by
Gaylord V. Skogerboe
Wynn R. Walker
James H. Taylor
Ray S. Bennett
Grant No. S-800278
Program Element 1BB039
Project Officer
Dr. James P. Law, Jr.
Robert S. Kerr Environmental Research Laboratory
P. O. Box 1198
Ada, Oklahoma 74820
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U. S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D. C. 20460
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ABSTRACT
Irrigation return flows carrying large salt loads as a re-
sult of contact with saline soils and aquifers have caused
serious salinity problems in the Colorado River Basin. The
Grand Valley in western Colorado is a major contributor to
the salinity problems in the basin and is therefore a logi-
cal region to test the effectiveness of salinity control al-
ternatives. This study has emphasized two on-farm measures;
namely, irrigation scheduling and tile drainage.
Two small farms were extensively investigated during the
two-year interval of the project to identify the structural
and non-structural requirements for maximizing water use ef-
ficiencies. Early season irrigations are shown to provide
significantly more salts in the river system than later ones.
Irrigation scheduling programs are shown to be effective
only when sufficient structural rehabilitation and labor in-
put are implemented to allow complete adherence to the sched-
uling suggestions. Because of the relatively inflexible
method of irrigation in Grand Valley, irrigation scheduling
must be implemented as a segment of a valley-wide salinity
control program.
This report was submitted in fulfillment of Grand No.
S-800278 by Colorado State University under the sponsorship
of the Environmental Protection Agency. Work was completed
as of March 31, 1974.
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CONTENTS
Page
Abstract . ii
List of Figures iv
List of Tables vi
Acknowledgements vii
Sections
I Conclusions 1
II Recommendations 3
III Introduction 5
IV Scientific Irrigation Scheduling 17
V Field Investigations 34
VI Results of Scheduling Evaluations 52
VII References 82
VIII List of Publications 85
IX List of Symbols 86
iii
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FIGURES
No. Page
1. The Colorado River Basin . . 6
2. Major sources of salinity in the Colorado
River Basin 7
3. The Grand Valley of Colorado 8
4. Normal precipitation and temperature at
Grand Junction, Colorado 10
5. Agricultural land use in the Grand Valley .... 11
6. Location of study farms in the demonstration
area 14
7. Irrigation scheduling components 20
8. Cumulative potential evapotranspiration in
Grand Valley for 1973 irrigation season 26
9. Relationship between moisture stress to moisture
percentage at different levels of soil salinity . 29
10. Relationship between crop transpiration rates
and soil water potential 29
11. Seasonal variation of water levels in two wells
monitoring the cobble aquifer 36
12. Seasonal and annual drainage flows for a
selected drain in the test area 39
13. The Bulla farm layout 43
14. The Martin farm-layout 44
15. Typical intake rates for the soils in
the test area 46
16. Typical furrow advance curves on the Bulla farm . 47
17. Cumulative intake functions for early and late
irrigations in the test area 48
18. Seasonal variations in water table elevations
on the Bulla farm during 1973 50
iv
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FIGURES (Continued)
No. Page
19. Seasonal variations in application efficiencies 61
for corn grown in the test area
20. Seasonal variation in application efficiencies
for sugar beets grown in the test area 62
21. Seasonal variation in application efficiencies
for barley grown in the test area 63
22. Comparison of root zone water supply and
evapotranspiration demands for sugar beets
in the test area 65
23. Seasonal percentage of deep percolation
attributed to various irrigations on the
farms studied 66
24. Seasonal variation in average field intake
rates for the Martin and Bulla farms 67
25. Comparison of application efficiencies for the
Martin farm before and after the implementation
of irrigation scheduling 69
26. Comparison of farm efficiencies for the Martin
farm before and after the implementation of
irrigation scheduling 70
27. Comparison of application efficiencies for the
Bulla farm before and after the implementation
of irrigation scheduling 71
28. Comparison of farm efficiencies for the Bulla
farm before and after the implementation of
irrigation scheduling 72
29. Seasonal distribution of salt pickup from the
farms in the test area 73
30. Soil moisture variations in the crop root zone . . 79
31. Schematic of requirements for effective
irrigation scheduling 80
v
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TABLES
No. Page
1. Description of farms in the demonstration
area included in the 1973 irrigation
scheduling studies 15
2. Variation of A/(A + y) with elevation and
temperature . ..... 22
3. Chart for estimating percent depletion of
readily available soil moisture by feel 30
4. USER irrigation scheduling service in Grand
Valley 32
5. Selected water quality from Well No. 3 in
the test area 38
6. Summary of water quality data taken from a
drain in the test area 41
7. 1972 Martin farm water budget (8.5 acres of corn) 55
8. 1973 Martin farm water budget (8.5 acres of corn) 56
9. 1972 Bulla farm water budget (25.7 acres of
sugar beets) 57
10. 1973 Bulla farm water budget (15.0 acres of corn) 58
11. 1973 Bulla farm water budget (10.7 acres of
barley) 59
VI
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ACKNOWLEDGEMENTS
The authors are indebted to the individuals who carefully
attended to the daily details of the field and laboratory
analyses. These people included Ms. Barbara Mancuso, Mr.
George Bargsten, Mr. Ted Hall, Mr. Chuck Binder, Mr. John
Bargsten, and Mr. Gregory Sharpe.
The cooperation of Mr. Frank Bulla and Mr. Don Arnold who
allowed this investigation to proceed on their lands is
also greatly appreciated.
The actual computer irrigation scheduling service was
provided by the Grand Junction Office of the U. S. Bureau
of Reclamation. The cooperative attitude of Mr. Bill
McCleneghan certainly contributed to the success of these
studies.
The writers would also like to thank Ms. Lee Kettering and
Ms. Betsy Zakely for typing the final drafts of this
report.
Finally, the efforts and advice given by the Project Of-
ficer, Dr. James P. Law, Jr., have been extremely helpful
in the successful pursuit of this demonstration grant.
Gaylord V. Skogerboe
Wynn R. Walker
James H. Taylor
Ray S. Bennett
Vll
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SECTION I
CONCLUSIONS
The irrigation of agricultural lands in the Colorado River
Basin is a significant cause of the salinity concentrations
encountered in the Colorado River. Emphasis towards stem-
ming further salinity increases has logically centered upon
improving the quality of irrigation return flows. This
emphasis, especially in the high salt contributing areas
like the Grand Valley in western Colorado, focuses upon re-
ducing the flows which pass through the saline soils and
aquifers, thereby reducing the salt pickup that occurs by
dissolution. Since a major fraction of the water contacting
local soils in this manner comes from over-irrigation,
measures aimed at improving irrigation efficiencies promise
good potential for controlling salinity. Among the methods
for achieving higher water use efficiencies on the farm,
"scientific" irrigation scheduling is possibly the most
important.
Irrigation scheduling consists of two primary components;
namely, evapotranspiration and available root zone soil
moisture. Evapotranspiration is calculated by using climatic
data. The other major category of required data pertains
to soil characteristics. First of all, field capacity and
wilting point for the particular soils in any field must be
determined. More importantly, infiltration characteristics
of the soils must be measured. Only by knowing how soil in-
take rates change with time during a single irrigation, as
well as throughout the irrigation season, can meaningful
predictions be made as to: (a) the quantity of water that
should be delivered at the farm inlet for each irrigation;
and (b) the effect of modifying deep percolation losses.
With good climatic data and meaningful soils data, accurate
predictions as to the next irrigation date and the quantity
of irrigation water to be applied can be made. In order to
insure that the proper quantity of water is applied, a flow
measurement structure is absolutely required at the farm
inlet.
The results of this demonstration project indicate that ir-
rigation scheduling programs have a limited effectiveness
for controlling salinity in the Grand Valley under existing
conditions. Excessive water supplies, the necessity for
rehabilitating the irrigation system (particularly the
laterals), and local resistance to change preclude managing
the amounts of water applied during successive irrigations.
To overcome these limitations, irrigation scheduling must be
accompanied by flow measurement at all the major division
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points, farm inlets, and field tailwater exits. In addition,
it is necessary for canal companies and irrigation districts
to assume an expanded role in delivery of the water. Also,
some problems have been encountered involving poor communi-
cation between farmer and scheduler, as well as certain de-
ficiencies in the scheduling program dealing with evapo-
transpiration and soil moisture predictions. These latter
problems can be easily rectified, however. Correcting these
conditions will make irrigation scheduling much more effec-
tive and acceptable locally.
Water budgets from which the study results were generated
resulted from intensive investigation on two local farms.
The selection of the two study farms was intended to be re-
presentative of conditions valley-wide. Analysis of the
budgets reveal that approximately 50% of the water applied
to the fields came during the April and May period when less
than 20% of the field evapotranspiration potential had been
experienced. Salt pickup estimates during this early part
of the season amounted to about 60% of the annual total for
each field. Another indication of the importance of early
season water management is presented in an analysis of ir-
rigation efficiencies. As the season progressed, the soils
became less permeable and the crop water use increased,
causing marked improvements in irrigation efficiency. Thus,
if irrigation scheduling is employed in its optimal format,
salt pickup from the two fields could have been reduced as
much as 50% or more.
The results of this demonstration project show that irriga-
tion scheduling is a necessary, but not sufficient, tool for
achieving improved irrigation efficiencies. The real strides
in reducing the salt pickup resulting from over-irrigation
will come from the employment of scientific irrigation sched-
uling in conjunction with improved on-farm irrigation prac-
tices. This combined effect could result in reduction of
300,000 tons annually of salt pickup from the Grand Valley,
depending upon the degree of improvement in present on-farm
irrigation practices.
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SECTION II
RECOMMENDATIONS
Although the results of this study indicate that existing
programs for irrigation scheduling in the Grand Valley to
control salinity are having only a marginal effect, the
potential for "scientific" irrigation scheduling has been
well established. The recommendations generated by ex-
perience on this project are basically two dimensional. The
first regards the improvements necessary to achieve an ef-
fective scientific irrigation scheduling program and the
second deals with future evaluations which could be under-
taken to remedy mounting salinity problems in the Colorado
River Basin.
Irrigation scheduling should not be taken individually as a
salinity control measure because its effectiveness is not
exclusive of the operations of the total irrigation system.
It is utter foolishness to advise an irrigator to apply
three inches of water when he does not have the capability
of measuring the amount of water applied. Past experience
and personal judgment are no substitutes for accurate
measurement. Consequently, irrigation scheduling should not
divorce itself from the improvement of flow measurement at
both the canal turnout and the field inlet. Also, if tail-
water runoff occurs (which is usually the case in Grand
Valley) a flow measurement structure should be installed at
each field outlet. In addition, when the soils are known to
be affected by year-to-year practices and climatic conditions,
the operation of the irrigation system, including suggestions
for timing and application from the scheduling printout,
should incorporate necessary changes for each condition.
The evaluation of conveyance channel linings completed in
previous studies, along with the investigation of irrigation
scheduling and field drainage in this effort, represent major
steps towards a completely developed salinity control tech-
nology. This work and that being undertaken by researchers
throughout the West can now begin to be integrated into re-
gional salinity control strategies. Salinity control stra-
tegies represent the integration of individual control mea-
sures into a coordinated program. Such policies depend not
only upon the effectiveness of each measure, but individual
costs as well; suggesting that the next phase of salinity
control investigation should develop and combine the cost-
effectiveness for individual salinity control measures into
strategies for achieving varying degrees of salinity control
in the Grand Valley. For example, both the cost and the
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expected salt pickup reduction should be established for
various on-farm irrigation improvements such as flow mea-
surement devices, increased labor for irrigation, automated
farm head ditches, gated pipe, cut-back furrow irrigation,
sprinkler irrigation, trickle irrigation, etc.
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SECTION III
INTRODUCTION
STATEMENT OF THE PROBLEM
In April of 1972, the seven basin states sharing the water
resources of the Colorado River Basin (Fig. 1) and the U. S.
Environmental Protection Agency (EPA) responsible for the
quality of such flows agreed to the necessity of maintaining
the concentrations of salts in the lower basin at or below
existing levels (U. S. Environmental Protection Agency,
1972). Further, the necessity to allow Upper Basin users to
proceed with the development of waters apportioned to them
under the Colorado River Compact of 1922 was realized.
These two segments of the statements emanating from the en-
forcement conference are, however, contradictory without
accompanying each new development with sufficient reductions
in existing salt loads to compensate for the effects of the
new water use.
The collective decisions regarding the control of salinity
in the basin have been induced by the mounting damages in-
curred by downstream users. Salinity problems are also of
international concern owing to the detriments being experi-
enced in the Mexicali Valley of the Republic of Mexico.
The methods available for controlling salinity include phre-
atophyte eradication, reducing evaporation, desalination,
elimination of mineralized point sources, importing supple-
mental water, and improving agricultural, municipal, and
industrial water use practices. While certain of these al-
ternatives may be either technologically impractical or
politically unacceptable, they represent the array of al-
ternatives from which an overall strategy must be generated.
Because of the limited effectiveness of each measure, such a
strategy for salinity control must be a combination of sev-
eral feasible alternatives. The first task is, therefore,
to develop the costs and the effectiveness of the individual
salinity control measures.
One examination of the sources of salinity in the basin,
shown in Fig. 2, reveals that of the man-made contributions,
irrigated agriculture has the largest effect. Consequently,
the major aspect of salinity control in the region must be
the effective use of irrigation diversions by improving the
efficiency of conveyance, farm, and wastewater systems. One
of the several important efforts funded by the U. S. Environ-
mental Protection Agency to develop salinity control tech-
nology is the Grand Valley Salinity Control Demonstration
Project in western Colorado (Fig. 3). This report describes
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/_ Wyorning__
Utah ^Colorado
Lake z'Arizona
Fig. 1. The Colorado River Basin,
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UPPER COLORADO RIVER
BASIN
AVERAGE SALT LOAD TONS/DAY
June 1965 - May 1966
NATURAL POINT SOURCES
AND WELLS
MUNICIPAL
AND
INDUSTRIAL
IRRIGATED AGRICULTURE
357 T/d)
LOWER COLORADO RIVER
BASIN
AVERAGE SALT LOAD TONS/DAY
November 1963 - October 1964
MUNICIPAL
AND
INDUSTRIAL
UPPER COLORADO
BASIf
INFLOW
72 %
(9833 T/d
T/d)
i/Q)
NATURAL
POINT SOURCES
NET RUNOFF
(610 T/d)
4%
(1180 T/d)
9%
IRRIGATED AGRICULTURE
Fig. 2. Major sources of salinity in the Colorado River
Basin.
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00
Grand Valley Salinity
Control Demonstration
Project
~-~r\% Gunnison
River
Fig. 3. The Grand Valley of Colorado.
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the project's role in testing irrigation scheduling as a
salinity control measure. Although irrigation scheduling
is a two-dimensional technique involving both the timing of
an irrigation and control of the quantities of water enter-
ing the root zone, many of the applications to date have
emphasized primarily the timing aspects. As the nation has
realized the necessity for improving water use practices to
stretch existing supplies and minimize water quality degra-
dation, the focus of irrigation scheduling is beginning to
achieve its full scope—the inter-relationship between im-
proved water management and improved water quality of irri-
gation return flows.
THE STUDY AREA
The Grand Valley area is one of the most significant salin-
ity inputs to the Colorado River system, and therefore, is a
logical area to develop salinity control technology. Water
entering the near-surface aquifers in the valley become
heavily laden with salts dissolved from the soils and aqui-
fers of marine origin which occur extensively in the area.
The primary source of these soils and aquifers is the Mancos
Shale formation which was formed as a result of the alter-
nate advance and recession of the great inland seas once
dominating the western United States. Since the water enter-
ing local aquifers comes mainly from irrigation channel
seepage and deep percolation from excessive and inefficient
irrigations, the emphasis of a salinity control technology
is to maximize the efficiencies of both the conveyance and
farm water use subsystems.
Agricultural Land_Use
Although the early explorers concluded that the Grand Valley
was a poor risk for agriculturally related activities, the
first pioneering farmers rapidly disproved this notion with
the aid of irrigation water diverted from the rivers (Colo-
rado and Gunnison) entering the valley. Through a long
struggle, an irrigation system evolved to supplement the
otherwise meager supply of precipitation (Fig. 4) during the
hot summer months. However, the futility of irrigation with-
out adequate drainage was quickly demonstrated in the valley
as low lying acreage became waterlogged with highly saline
groundwater. Today, the failure to completely overcome these
conditions is still evident as illustrated by a summary of
land use in the valley presented in Fig. 5. For example, of
the more than 70,000 acres of irrigable cropland, almost one-
third is either in pasture or idle. An examination of land
use in Grand Valley by Walker and Skogerboe (1971) indicates
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GRAND JUNCTION, COLO.
Alt. 4843 ft.
iin.
8.29" annual
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H
0
o
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o
, 190 frost -free days
Apr. 16
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Oct.23
80° F
UJ
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tr
UJ
a.
S
UJ
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<
UJ
60
Fig. 4.
52.5 °F annual
JFMAMJJAS
N
Normal precipitation and temperature at Grand
Junction, Colorado.
10
-------
I20r
100
CO
o>
o
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-------
a fraction of the 30-40 thousand acres of phreatophytes and
barren soil were also once part of an irrigated acreage.
Evidence exists that these same lands were once highly pro-
ductive and subsequently ruined by over-irrigation and in-
adequate drainage.
Irrigation Practices
The prevalent method of applying water to croplands in the
valley is furrow irrigation. Small laterals carrying one
to five cubic feet per second (cfs) divert water from the
company or district operated canal systems to one or more
irrigators. Water then flows into field head ditches where
it is applied to the lands to supply the growing crops and
maintain a salt-free root zone environment. The predominant
alfalfa, corn, sugar beet, orchard, and small grain economy
is served by a more than adequate water supply. The 70,830
acres of irrigable cropland encompassed within the irrigation
system enjoys a total diversion of more than eight acre-feet
per acre during normal years. Considering that the potential
evapotranspiration of these croplands is usually less than
three acre-feet per acre, it is obvious that existing water
use efficiencies are low.
Enough variation in climate exists in the valley to separate
the agricultural land use into three primary regions. In
the eastern end of the valley, the protective proximity to
the abrupt Grand Mesa results in extended periods of frost-
free days which allows apple, peach, and pear orchards to
abound. In the western half of the valley, the primary em-
phasis is on producing corn, alfalfa, sugar beets, and small
grains. Between these two regions is a transition zone of
small farms and the urban setting of Grand Junction—the
population center of the area.
The farms in this transition area are particularly affected
by adverse conditions, and high salt contributions are being
returned to the Colorado River. The intensive study area
for the Grand Valley Salinity Control Demonstration Project,
illustrated in Fig. 3, was selected here. The primary ad-
vantage for undertaking the studies in this area was that
earlier phases of the Grand Valley Salinity Control Demon-
stration Project were conducted here and thus a great deal
of da'ta was already available to facilitate this investiga-
tion. Also, accomplishments achieved under adverse condi-
tions are much more meaningful than improvements on the
better agricultural lands.
12
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PURPOSE OF STUDY
The central purpose of this project has been to further the
development of technology needed to improve the quality of
flows returning to the Colorado River. Because the agricul-
tural water uses have been the largest contributors to the
conditions which now exist, the emphasis has been primarily
directed at improving both structural and non-structural
components to minimize the groundwater flows contacting the
saline subsurface geologic formations. The initial phase
was begun in 1968 to evaluate canal and lateral linings as
salinity control measures. Those results, reported by
Skogerboe and Walker (1972) , demonstrated the need for fur-
ther investigations concerning on-farm water management
practices. As a result, a second and third phase program
was undertaken to determine the feasibility of two farm
management improvements—irrigation scheduling and drainage
—in controlling salinity. This report is presented to sum-
marize the findings related to irrigation scheduling as a
salinity control measure.
SCOPE OF STUDY
The experience and data generated by the first phase of this
project have been important prerequisites to the successful
completion of this study. The scope of the field investi-
gations, however, was changed to reflect the increased inter-
est on the farm itself. This allowed a more intensive
evaluation of irrigation practices with less emphasis on the
area-wide hydro-salinity flow system because of previous
knowledge gained from earlier studies.
Five fields in the demonstration area were incorporated into
the study to represent a cross-section of agricultural prac-
tices in the Grand Valley. These farms, located in Fig. 6
and described in Table 1, were included in an irrigation
scheduling service implemented in the valley by the local
U. S. Bureau of Reclamation (USER) office, thereby allowing
the efforts of this investigation to be coordinated with the
USBR irrigation scheduling program.
Since the test area is characteristically operated by small
unit farmers and the soils are severely affected by the high
water table conditions, agricultural productivity is not
presently sufficient to support most of the occupants, and
many have outside jobs in local businesses or industry. One
of the concerns of the investigators was in demonstrating
the value of the scheduling service to these individual land
owners. In addition, these lands were once among the
13
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Stub Ditch
Government
Highline -
Canal
Scale I Mile
Fig. 6. Location of study farms in the demonstration area,
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Table 1. Description of farms in the demonstration area
included in the 1973 irrigation scheduling studies.
Farm
Martin
Bulla
Canaday
Kelleher
Wareham
Crop
Corn
Barley
Corn
Barley
Oats
Pasture
Alfalfa
Crested
Wheatgrass
Crop
Acreage
9.2
10.7
15.0
17.1
9.8
1.0
17.4
13.6
Field
Capacity
22.3
24.2
23.9
26.5
25.0
25.8
30.5
29.3
Wilting
Point,
%
10.7
13.4
12.1
12.8
12.2
14.1
16.7
16.7
15
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valley's most productive (at the turn of the last century)
and a significant impetus could be generated locally in
support of salinity control programs if such measures were
effective in returning these lands to a high level of agri-
cultural productivity.
In this study, an effort has been made to evaluate irriga-
tion scheduling under various management conditions ranging
from little or no improvements in on-farm irrigation prac-
tices to maximum use of the scheduling recommendations.
Three of the farms, namely the Kelleher, Canaday, and Ware-
ham farms, were included primarily as part of the third
phase of this project involving field drainage, but were
included in the scheduling program to yield a linkage be-
tween the two studies. The two fields referred to as the
Martin and Bulla farms were intensively studied to evaluate
the potential for salinity control resulting from irrigation
scheduling, along with the requirements for maximizing this
potential in the Grand Valley with structural and non-struc-
tural improvements in the agricultural system.
In addition to the on-farm evaluations, the data collection
and analysis of important hydro-salinity parameters in the
study area were continued on a reduced level to provide con-
tinuity with earlier data. The investigators concluded that
such efforts were necessary to detect any changes relative
to the improvement of water management practices in the area,
while providing a unique opportunity to refine the results
of the earlier study, if desired. Of course, the size,
scope, and detail with which any research and/or demonstra-
tion effort is conducted must be compromised with the time,
talent, and funding provided to the researchers. A workable
balance was achieved in this project while maintaining a
high degree of sensitivity towards the goals of the project.
16
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SECTION IV
SCIENTIFIC IRRIGATION SCHEDULING
HISTORICAL PERSPECTIVE
Since the days in the Tigris and Euphrates Valleys centuries
ago, fanners engaged in irrigated agriculture have had to
face the critical decisions of how and when to irrigate
their lands. Consequently, irrigation scheduling in its
most primitive form emerged long before the advent of digi-
tal computers, climatological monitors, or instantaneous
communications systems. Present day investigations and ap-
plications of this long known concept have produced great
strides towards broadening the scope of irrigation schedul-
ing to meet the requirements of a modern agricultural tech-
nology. Important and new dimensions are necessities if
modern agriculture is to effectively challenge such threat-
ening problems as world food and fiber shortages, deterio-
rating environmental quality, and the increasing demands
upon limited supplies of natural resources.
Until the development of the present irrigation scheduling
technology, an irrigator was primarily dependent on the
judgment and experiences of his immediate associates and
himself. For example, the farmers who produced the most
were often able to judge the need for an irrigation based on
the appearance of the crops. In fact, this requirement re-
mains as one of the most effective tools in the production
of specialized crops (e.g., alfalfa seed) today. However,
the fallibility of experience and judgment has been demon-
strated throughout the world, as over-application of water
has led to yield reductions due to fertilizer leaching,
poor soil aeration, and plant diseases. In addition, such
inefficient irrigation practices have created regional water
quality problems, mosquito nuisances, public health problems,
and local property damage due to waterlogging of soils and
seepage of water into basements of homes and businesses.
The refinement in irrigation scheduling techniques has pro-
vided interested farmers with a tool for minimizing the
deleterious effects of poor water management. By basing
the scheduling service on a thorough understanding of the
primary parameters affecting crop water demands, a consis-
tent and reliable program can be implemented. The compo-
nents of the current scheduling services were developed and
evaluated in 1966 and 1967 in southern Idaho by personnel of
the Agricultural Research Service, U. S. Department of Agri-
culture. Beginning in 1968 and 1969, the program found wide
acceptance in Arizona, Nebraska, Kansas, Washington, and
17
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California (Jensen, 1972). The objectives of the irrigation
scheduling programs are five-fold (U. S. Department of the
Interior, 1971) :
1. Increase crop yields and quality;
2. Reduce fertilizer losses;
3. Reduce the amount of unnecessary water and labor
inputs to farming;
4. Reduce drainage problems; and
5. Improve the quality of return flows.
The success in developing this scheduling program was assured
by the years of study of crop water requirements and irriga-
tion methods.
The studies of irrigation water requirements in the United
States were initiated immediately following the establish-
ment of the agricultural experiment stations in 1887 and
have been summarized by Jensen (1973). For example, the
amount of water applied to wheat, barley, oats, corn and
garden crops during the summer of 1887 was measured by Mead
(1887). The amounts of water applied to such crops as al-
falfa, corn, flax, oats, peas, potatoes, rye, sugar beets,
timothy, and wheat from 1893 to 1898 in Wyoming was summa-
rized by Buffam (1900).
Similar studies were started in Utah by Mills (1890) along
with other studies initiated throughout the West on funds
provided by the Appropriation Act of 1898. Most of the
early studies involved the measurement of water delivered to
irrigated farms.
Extensive plot and field studies were started during the next
two decades to determine seasonal consumptive use using soil
sampling techniques. This was started by Widstoe (1912) ,
by Harris (1920), by Lewis (1919), and by Hemphill (1922) to
note just a few of the more prominant investigations.
Studies by Briggs and Shantz (1913) were initiated near
Akron, Colorado, in 1910-1913 involving up to 55 species
and various plants. Since these studies were not conducted
under field conditions, the authors stated that "the water
requirement measurements must therefore be considered rela-
tive rather than absolute." This basic data qualification
was often overlooked or disregarded for the next half-cen-
tury. In recording meteorological data, Briggs and Shantz
also recognized that solar radiation was the primary cause
of the cyclic changes of environmental factors and that
sensible heat from the air contributed materially to energy
dissipated by transpiration.
18
-------
During the next two decades more emphasis was placed on
methods of estimating seasonal consumptive use. Problems of
nonrepresentative environmental conditions around pots or
containers, and unsaturated drainage between sampling dates
were recognized. The energy balance concept, which was ap-
plied to evaporation from water surfaces in the 1920's and
1930's, was applied to crop surfaces by Penman and Budyko in
the 1940's. Penman combined an energy balance and aerodyna-
mic equation into what is commonly known as the combination
equation which is now used in modified forms extensively
throughout the world (Penman, 1948).
The modern irrigation scheduling programs which evolved from
these early experiments consist of six primary steps (Fig.
7). First, an inventory of the soil and crop characteris-
tics for each field is made to gain an understanding of the
essential requirements for efficient irrigation. Then a
calculation of the water needs of the growing crops is made,
specifically at what rates are the crops and soil surfaces
utilizing water from the soil moisture reservoir. Such
estimates are generally based on well tested empirical tech-
niques, rather than actual measurements, because of conven-
ience and the established reliability of the computational
procedures. The next aspect of irrigation scheduling is to
determine the availability of moisture in the root zone for
meeting crop needs between irrigations. This step is ac-
complished by initially sampling the soil profile to measure
the available soil moisture storage, and then periodically
sampling to re-assess the available moisture and update the
consumptive use estimates. Upon determining the amount of
soil moisture available, the interval between irrigations is
projected. In addition, by knowing the soil and irrigation
system characteristics, the amount of water to be applied
(as well as the means to accomplish the suggested applica-
tion) can be determined. Finally, the results of the pre-
vious steps must be delivered to the irrigator in order to
implement the suggestions. This program is subsequently re-
peated throughout an irrigation season.
ESTIMATION OF EVAPOTRANSPIRATION
Several techniques have been developed as means for estima-
ting the evapotranspiration from growing crops including
soil sampling, lysimeters, large scale hydrologic balances,
pan evaporation, and empirical computational procedures.
The two methods most used in scheduling programs are compu-
tational methods known as the Penman and Jensen-Haise
methods. These procedures provide daily estimates using
local climatological information. The accuracy varies ac-
cording to the ability to represent meteorological conditions
19
-------
Inventory Soil and
Crop Characteristics for
Each Field
Compute Expected
Values of Evapotranspiration
During Next Seven Days
Measure Soil Moisture
Availability
Determine Date of
Next Irrigation
Determine How Much Water
to be Applied and the Means
to Accomplish the Specified
Application
Communicate Scheduling
Suggestions to Individual
Irrigators
;s This the
rinal Irrigation ?
Irrigotor Communicates
Date and Amount of
His Last Irrigation to
Scheduling Service
(stop)
Fig. 7,
Irrigation scheduling components
20
-------
properly, but generally the accuracy can be within 10 to
15% (Jensen, 1972).
Penman Equation
In 1948, Penman combined an energy balance equation with an
experimentally derived aerodynamic function for water vapor
movements to yield an expression for estimating the evapora-
tion from an open water surface. The values determined from
the relationship were multiplied by empirical constants to
arrive at the evapotranspiration rates from well-watered
grass. However, in 1963 Penman determined that by using a
proper value of albedo (a = 0.25), the potential evapotrans-
piration for grass could be evaluated directly.
The form of the Penman combination equation employed today
can be written,
Etg = ATY" (Rn+G) + A+Y~ d5-36**1-0 + 0.0062u2)(e° - ez) ..(1)
~2
in which Etg is the potential evapotranspiration in cal cm
day"1 from a well-watered short-grass surface, A is the
change in saturation vapor pressure with temperature in mb
deg , y is a property of dry air in mb deg"1, Rn is the net
radiation in cal cm"2 day"1, G is the sensible heat flux to
the soil in cal cm"2 day"1, U2 is the wind speed at a height
of two meters expressed in miles per day, e| is the satura-
tion vapor pressure in mb for atmospheric conditions z meters
above the crop surface, ez is the existing vapor pressure in
mb at a height of z meters, z is the height at which the
wind velocity is actually measured.
The terms in Eq. 1 may not be readily available, thus re-
quiring additional computations. First, the terms A/(A+y)
and Y/(A+Y) can be calculated, but are usually expressed in
convenient tables such as shown in Table 2. Net radiation,
Rn, is determined either by direct measurement or by the re-
lationship,
(2)
where a is the albedo (usually = 0.225-0.25 for green crops),
Rs is the solar radiation in cal cm"2 day"1 which is general-
ly measured, and Ri., is the net outgoing thermal radiation in
cal cm"2 day"1. The value of R^ is calculated with the re-
gression expression,
21
-------
Table 2. Variation of A/(A+Y) with elevation and temperature.
to
Temp. °C
Temp. °F
Elevation
(ft)
0
2000
4000
6000
8000
10,000
-6.7
20
.296
.309
.324
.339
.357
.376
-1.1
30
.377
.392
.408
.425
.444
.464
4.4
40
Values
.461
.477
.493
.500
.530
.551
10
50
of
.534
.559
.576
.592
.610
.630
15.6
60
A/A+Y
.620
.634
.649
.665
.682
.699
21.1
70
.687
.700
.714
.728
.743
.758
26.7
80
.745
.757
.769
.781
.794
.807
32.3
90
.794
.804
.814
.824
.835
.846
37.8
100
.833
.842
.851
.859
.868
.877
A+Y A+Y
-------
a s
Rso
+ b Rbo «3>
in which a and b are the regression coefficients usually
taken as 1.0 and -0.2 for arid areas respectively, Rso is
the clear day solar radiation taken from existing charts
and expressed in cal cm"2 day"1, and Rj-,o is the clear day
net outgoing thermal radiation in cal cm"2 day"1 calculated
by,
"bo = («
= ai +b1/e, 111.71 x io-» IT * .... (4)
where a-^ and b^ are again empirical constants (0.39 and
-0.042 in arid regions), e^ is the saturation vapor pressure
at mean dew point temperature in mb, and T^ is the average
daily temperature, °K. The final parameter in Eq. 1—wind
speed at two meters—is generally not available at this
height. Therefore, the wind speed at two meters can be ap-
proximated by the following logarithmic relationship,
u, = u (2/z)°-2 (5)
in which uz is the wind speed in miles per day at a height
of z meters.
Upon generating the value of Etg, an equivalent depth can be
determined by multiplying by 0.00171 to obtain mm/day and by
0.000673 to obtain in/day.
Although the Penman relationship includes most of the para-
meters affecting evapotranspiration rates and is sensitive
to a wide variety of climatic changes, it remains primarily
a tool for experimental work. Data for the function must
be collected in sites representative of the crop surfaces
being evaluated, thus requiring instrumentation needs far
beyond the capability of most agricultural areas. However,
as part of large irrigation scheduling services, instrumen-
tation for these data needs are often justifiable in terms
of reliability and consistency.
Jensen-Haise Method
The second method which is of great use in irrigation sched-
uling programs is the Jensen-Haise Method of estimating
23
-------
evapotranspiration . In 1963, Jensen and Haise re-evaluated
about 3,000 published and unpublished short-period measure-
ments of evapotranspiration based on soil sampling procedures
during a 34-year period in the western United States (Jensen
and Haise, 1963). These data were correlated with solar
radiation (the main component of the energy balance equation)
and mean air temperature in order to develop an equation for
potential evapotranspiration. Potential evapotranspiration
as used here represents the upper limit, or maximum, evapo-
transpiration under given climatic conditions that occurs
within a field having a well-watered crop of alfalfa with
about 12 to 18 inches of top growth. The equation to com-
pute this potential evapotranspiration is,
Eta = CT(T " VRs
where Cy is an air temperature coefficient which is constant
for a given area and is derived from the long term mean max-
imum and minimum temperatures for the month of highest mean
air temperature, T is the daily mean temperature, T is a
constant for a given area, Rs is daily solar radiation ex-
pressed as cal cm~2 day'1, and Eta is the potential evapo-
transpiration of the alfalfa, in cal cm~2 day-1.
When accurate evapotranspiration data are available for an
area, Cip and Tx can be determined by calibration. When the
data are not available, then the coefficients are estimated
by empirical equations. The equation for the temperature
coefficient Crp is,
CT = Cj + C2C
'H
where CH is a humidity index and is evaluated as follows,
r - 37.5 Hg ,_ 50 mb ,0.
CH - e£ - e? " eg - e? <8)
in which eg is saturation vapor pressure in mm Hg (or in mb)
at mean maximum air temperature during the warmest month and
ef is the saturation vapor pressure at mean minimum air tem-
perature during the same month. The value of Ci is found as
follows:
Ci = 68°F - (3.6°F x elevation in feet/1000 feet). .(9)
24
-------
The value of C2 is 13°F (or -10.6°C) while the coefficient Tx
is calculated as:
T = 27.5°F - 0.25(ef - e?)°F/mb - (elev./lOOO feet)°F (10)
A
The equation coefficients are primarily a function of specif-
ic areas and are adjusted for elevation to correct for dif-
ferences in atmospheric pressure, which affects vapor pres-
sure values.
Although this method is simpler to use in a scheduling pro-
gram because it requires less meteorological data than the
Penman equation, it may be somewhat less sensitive to cli-
matic changes such as windy weather. A comparison of the
Penman and Jensen-Haise methods (both adjusted and unadjust-
ed for elevation at the Grand Valley) with pan evaporation
data for the Grand Valley area is presented in Fig. 8 from
data supplied by the Bureau of Reclamation. It should be
remembered in viewing Fig. 8 that the estimates represent
evapotranspiration from different surfaces. Thus/ the
Jensen-Haise (both standard and adjusted) is for well-water-
ed alfalfa, while the Penman is for short grasses. The
variation in the Jensen-Haise methods with altitude should
be carefully examined since the existing practices in Grand
Valley suggest the elevation corrections introduce, rather
than correct, error. However, a recently released study at
an altitude of 10,000 feet indicates the elevation correction
is accurate and, therefore/ deserves further evaluation
(Kruse and Haise, 1974).
EVALUATION OF SOIL MOISTURE
Water provides the structural integrity of the plant tissues,
as well as performs an important role in the transport of
nutrients throughout the plant body. Within the soil-water-
plant system, a liquid phase provides a continuous link be-
tween soil, plant, and atmosphere. The details character-
izing soil-water-plant relationships are far too vast to
describe herein, rather a brief review of some of the more
important aspects will be touched upon. Specifically, the
aspect of the soil-water-plant system of primary interest in
this study is the soil water availability.
The availability of soil moisture is governed by several im-
portant parameters including (Buckman and Brady, 1969):
25
-------
8Q
in
a>
60
c
9
& 40
o
o
w
"5.
v>
o
CL
>
UJ
Ap7 ' May ' June ' July ' Aug ' Sept ' Oct
100
Jensen
(adjusted for elevation)
Penman
Pan Evaporation
Jensen (not adjusted for elevation)
I—i
' ' 1 1 1 1 1 1
150 200
Julian Days
250
300
Fig. 8. Cumulative potential evapotranspiration in
Grand Valley for 1973 irrigation season.
26
-------
1. Soil texture, structure, and organic matter, all
of which affect the moisture tension relationship;
2. Osmotic potentials governed by the solutes in the
soil solution and the rates of evapotranspiration;
and
3. Soil depth included within reach of the plant root
system.
Soil texture determines to a large extent the amount of
water which can be stored in the profile for use by crops
and the rates with which water enters and moves through the
soil. Fine textured soils, those with high clay contents,
hold much more moisture than do coarse textured soils such
as those with high sand contents. In addition, the coarse
soils drain faster and thus require more frequent irrigation
and greater care with respect to the quantities applied.
The moisture generally available for use by the plants is
usually taken as the difference between field capacity and
the wilting point. However, to avoid placing stress on the
crops, irrigations are usually scheduled when the available
soil moisture is depleted by 50% (Buckman and Brady, 1969).
The size and shape of the soil aggregates play an important
role in the productivity of different soils, by affecting
the ease with which roots penetrate the soil, along with
the rate at which water enters and moves through the soil.
Soils with poor structure impede the intake of water into
the soil and consequently reduce the water available for the
plants. Soil structure also affects the moisture holding
capacity and the distribution of roots. Often, farming
practices which are not suitable for the local soils will
reduce yields by diminishing the soils capability to op-
timally support crop growth.
The percentage of organic matter in the soil and its in-
fluence on moisture holding capacity has been the subject of
debate for a number of years. In the Central Valley of
California, several studies were made on soils with various
quantities of organic matter added to the soil. The con-
clusions drawn from these studies indicated that although
large additions of organic matter did in some cases increase
field capacity percentage, they also raised the wilting
point percentage; hence little affect on the additional
available moisture capacity was realized (Willardson, 1972).
Other studies were made which stated that the quantity of
water that soils can hold in a form available for plant use
is influenced by both the quantity of clay and organic mat-
ter in the soil. In general, soils which are low in clay
and organic matter have low water holding capacities, while
27
-------
soils having large amounts of these materials usually have
increased capacities for holding water. Sandy soils show
appreciable increases in available storage capacity when in-
creased amounts of organic matter are used.
Among the more influential parameters which affect the soil
water availability are those which affect the osmotic po-
tential. Probably the most critical of these factors is the
concentration of dissolved salts in the soil water. As the
salt concentration increases, the osmotic potential must in-
crease to extract the moisture from the soils such that the
wilting point may vary quite widely from its usual value of
-15 bars. In severe cases, the moisture content of the soil
must be maintained near field capacity to allow sufficient
water uptake by the crops. A typical plot of the salinity
effects on osmotic potential is illustrated in Fig. 9.
Another of the primary factors affecting the rate of crop
transpiration is the osmotic potential as demonstrated in
Fig. 10. From this illustration, it is important to note
that irrigation scheduling should vary allowable soil mois-
ture depletion percentages to reflect the rates of water up-
take by the plants.
Finally, a soil parameter having a pronounced effect on a
scheduling program is the depth of soil penetrated by the
root system. Soils which allow deep penetration by roots
increase the volume of the soil moisture reservoir available
to crops. In addition, the nutrient resources are more
effectively utilized, (if sufficient nutrients are not avail-
able near the ground surface), thereby increasing crop
yields. As the season progresses, rooting depths increase
on the annual crops, resulting in more available water and
plant nutrients.
The amount of soil moisture that has been depleted from the
root zone is determined by using gravimetric techniques, or
a "feel" method with a corresponding chart. Although the
sampling is more accurate, it requires more time to take the
samples and weigh them, dry them, and calculate a value.
The percent depletion is then obtained by interpolating be-
tween the wilting point moisture content and field capacity.
The other method, which is quicker but less accurate, is the
"feel" method used by the Bureau of Reclamation. The mois-
ture depletion is determined by taking a sample with an Oak-
field probe and manipulating it in the hand and observing
whether it will make a ball or a ribbon of certain length,
thus fixing the moisture depletion. The depletion is de-
termined either from charts such as illustrated in Table 3,
or by calculation. In the latter case, the summation of
the evapotranspiration minus the summation of effective
28
-------
£
o
c
V)
-------
Table 3. Chart for estimating percent depletion of readily available s>oil moisture by
feel.
Per Cent
Depletion
100-75
75-50
50-25
25-0
0
(Field
Capacity)
Feel or appearance of soil by general texture groups
LIGHT
Loamy fine sand- fine sandy loam
Dry, loose, flows through
fingers
Appears to be dry will not
form a ball.*
Tends to ball under pressure
but seldom holds together.
Forms a weak ball, breaks
easily, has no slick feeling.
Upon squeezing no free water
appears on soil but wet out-
line of ball is left on hand.
MEDIUM
Very fine sandy loam-silt
Powdery, dry, will not
form a ball.*
Dry almost powdery. A
ball can be formed under
pressure. The ball is very
crumbly and hardly holds
its shape. Will not rib-
bon; soil too crumbly.**
Forms a pliable ball. Soil
doesn't stick to hand. No
moisture on hand just damp
feeling. Ribbons readily.
Soil has no slick feeling.
Forms tight plastic ball;
slightly sticky. Ribbons
easily; solid. Slick
feeling with moist par-
ticles left on hands.
Same as light.
FINE AND VERY FINE
Sandy clay loam-clay
Hard , baked , cracked
sometimes has loose
crumbs on surface.
Somewhat pliable. Will
ball under pressure.*
Forms a pliable ball.
Ribbons readily.
Soil sometimes has a
slick feeling.
Easily ribbons out
between fingers; has
a slick feeling.
Same as light.
* Ball is formed by squeezing a handful of soil firmly.
** Ribbon is formed by squeezing soil out between thumb and forefinger.
Supplied by Bureau of Reclamation, Grand Junction Projects Office
-------
rainfall which occurs during the period of interest is the
moisture depleted from the root zone. A promising alter-
native to these methods, generally limited to research
studies, is the use of neutron probes which are calibrated
by gravimetric sampling. This method generally is the most
accurate because of the consistency of the measurements.
PREDICTING SUBSEQUENT IRRIGATIONS
The expected date of the next irrigation is determined by
knowing the maximum allowable depletion of soil moisture,
MQ, the depletion which has occurred to date, M^, and the
average expected evapotranspiration, E^., for the three
preceding days and the three forecast days. The number of
days N till the next irrigation is calculated by,
N = (MQ - Md)/Et .............. (11)
where N equals O if M MQ ............ (13)
where Wa is the total depth of water per unit area to be de-
livered to the field; and Ef is the farm efficiency which
is the percentage of water supplied to the farm that is con-
sumptively used. The amount of water that must be applied
to the root zone will be determined by the method of irriga-
tion which the farmer uses and his skill and willingness to
apply water efficiently to reduce the losses below the root
zone (deep percolation) and the tailwater runoff from the
field. The irrigation scheduling program utilized by the
USSR in Grand Valley is depicted in Table 4.
IRRIGATION SCHEDULING REQUIREMENTS
As a summary of this section, the authors would like to dis-
cuss the requirements for successful irrigation scheduling
programs in the light of an overall salinity control ob-
jective. In previous segments of this section, the
31
-------
Table 4. USBR irrigation scheduling service in Grand Valley.
DATE OF COMPUTER UPDATE-
SECTION A-
-June 7, 1971
(Update 3)
COMPUTER UPDATE NUMBER
^-SECTION B
u>
to
Recorded Dally Climatic Data
and Potential Evapotranr;plratlon
for the Update Period
Date
601
602
603
604
605
606
2
Ave.
Tecp.
Deg.F
49.5
52.0
50.5
45.0
46.5
47.0
J
Solar
Rad.
In./D
.37
.40
.41
.40
.41
.21
4
Daily
ETP.
In./D
.20
.23
.23
.19
.20
.10
5
Effect
Precip
Inches
.00
.00
.00
.00
.00
.00
6
Wind
Run
Kile*
70.0
30.0
90.0
90.0
12U.O
5.0
FARM NAME-
Forecast Potential
Evnpotransptrat Ion
Inches Per Day
1
Date
607
608
6CJ9
610
611
612
613
^ 'FARM
2
ETP.
In./D
.29
.30
.30
.30
.30
.31
.31
'ffDMWR
1
Dote
614
615
616
617
618
619
620
2
ETP.
In./D
.31
.32
.32
.32
.32
.32
.33
SECTION C
SAMPLE FARM
Farm Code
Irrigation Schedullcs— Field Statue as of June 7
1971
123
Fid Code
Crop and
Sice Ac.
1 Pas 40
2 Alf 47
3 WHE 22
4 5
Dates
Plant &
Cover
505 605
510 710
514 720
6
Crop
Coef
Ave
.87
.69
.24
7
RT
ZN
FT
3.0
8.5
2.0
e
Soil
Type
Silt L
9
Hold
Cap.
Inch
3,8
10.4
2.6
to
Allow
Depl.
Inch
2.7
7.3
1.0
//
Hep I
Molsl
Beg
.0
.0
.0
"f? '
Soil
«.tr«
End
1.0
.1
.5
13 14
15
Irrigations
Last
Date /iint
In.
518 1.8
519 1.5
520 3.5
16
Next
Date
613
701
612
Ant
in.
3.7
11.5
1.5
-------
essential aspects of a scheduling program included determi-
nation of evapotranspiration rates, soil moisture avail-
ability, and intervals between irrigations of specified
application. In addition, the communication of data between
irrigator and scheduler was noted. When these tasks are
integrated in a manner that maximizes the efficiency with
which a farmer applies water to his land, an effective ir-
rigation scheduling program is considered implemented.
In order to achieve any reduction in the salt load reaching
the Colorado River, irrigation scheduling must accomplish
more than just specifying a quantity of irrigation water to
be applied at a specific date. The quantity of irrigation
water applied must result in reduced deep percolation losses,
Therefore, the problem at hand is to determine the essential
elements of an irrigation scheduling program that will mini-
mize deep percolation losses.
33
-------
SECTION V
FIELD INVESTIGATIONS
EXPERIMENTAL DESIGN
The efficiency with which water is utilized by the irriga-
tors in the Grand Valley varies widely, depending on soil
and crop factors as well as the skill and concern applied
by the farmer to each irrigation. Since the more acute
salinity problems are traceable to the most inefficient ir-
rigation practices, irrigation scheduling as a salinity
control measure will also exhibit a wide range of effects.
The principle aim of this project is directed towards estab-
lishing this range of effectiveness. In order to success-
fully evaluate irrigation scheduling as a salinity control
alternative, the scope of the project has included a more
detailed examination of segments of the hydro-salinity sys-
tem. Specifically, such parameters as soil, crop, and field
characteristics, along with irrigation efficiencies, surface
and groundwater outflows, and root zone moisture and salt
movements are of special concern.
To accurately describe the important variables in the study,
the design of the field investigations were formulated into
three phases. The first phase was continued monitoring of
essential water and salt movements within the hydrologic
framework surrounding the farming environments. Next, an
effort was made to evaluate some of the operational problems
inherent in implementing irrigation scheduling on croplands
in the valley. This aspect of the study was included to
provide some experience regarding actual field problems on
farms in serious need of structural rehabilitation and im-
proved water management. The primary emphasis of the study
was centered in the final phase which involved a concerted
effort to evaluate the on-farm factors determining the ef-
fectiveness of irrigation scheduling.
In this section of the report, a description of the field
work undertaken to accomplish the three work phases will be
presented. Since the study involved a great deal of data
collection and analysis to arrive at the final results, a
representative sample will be periodically included so the
reader will develop some appreciation for the results.
Some of the data is descriptive of the more significant
segments of the hydro-salinity network throughout the valley
and thus serves as a good illustration of these conditions.
Hopefully, the inclusion of some field data will provide a
smoother transition between this section and the presenta-
tion of the findings in the following section.
34
-------
AREA HYDROLOGY
Most of the extensive instrumentation employed during the
initial evaluation of canal and lateral linings was main-
tained and utilized during this project. Since the scope
of this study was altered slightly to facilitate the project
objectives, this data collection effort was generally re-
duced. The measurements of canal inflows and outflows were
discontinued, and the measurements of drainage discharge,
piezometer fluctuation, and subsurface exploration were re-
duced. During the years of investigations in the area,
project personnel were able to equip substantial laboratory
facilities for detailing the chemical characteristics of
both soil and water. In this project period, the number of
samples analyzed was expanded to provide a broader basis for
stating the chemical quality characteristics of the area.
Among the additional samples collected and analyzed were
those taken from the local canals and ditches which were
compared to data published in the U. S. Geological Survey
surface water records.
Groundwater Movements and Quality
The analysis of data collected from wells extending into the
cobble aquifer indicates the magnitude of the flows return-
ing to the river, as well as the salts being transported as
dissolved minerals. The well installations, numbering 17
in all, were located throughout the test area as part of the
earlier investigations.
Evaluations made earlier indicate that the regional effec-
tiveness of a salinity control program will be manifested
primarily in the hydraulic characteristics of the aquifer.
A comparison of data relating the hydraulic conductivities
of both the area soils and the aquifer indicate that the
cobble is many times more permeable than the upper soils.
Since the soils and cobble are generally saturated and ex-
hibit only slight differences in hydraulic gradients, nearly
all of the subsurface return flows are occurring in the
cobble aquifer. Within the period of study in the area,
the water levels in the wells have not varied significantly
over the annual cycle. For example, data from two wells,
one near the river {Well No. 3) and the other (Well No. 12)
two miles further north, have been plotted in Fig. 11 to
illustrate both the seasonal and annual variation. The
changes in hydraulic gradient occurring during the period of
this study can be evaluated by dividing the elevation dif-
ferences by the distance between the two wells. For example,
at the maximum and minimum points during each year, the
hydraulic gradients are shown on Fig. 11. It is apparent
35
-------
OJ
Well Elevation Data, Well No.12
^T I V-/^T —- * , _^ , ^> K • A •« i t A ft
ONDJFMAIYIJJASONDJ FMAMJJASONDJ FMAMJJA.S
Well Elevation Data, Well No. 3
ONDJ FMAMJJASONDJ FMAMJJASONDJ FMAMJJAS
197 I
I 972
1973
Fig. 11. Seasonal variation of water levels in two wells monitoring the cobble aquifer,
-------
that the minimum and maximum points for each well occur at
different times, pointing out the time lag which may exist
as the water flows from one point to another. Another im-
portant observation is that the hydraulic gradient changes
only 5-8 percent during the season; and over the three years
of available data, the variation was small.
The water quality samples taken from these wells are the
most conclusive demonstration of the salt pickup occurring
in the Grand Valley. A summary of data collected from Well
No. 3 near the Colorado River, presented in Table 5, indi-
cates several important characteristics of the flows being
evaluated by the wells. The value of total dissolved solids
in this well range between 6,000 ppm and 8,000 ppm with an
average of about 6,700 ppm. The ratio between the concen-
tration of TDS and specific conductance for the irrigation
water ranges between 0.6 to 0.7, but the data from the well
indicates a ratio of about 1.09. This abrupt change is due
to the high sulfate content of the groundwater. Well quali-
ty data also depict the effect of irrigation in Grand Valley
upon specific salt constituents being added to the Colorado
River. Skogerboe and Walker (1972) reported that the average
cation composition of the irrigation water in terms of meq/1
is 41% calcium, 24% magnesium, 34% sodium, and 1% potassium.
Data in Table 5 indicate an average of 25% calcium, 43%
magnesium, 31% sodium and 1% potassium. The examination of
anionic constituents is even more revealing in that the per-
centage of sulfate increases from 58% in the irrigation
water supply to 79% in the groundwater return flows.
Drainage Outflows
The combination of tight soils and a ground slope of between
1 and 2 percent results in a large percentage of the farm
irrigation applications being wasted as field tailwater.
These flows, along with groundwater intercepted by the
drainage system, were measured with one-foot or two-foot
Cutthroat flumes and nine-inch Parshall flumes. Each drain
conveying flows from the study area was continuously moni-
tored by equipping the flumes with stilling wells and water
level recorders. Water quality samples were also collected
periodically from these drains in order to evaluate the
linkage between surface and subsurface hydrology.
The flows recorded during this study indicate that the
principal use of the drainage system is for conveying field
tailwater to the Colorado River. For example, the seasonal
variation in discharge for one of the most significant
drains, shown in Fig. 12, indicates a very rapid change
which corresponds to the beginning (April 1) and end
37
-------
Table 5. Selected water quality from Well No. 3 in the test area
OJ
oo
Cations
Sampling
Date
10-30-69
11-26-69
2-05-70
5-12-71
6-22-71
7-20-71
8-02-71
3-27-72
7-10-72
8-01-72
9-14-72
10-03-72
11-15-72
1-02-73
1-29-73
3-05073
4-02-73
4-30-73
7-02-73
7-30-73
TDS
(ppm)
6664
5904
7780
6400
6648
6500
6584
6604
6796
6884
6612
6476
6524
6472
6504
6544
6476
6408
8356
5573
EC
(w mhos/cm)
5900
4592
6136
5900
6800
6000
6335
6401
6489
6507
C533
6522
6381
6240
6355
6355
6355
5720
5497
5140
Ca
(ppm)
447
485
409
461
477
487
822
461
471
431
551
521
377
481
451
488
561
471
400 •
486
Mg
(ppm)
513
533
528
498
440
512
770
498
522
516
474
456
586
492
571
520
437
504
426
460
Na+
(ppm)
750
720
650
637
1,030
317
495
713
644
736
725
684
644
667
690
753
782
748
729
600
K+
(ppm)
17
18
17
14
15
14
17
15
13
13
14
14
15
14
14
17
16
10
12
10
HCOj
(ppm)
586
593
600
122
409
593
583
534
583
586
581
564
583
647
683
622
647
884
659
598
Anions
Cl~
(ppm)
332
328
328
158
111
299
306
309
304
302
330
278
307
313
342
488
362
300
400
328
Analysis*
soT
(ppm)
3525
3525
3063
4055
5380
3170
3540
3696
3240
4512
3552
4560
3648
3888
4032
3696
3600
3504
3024
3168
NO 3
(ppm)
163
137
90
62
157
149
130
130
124
126
129
114
114
122
124
127
119
105
114
139
Error
(%)
2
3
5
0
9
4
12
0
3
9
1
11
0
4
3
1
10
10
10
10
Average
6679
(Ications
( Ecations
6094
- lanions) x
+ lanions)
488
100.0
513
686
14
583
311
3719
129
1%
Note - Zcations and Eanions are expressed in meq/1.
-------
9
8
7
U)
Measured Drainage
Estimated Subsurface Contribution
(Based on Inflow-outflow)
ONDJ FMAMJJASONDJFMAMJJASONDJ FMAMJ JAS
Fig. 12. Seasonal and annual drainage flows for a selected drain in the test area,
-------
(October 31) of the irrigation season. The fraction of
these flows which can be attributed to either groundwater
interception or field tailwater is identified by the water
quality of the flows (Table 6). In comparing salinity
levels before and after the end of the irrigation season,
it is apparent that field tailwater accounts for about 80-90
percent of the flows in this drain.
Quality of Irrigation Water
As a means of rapid identification of certain segments of
the salt flow system near the farm environment, periodic
sampling for water quality was undertaken on the flows being
diverted for irrigation. Since these data, with the excep-
tion of the Mesa County Ditch, do not differ significantly
from the data collected by the U. S. Geological Survey at
the local river stations, a summary will not be presented
herein. Water for the Mesa County Ditch consists primarily
of spillage from the Grand Valley Canal into Lewis Wash
along with spillage from other canals, tailwater return
flows, and groundwater. Thus, the quality of irrigation
water supplied to croplands under the Mesa County Ditch is
significantly deteriorated as indicated by Skogerboe and
Walker (1972).
Use of Data
Earlier reports by Walker (1970) and Skogerboe and Walker
(1972) employed canal, drain, and groundwater flow data to
arrive at segmented budgets of the hydrology in the project
area. These studies were based on other investigative
efforts and modeling programs. Certain of those earlier
results have been employed to extend conclusions to the
broader context of the entire Grand Valley, with good cor-
relation.
For the purposes of this study, a recalculation of budgets
for the project area will not be made, because the effect of
irrigation scheduling can be established from the data pre-
viously presented. For example, each acre-foot of water
entering the groundwater can be expected to return to the
river containing 6,000 to 8,000 ppm of total dissolved
solids. Some of the salt in these flows are those initially
carried by the irrigation water, so the magnitude of salt
pickup is a fraction of the range noted above. Rather than
using flows and concentrations to determine salt pickup as
was done previously, data relating quantities of salts will
be used.
40
-------
Table 6. Summary of water quality data taken from a drain in the test area.
Sampling
Date
3-04-69
5-15-69
6-11-69
6-30-69
8-12-69
9-02-69
10-28-69
12-31-69
2-05-70
6-08-70
6-15-70
7-27-71
8-12-71
7-11-72
7-31-72
9-04-72
10-03-72
11-15-72
1-02-73
1-29-73
3-05-73
4-12-73
4-30-73
7-02-73
7-30-73
i
Average
Average
TDS
(ppm)
8056
2428
2148
1340
2388
2724
1596
7588
7880
2492
1788
1392
956
1692
2868
2256
1700
7472
7548
7692
7608
7368
1660
1460
1680
1916
7651
EC
(ymhos/cm)
5973
2282
1951
1897
2615
2349
1180
7168
7100
2564
2000
1856
2277
2163
3264
2714
2342
7151
7269
7246
7200
7175
2057
1816
2081
2200
7035
Ca++
(ppm)
471
128
120
134
144
164
196
489
491
168
160
88
170
140
207
176
152
417
371
461
581
581
124
154
115
149
483
Mg*"1"
(ppm)
529
141
117
85
161
129
73
503
547
140
92
117
79
107
182
129
100
567
577
492
378
431
104
61
92
112
503
Na"1"
(ppm)
425
290
200
195
356
270
245
975
950
225
260
162
251
230
451
319
253
909
960
932
1144
1093
260
87
275
255
924
K*
(ppm)
16
8
5
6
11
9
6
18
16
7
2
7
7
5
9
a
8
14
13
13
16
16
6
5
5
7
15
HCOi
(ppm)
403
215
200
205
268
288
234
547
493
257
254
207
257
215
2R8
261
251
517
659
610
561
599
415
232
232
252
549
Cl
(ppm)
340
144
100
92
192
184
148
324
326
155
41
115
194
127
187
207
160
310
313
367
427
322
139
62
122
139
341
SO,.
(ppm)
3000
1175
913
850
1275
1275
900
4050
4050
840
973
750
913
780
1368
1008
1008
4608
4176
4464
4086
4464
816
640
804
958
4112
NO 3
(ppm)
186
1
41
16
27
40
38
128
5
54
40
30
39
39
74
42
29
94
141
165
36
165
29
32
24
33
115
Analysis*
error
2
2
3
3
1
10
1
2
4
5
1
3
6
3
6
2
5
3
0
5
1
1
0
8
0
'Average of irrigation season samples
2Average of non-irrigation season samples
*Error = (Ecations - £anions)x 100.0
(Ecations + Eanionsj
2 /
Note - Ecations and Sianions are expressed in meq/1
-------
FACTORS AFFECTING IRRIGATION SCHEDULING
Since the purpose of this phase of the project was to
evaluate irrigation scheduling as a salinity control mea-
sure, two farms were selected upon which a detailed analysis
of the factors affecting the success or failure of sched-
uling irrigations was undertaken. In accomplishing this
work, three divisions in the analysis developed. The first
of these was the efforts to understand the operation of the
individual farms. Secondly, several investigations were
undertaken to determine the field characteristics describing
the inter-relationships between water management and salinity.
Finally, water and salt budgets were developed using data
collected over two years of study.
Field Descriptions
The two farms, (Buila and Martin farms), represent con-
trasting farming practices in an area where farming is not
the primary source of income. The Bulla farm was incorpora-
ted into this study to represent the valley-wide conditions
rather than local water management practices. Careful at-
tention is given to the details of farming since this field,
along with several others, is the primary enterprise of the
farmer. Because of the well operated nature of the farm,
soil conditions are well maintained allowing a variety of
crops to be grown including tomatoes, corn, barley, and
sugar beets. The agricultural variety of this farm is
particularly significant because the investigators have been
able to evaluate a wide range of agricultural conditions.
The Bulla farm, shown in detail in Fig. 13, consists of
about 25 acres of land supplied with water from a diversion
from the Price Ditch to the north. Excess irrigation water
supplied by a concrete head ditch at the northern boundary
of the field results in field tailwater, which is removed
from either corner at the southern boundary and then flows
into the Grand Valley Canal.
During all of the years that the Martin farm, shown in Fig.
14, has been investigated, the 8.5-acre field has been cul-
tivated exclusively in corn silage. This farm is one part
of a 200-acre farming operation conducted by Mr. Don Arnold
through a rental agreement with several local land owners.
As a result, the time spent in irrigating this field has
been minimized in order to facilitate the overall efficiency
of the total enterprise. These conditions presented a uni-
que opportunity during the 1973 irrigation season, as the
renter agreed to turn the irrigations of the Martin farm
over to the project personnel. Thus, while the Bulla farm
represented intensive analysis of an existing practice, the
42
-------
Build Form
Topography
o so 100 200
Scale, in f«et
4672.54
790'
Field Tailwater
Removal
4666.0 Water Surface Grand Valley Canal
Fig. 13. The Bulla farm layout.
43
-------
Irrigation
Water Supply
Martin Farm
Topography
0 50 100 200
Scale, in feet
Field Tail water
Removal
161
Water Surface 206*
Elevation
332
Fig. 14. The Martin farm layout.
44
-------
Martin farm allowed the investigators to explore the poten-
tial of various irrigation procedures.
Field Characteristics
In an effort to expand the scope of this study beyond simply
evaluating existing on-farm water management practices, the
investigators studied several of the important characteris-
tics of each farm to see where improvements might be made
to increase irrigation efficiencies. In addition to the in-
flow and outflow measurements, other parameters evaluated
included soil intake rates, furrow advance rates, field
levelness, and field water table elevations and chemical
quality of surface and subsurface flows.
The procedure employed to determine intake rates involved
carefully measuring the water into and out of a 200-foot
section of furrow with the time-varying differences recorded.
Then, the data are formulated into intake rate functions as
shown for two tests in Fig. 15. Intake rates vary consider-
ably throughout the irrigation season reflecting changes in
soil conditions and tillage practices. Generally, intake
rates were shown to decrease significantly as the season
progressed, indicating the necessity to modify irrigation
practices during the season.
Furrow advance rates, described by Skogerboe and Walker
(1972), were determined for a wide range of furrow flow
rates during this study. One hundred foot intervals were
marked along a furrow, with the time being recorded for the
flow to traverse each interval. The results, illustrated
for a typical set of data in Fig. 16, have only cursory
value in this study, but do indicate some interesting facts
concerning the time water should be left in the furrows
during each irrigation set. For example, suppose a furrow
on the Bulla farm is supplied with about 8.5 gpm in which
the advance rate and intake functions are similar to the
functions shown in Figs. 15 and 16. After about five hours
under normal conditions, the flow would have reached the end
of the field and began wasting a fraction to field tailwater,
If the flow is to remain in the furrow 24 hours, then the
fraction of the flows resulting in root zone applications
and field tailwater can be computed.
An example of these results can be plotted to demonstrate
the decisions a farmer may face in operating his irrigation
system to maximize returns from the land. In Fig. 17, a
cumulative intake is plotted against time for typical early
and late season conditions. During the initial stages of
an irrigation, the bulk of the water is applied to the root
45
-------
*»
a\
o
_c
c
o
cr
-------
300
200 -
to
-------
00
8
to
-------
zone. As the irrigation progresses, less and less water
infiltrates the soil surface, thereby resulting in most of
the flow being field tailwater.
These results also indicate possible methods for improving
the effectiveness of irrigation scheduling as a salinity
control measure by allowing the farmer to apply the proper
amounts of water to his soil by regulating the time that
water is allowed to remain in each furrow. For example/
suppose an early irrigation requires that only 5.5 inches
be added to the root zone storage in the field. If the
length of each irrigation set is reduced from 24 to 12 hours,
the deep percolation loss can be reduced from 2.5 inches to
0.3 inches. Thus, a great deal of salinity control would be
realized.
In addition to evaluating soil characteristics, a new di-
mension to this study was the observation of water table
levels and water quality within the farm confines. An
example of the elevation data, shown in Fig. 18, illustrates
the season variations of the on-farm water table heights.
In budgeting the root zone flows and monitoring water table
elevations around the farm, is was obvious that much of the
variations were due to changes in the groundwater system
outside the fields. The quality of the farm water tables
averages about 2000-3000 mg/1 less than deeper groundwater
flows, thereby suggesting the qualitative nature of the
salt pickup system. It may be worth noting, however, that
the effects of poor field levelness is apparent on the
Martin farm as the quality of groundwater under the upper
and lower ends of the field is 7000-8000 mg/1 higher (i.e.,
15,000-16,000 mg/1 TDS) than the aquifer flows pointing to
the possibility of poor leaching.
Irrigation Efficiencies
For the purpose of this study, irrigation efficiency has
been divided into two more specialized measures of efficien-
cy; namely, farm efficiency and application efficiency.
Farm efficiency is defined as the percentage of the water
supplied to the farming confines which was either consumed
by the crop or stored within the root zone for subsequent
use. This measure of efficiency represents the overall ef-
fectiveness of the irrigator in using his water supply on
his land. Application efficiency is the percentage of the
water entering the environment of the root zone which is
used by the crop or remains in root zone storage. This ef-
ficiency is intended to demonstrate the adequacy of the
irrigation system in supplying the crop with suitable mois-
ture. The difference between the two efficiencies is that
49
-------
Ul
o
0>
o
to
£
O
a.
a>
O
2
3
4
9
10
£ 12
13
April
May
June
July
Aug.
I 9 16 23 I 5 12 19 26 I 9 15 23 29 6 13 20 27 3 10 17 24 31
Dote
Fig. 18. Seasonal variations in water table elevations on the Bulla farm
during 1973.
-------
the latter does not account for field tailwater or seepage
from the field head ditch.
To determine values for either measure of efficiency, it is
necessary to measure inflows, outflows, and soil moisture
storage as well as calculate evapotranspiration. Deep
percolation is determined as the residual in a water budget-
ing procedure. The inflows to the fields and the field
tailwater were continuously measured with recorder equipped
Parshall flumes on either end of the field. The calculated
difference between the two flows during an irrigation period
is the quantity of water applied to the root zone. Once
within the root zone, the water is either consumed by the
crops, percolates below the root system, or remains in
storage. To account for this delineation, soil moisture
samples were taken before and after each irrigation. The
quantities of water transpired by the crops or evaporated
from soil surfaces were calculated by the methods described
in the previous section.
Based upon these data, water and salt budgets for the farm
environment were prepared. Farm and application efficiency
were then determined for each irrigation, as well as season-
al averages in order to evaluate the variation of these ef-
ficiencies throughout the irrigation season.
From the derived water and salt budgets, the effects of ir-
rigation scheduling can be ascertained. For example, the
relative effects of early irrigation are shown in the budgets
and can be compared with later water application efficien-
cies to determine where improvements are most needed.
51
-------
SECTION VI
RESULTS OF SCHEDULING EVALUATION
In the Grand Valley, irrigation scheduling along with field
drainage, conveyance channel linings, structural rehabili-
tation of the irrigation system, and non-structural incen-
tives for better water management appear to be the most
feasible array of salinity control alternatives. Of these
measures, irrigation scheduling may yield the most substan-
tial results because of the large number of parameters in
the hydro-salinity flow system which are influenced. Two
years of evaluating irrigation scheduling have produced
several important results with respect to salinity control
in the area which logically divide into the conclusions de-
rived from the study and the recommendations for strength-
ening the impact of irrigation scheduling. These two seg-
ments will be developed in this section by first presenting
an analysis of the water and salt budgets and then detailing
the requirements for successful irrigation scheduling in the
valley.
ANALYSIS OF WATER AND SALT BUDGETS
Budgeting Procedure
During this study as in those previous, the investigators
have identified the water and salt flows in terms of budgets
as a means of evaluating the impact of improvements. The
budgets generated for this study are narrow in scope and in-
clude only the irrigated field from the head ditch and waste-
water ditch to the bottom of the root zone (assumed to be
four feet). For each irrigation and the periods between the
water flows are determined. This process gives a complete
budget accounting for each budget interval and thus allows
the results to be examined in more detail. Because of the
scope of this evaluation, the budgets presented are primari-
ly of the water flows with the salts described only in the
context of salt pickup from deep percolation.
The budgets to be presented in the following pages include
four segments: (1) Farm Inflows; (2) Root Zone Water Bud-
get; (3) Irrigation Efficiencies; and (4) Salt Pickup. In
the following paragraphs, the discussion will focus upon
the definition of these budget items and note the data from
which they are identified.
Inflows to the farm take the form of either diversions from
irrigation canals and ditches or effective precipitation
52
-------
(moisture added to the root zone). These flows are applied
to the crop surfaces and subsequently infiltrate into the
soil or become surface runoff from the field into waste
ditches or drains. Each of the inflows to the field and the
surface outflows are measured as described in the previous
section.
The root zone water budget accounts for moisture within the
reach of the crops. A depth of four feet was assumed to
encompass this region even through root depths often exceed
this value at maturity. Since the crops being grown on the
two farms were corn, sugar beets, and barley, the assumed
depth is not unreasonable. The first segment of the root
zone budget is the total water additions determined by com-
bining the water supplied by an irrigation with effective
precipitation. It should be noted that groundwater might
also be available to the deep rooting crops such as alfalfa,
although not to the two farms in this study. The moisture
supplied to the root zone replenished the soil moisture
storage reservoir. The change in this storage volume is
determined from soil moisture sampling at the beginning and
end of each budget interval. During the budget interval,
the crops transpire water which is calculated using the
Jensen-Haise method for estimating evapotranspiration. Fi-
nally, the residual flow, deep percolation, is arrived at
through a mass balance.
As a means of applying these results to the broader context
of the Grand Valley, two measures of irrigation efficiency
were defined earlier. The first of these, farm efficiency,
is the percentage of the farm water supply utilized by the
crops. Thus, farm efficiency is the consumptive use minus
the root zone storage change divided by the total root zone
addition plus field tailwater. Because farm efficiency
values are affected by both the quantities of field tail-
water and deep percolation, it serves as a general indica-
tion of water management on the farm. The second measure,
application efficiency, is the percentage of the root zone
addition utilized by the crops and thus is determined by
the formula for farm efficiency except that field tailwater
is omitted from the denominator. Consequently, application
efficiency describes the suitability of the irrigation de-
sign and the irrigation practices.
The final item in the budgets is the expected salt pickup
resulting from the deep percolation losses. These values
are determined by comparing the quantity of salts carried
by the irrigation diversions with the quantity in the
groundwater as discussed in the previous section. Samples
from the local canals and the U. S. Geological Survey
samples from the river inflows were compared with well data
53
-------
such as that presented previously in Table 5. It should be
noted that values in Table 5 are slightly low with respect
to the average groundwater qualities as determined by the
wells.
The 1972 and 1973 water and salt budgets for the Martin and
Bulla farms are presented in Tables 7, 8, 9, 10, and 11.
For purposes of clarifying the budgeting procedures, some
numerical segments from the tables can be abstracted for
demonstration. Consider first the initial irrigation on the
Martin farm during the 1972 irrigation season (Table 7).
Flows passing the inlet flume were recorded as 129.48 acre-
inches, while the flows passing the two field exits were
measured as 52.80 acre-inches, thereby leaving 76.68 acre-
inches in the soil profile (root zone addition). During
this period between April 18th and April 27th, 5.1 acre-
inches of rain fell on the field of which three-fourths, or
3.83 acre-inches, were assumed effective in infiltrating
the soil surface.
Of the 80.51 acre-inches of moisture added to the root zone
region (irrigation plus precipitation) only a negligible
amount evaporated from the surface because of climatic con-
ditions and the crop had not yet emerged. However, the soil
moisture samples indicated that 35.46 acre-inches had been
stored in the root zone leaving 45.05 acre-inches as deep
percolation.
If the two measures of efficiency as defined earlier are
calculated, it is apparent that this first irrigation was
highly inefficient in leading to 31.4 tons of salt being
picked up. Specifically, the 76.68 acre-inches of moisture
added by way of irrigation to the root zone transported 2.8
tons of salt (322 ppm) and the 45.05 acre-inches of deep
percolation upon reaching equilibrium with the 6700 ppm
groundwater would carry an expected 34.2 tons, or a net in-
crease of 31.4 tons.
During the interval between April 27th and June 17th when
no irrigations were made, the crops essentially drew water
from the soil moisture reservoir. The intervals between
irrigations often indicates additional unsaturated deep
percolation, making the efficiency values in the analysis
slightly optimistic.
Analysis of Irrigation Efficiencies
Probably the best indication of the effectiveness of a
change in irrigation practices can be gained from analyzing
irrigation efficiencies. During the two years of this
54
-------
Table 7 . 1972 Martin farm water budget. (8.5 acres of corn)
BUDGET INTERVAL (All values in units of acre-inches)
Budget Item
Farm Inflows
Field Tailwater
Root Zone Additions
Precipitation (effective)
Root Zone Water Budget
Total Additions
Consumptive Use
Storage Change
Deep Percolation
Irrigation Efficiencies
Farm Efficiency
Application Efficiency
Salt Pickup (tons)
(tons/acre)
4/18
to
4/27
52.80
76.68
3.83
80.51
0
35.46
45.05
26.6%
44.0%
31.4
3.69
4/27
to
6/17
0
0
0
0
21.99
-22.13
0.14
__
—
0.1
0.01
6/17
to
6/28
21.60
31.68
0
31.68
25.16
b/28
to
7/5
10.00
22.31
1.70
24.01
20.27
-11.17 i -10.96
17.69
26.3%
44.2%
11.8
1.39
14.70
27.4%
38.8%
9.9
1.16
7/5
to
7/12
0
0
0
0
18.0
-18.0
0
__
—
0
0
7/12
to
7/17
12.36
28.03
0
28.03
17.85
-3.40
13.58
35.8%
51.6%
8.9
1.05
7/17
to
7/24
0
0
0
0
18,15
-18.20
0.05
__
—
0
0
7/24
to
7/31
12.59
33.64
0
33.64
20.32
9.18
4.14
63.8%
87.7%
1.7
0.20
7/31
to
8/8
17.52
20.04
0
20.04
22.70
-5.53
2.87
45.7%
85.7%
1.0
0.12
8/8
to
8/21
54.12
54.72
2.13
56.85
31.88
14.79
10.18
42.1%
82.1%
4.4
0.52
8/21
to
9/10
0
0
0.94
0.94
32.73
-31.79
0
—
— —
0
0
Total
180.99
267.10
8.60
275.70
229.05
-61.75
108.40
36.6%
60.7%
69.2
8.14
01
-------
Table 8 . 1973 Martin farm water budget. (8.5 acres of corn)
BUDGET INTERVAI {All values in units of acre-inches)
Faro Inflows
Field Tailwater
Root Zone Additions
Precipitation (effective)
Root Zone Hater Budget
Total Additions
ConsuBptive Use
Storage Change
Deep Percolation
Irrigation efficiencies
Fam Efficiency
Application Efficiency
Salt Pickup ( tons >
(tons/acre)
5/1
to
6/17
44.04
70.08
22.27
92.35
20.00
6.38
65.97
19.3%
28.6%
47.0
5.53
6/17
to
6/25
28.32
33.36
0
33.36
12.32
3.06
17.98
24.9%
46.1%
12.5
1.47
6/25
to
7/1
0
0
0
0
13.02
-13.02
0
—
— —
0
0
•7/1
to
7/10
36.38
23.97
0
23.97
24.10
-3.82
3.69
33.6%
84.6%
1.8
0.21
7/10
to
7/16
0
0
2.21
2.21
14.26
-12.05
0
—
— —
0
0
7/16
to
7/20
7.22
8.67
3.86
12.53
7.36
5.00
0.17
62.6%
98.6%
0
0
7/20
to
7/26
0
0
2.94
2.94
10.38
-7.44
0
—
— —
0
0
7/26
to
7/30
8.58
9.01
0
9.01
11.33
-2.80
0.48
48.5%
94.7%
0
0
7/30
to
8/3
0
0
0
0
7.68
-7.68
0
--
--
0
0
8/3
to
8/9
6.04
9.78
2.76
12.54
12.00
0.54
0
67.5%
100.0%
0
0
8/9
to
8/14
0
0
0
0
8.00
-8.00
0
—
—
0
0
8/14
to
8/17
4.25
6.38
0
6.38
5.40
0.98
0
60.0%
100.0%
0
0
8/17
to
8/21
0
0
0.64
0.64
6.28
-5.64
0
—
—
0
0
8/21
to
8/28
30.60
36.89
0
36.89
7.17
24.72
5.0
47.3%
86.4%
1.6
0.19
8/28
to
9/10
0
0
0
0
2.92
-2.92
0
—
—
0
Total
165.43
198.14
34.68
232.82
162.22
-22.69
93.29
35.0%
59.9%
62.9
7.4
(J\
ON
-------
Table 9
1972 Bulla farm water budget . (25.7 acres of sugar beets)
Budget Item
Farm Inflows
Field Tailwater
Root Zone Additions
Precipitation (effective)
Root Zone Water Budget
Total Additions
Consumptive Use
Storage Change
Deep Percolation
Irrigation Efficiencies
Farm Efficiency
Application Efficiency
Salt Pickup (tons)
(tons/acre)
BUDGET INTERVAL '(All values in units of
4/7
to
4/25
156.24
226.32
3.83
230.15
18.06
16.20
195.89
8.9%
14.9*
139.2
5.42
- 4/25
to
5/1
0
0
0
0
5.51
-7.31
1.80
--
1.3
0.05
5/1
to
5/11
28.20
24.60
0
24.60
10.58
+11.00
3.02
40.9%
87.7%
1.7
0.08
5/11
to
5/22
44.88
135.36
0
135.36
22.33
+78.30
34.73
55.8%
74.3%
22.5
0.87
5/22
to
6/16
0
0
12.15
12.15
110.97
-103.00
4.18
—
3.1
0.12
6A6
to
6/28
96.24
65.76
0
65.76
80.00
-20.00
5.76
37.0%
91.2%
1.6
0.06
6/28
to
7/6
0
0
0
0
72.63
-75.00
2.37
__
1.7
0.07
7/6
to
8/1
173.16
312.48
5.67
318.15
223.29
+13.50
81.36
48.2%
74.4%
47.5
1.85
acre- inches)
8/1
to
8/8
56.52
69.60
0
69.60
58.32
-15.66
26.94
33.8%
61.3%
17.1
0.67
8/8
to
8/28
113.16
113.76
6.75
120.51
137.16
-20.00
3.35
50.1%
97.2%
0
0
8/28
to
9/14
139.56
73.20
2.97
76.17
90.72
-20.00
5.45
32.8%
92.8%
0
0
VI4
to
9/ia
65.64
60.84
0
60.84
31.32
+27.00
2.52
46.1%
95.9%
0
0
Total
873.60
1,081.92
31.37
1.113.29
860.89
-114.97
367.37
37.5%
67.0%
235.7
9.18
-------
Table 10. 1973 Bulla farm water budget. (15.0 acres of corn)
BUDGET INTERVAL (All values in units of acre-inches)
Budget Item
Farm Inflows
Field Tailwater
Root Zone Additions
Precipitation (effective)
Root Zone Water Budget
Total Additions
Consumptive Use
Storage Change
Deep Percolation
Irrigation Efficiencies
Farm Efficiency
Application Efficiency
Salt Pickup (tons)
(tons/acre)
5/22
to
6/27
69.24
52.30
19.50
71.80
41.99
9.60
20.21
36.6%
71.9%
14.1
0.94
6/27
to
7/7
9.12
36.36
0
36.36
31.39
4.00
0.97
77.8%
97.3%
0.7
.05
7/7
to
7/14
0
0
2.60
2.60
23.74
-25.60
4.46
—
—
3.3
0.22
7/14
to
7/23
59.52
58.80
12.20
71.00
29.30
24.45
17.25
48.8%
89.8%
10.4
0.69
7/23
to
7/30
8.88
9.84
0
9.84
32.08
-28.05
5.81
21.5%
41.0%
3.9
0.26
7/30
to
8/13
85.80
120.24
4.50
124.74
57.15
34.15
33.44
43.4%
73.2%
17.4
1.16
8/13
to
8/27
67.92
84.48
1.05
85.53
56.68
-4.65
33.50
33.9%
60.8%
19.9
1.33
8/27
to
9/2
0
0
0
0
12.71
-22.50
9.79
—
—
7.2
0.48
9/2
to
9/10
57.96
74.88
0.45
75.33
26.00
-18.20
31.13
33.2%
59.5%
18.2
1.21
Total
358.44
436.90
40.30
477.20
311.04
9.60
156.56
38. 4%
67.2%
95.1
6.34
oo
-------
Table 11. 1973 Bulla farm water budget. (10.7 acres of barley)
BUDGET INTERVAL (All values in units of acre-inches)
Budget Item
Farm Inflows
Field Tailwater
Root Zone Additions
Precipitation (effective)
Root Zone Water Budget
Total Additions
Consumptive Use
Storage Change
Deep Percolation
Irrigation Efficiencies
Farm Efficiency
Application Efficiency
Salt Pickup (tons)
(tons/acre)
4/23
to
4/30
33.76
77.76
12.84
90.60
2.93
30.92
56.75
16.9%
23.2%
40.0
3.74
5/1
to
5/30
0
0
12.52
12.52
32.44
-40.83
20.91
—
—
13.4
1.44
5/30
to
6/7
12.60
29.28
6.96
36.24
16.13
20.00
0.11
59.7%
80.5%
0
0
6/7
to
6/13
0
0
0
0
20.31
-26.15
5.84
—
—
4.3
0.40
6/13
to
6/19
45.12
28.44
0.54
28.98
13.79
10.49
4.52
32.3%
82.5%
2.7
0.25
6/19
to
6/24
0
0
0
0
18.32
-19.80
1.48
—
—
1.1
0.10
6/24
to
6/30
18.00
34.92
0
34.92
17.98
10.06
6.88
53.0%
80.3%
4.3
0.40
6/30
to
7/7
51.12
61.92
0
61.92
28.03
-8.99
42.88
16.8%
30.7%
30.2
2.83
7/7
to
7/10
0
0
0
0
11.56
12.00
0.44
—
— —
0.3
.03
7/10
to
7/17
28.32
32.16
2.57
34.73
21.62
10.38
2.73
46 .7%
84 7%
O •» * 1 v
1.3
0.12
Total
188. 91
264.48
35.4:
299.91
183. 2S
-25. 92
142.54
24. 9*
40 . 71
99.6
9.31
cn
vo
-------
study, and the 1971 data presented by Skogerboe and Walker
(1972), efficiency determinations have been made for three
of the most common crops in the valley; namely, barley, corn,
and sugar beets. These data relating to application effi-
ciencies were plotted against time to determine the seasonal
variations and the effect of the crop being grown.
In Fig. 19, the seasonal distribution of application ef-
ficiency for corn in the test area is plotted. The selected
data points represent the three years of data collected on
the two farms from which a smooth curve has been sketched in
to illustrate the general nature of the seasonal application
efficiencies. Although it is apparent that efficiencies
increase significantly as the season progresses, a great
deal of variation is encountered as a result of year to year
changes in soil conditions, irrigation practices, and crop
rotations. In addition, some variation may be hopefully
attributed to the improvements in water management practices
being implemented by the irrigators as a result of the
salinity control efforts in the valley.
An interesting comparison which comes from being able to de-
fine such curves as Fig. 19 for other crops is the effect on
salinity due simply to the variety of crop grown. Efficien-
cy data for sugar beets and barley were plotted against time
for several irrigation tests in Figs. 20 and 21. On the
Bulla farm during 1972 and 1973 all three crops were grown.
From Table 9, the Bulla farm contributed 235.7 tons of salt
as a result of growing sugar beets, or 9.2 tons per acre,
while the corn and barley cropping contributed 9.3 tons per
acre and 6.3 toms per acre, respectively, as indicated in
Tables 10 and 11. It is also interesting to note that the
1972 irrigation season produced 21% (41 tons) more salt than
the combined total for the 1973 year. Although these fi-
gures are few, it is apparent that crop selection may have
a pronounced effect on salt loading and additional investi-
gation in this area may be indicated.
The average application or farm efficiency value presented
in the "total" column is misleading and often incorrectly
used in salinity control discussions. As an illustration,
the first two irrigations on the Martin farm during 1972
(Table 7) both had application efficiencies of about 44%,
yet the first irrigation caused almost three times as much
salt pickup. Thus, an important point to be drawn from
Tables 7, 8, 9, 10, and 11 is that the impact of individual
irrigations may be drastically different but be of compar-
able efficiency. Likewise, the obvious importance of the
early season irrigation in formulating salinity control
strategies is not illustrated. A possible method of ad-
justing efficiency values by weighting them by the
60
-------
100
90
80
70
<£
^60
o
c
0>
150
UJ
c 40
o
ci
^ 30
Q.
a
20
10
Apr
May
Jun Jul
Irrigation Season
Aug
Sept
Fig. 19. Seasonal variations in application efficiencies for corn grown in
the test area.
-------
100
80
60
10
.2 40
o
o
"5.
Q.
20
Apr
Fig. 20.
May
Jun Jul
Irrigation Season
Aug
Sept
Seasonal variation in application efficiencies for sugar beets grown
in the test area.
-------
u>
100
80
o 60
g>
o
LJ
o 40
o
o
"5.
Q.
20
0
Apr
May
Jun Jul
Irrigation Season
Aug
Sept
Fig. 21. Seasonal variation in application efficiencies for barley grown in
the test area.
-------
magnitude of salt pickup may improve the interpretation of
irrigation efficiencies as they apply to salinity.
The results of the budget data in Tables 7-11 show that the
goal for salinity control on the irrigated lands of the
Grand Valley should concentrate heavily on improving the
first two or three irrigations. A further demonstration of
this conclusion is shown in Pig. 22 where cumulative appli-
cations are compared to cumulative potential evapotranspira-
tion for the sugar beets grown on the Bulla farm in 1972.
During April and May, approximately 50% of the water supplied
to the root zone is added at a time when less than 20% of
the demand is experienced. The comparison in Fig. 22 does
not, of course, account for soil moisture storage changes
occurring in the root zone, which would change the curves
only slightly. In examining the effects of these conditions,
it can be seen that salt pickup is also accelerated during
this period. For example, the data in the budget tables
was plotted in a similar manner in Fig. 23 to illustrate
the seasonal variation in deep percolation losses. These
results clearly indicate the effects of early irrigations in
the Grand Valley, where about 60 percent of the deep perco-
lation occurs in the first two months of the season.
The effects of only the simplest adjustments in irrigation
practices during the early segment of the season could pro-
duce significant reductions in the salt loadings to the
Colorado River. If the quantities of water added to the
root zone during each irrigation in Tables 7-11 are divided
by the duration of each irrigation, an estimate of the
average intake rate can be determined as shown in Fig. 24
for the two study farms. During the first irrigation on the
Martin farm in 1972, 80.5 acre-inches were added to the
root zone soil moisture storage as a pre-irrigation by ap-
plying the water in 24-hour sets. In recalling the shape
of the accumulative intake relations shown previously in
Fig. 17, the intake after 12 hours would have been about
60% of the 24-hour values for early irrigations. Consequent-
ly, a 12-hour rather than the 24-hour set would have cut the
application about 40%. Examining data on the other budget
variables in Table 7 reveals that deep percolation losses
would have been reduced to about 40% (or 18.09 acre-inches
of deep percolation rather than 45.05 acre-inches), thus
illustrating the scope of the salinity control policy in-
volved in on-farra water management.
The implementation of irrigation scheduling in conjunction
with improved irrigation practices on the two farms can
also be demonstrated from the efficiency data presented in
the budget tables. Application and farm efficiency data for
three years on the Martin farm are compared monthly in
64
-------
100
en
en
Root Zone
Additions
Evapotranspiration
(Sugar Beets)
Jun Jul
Irrigation Season
Aug
Sept
Fig. 22
Comparison of root zone water supply and evapotranspiration demands
for sugar beets in the test area.
-------
100
3
O
<5
a.
O.
9
o
<5
a.
May
Jun Jul
Irrigation Season
Aug
Sept
Fig. 23. Seasonal percentage of deep percolation attributed to various irrigations
on the farms studied.
-------
CO
0>
1.0
0.9
0.8
0.7
0.6
_r 0.5
o
CC
0.4
o>
JC
o
^ 0.3
0.2
O.I
o Bulla
• Martin
Note - Average 24 Hour
Intake Rate
Apr
May
Jun
Irrigation Season
Jul
Aug
Fig. 24. Seasonal variation in average field intake rates for the Martin and
Bulla farms.
-------
Figs. 25 and 26. Except for the May irrigations, a notice-
able improvement is demonstrated in these graphs because
irrigations were conducted by project personnel. Careful
attention given to actual field conditions resulted in
considerable success. In fact, a comparison of 1971 and
1973 data indicates a general improvement in application ef-
ficiency resulting in significant reductions in salt pickup.
However, insufficient care was taken during the initial ir-
rigations due to inexperience, which resulted in large salt
loadings.
Data from the Bulla field, shown in Figs. 27 and 28, do
not show significant improvement. Although these data re-
present practices under different cropping patterns, the
improvement due to irrigation scheduling remains in doubt.
In talking with the farmer, it appears that the irrigation
scheduling is having only a marginal effect on his water
use practices.
Analysis of Salt Pickup
The last line in each of the budget tables represents the
total quantities of salts picked up by the water percolating
below the root zone. These values are determined by com-
paring the total quantities of salts added to the root zone
on each individual farm with the flows (water and salt)
leaving the study area. Typical salinity concentrations
employed in this determination were taken from inflow data
and data presented in Table 5.
The data from the two years of this study on each farm were
aggregated in a manner shown in Fig. 23 to indicate the
seasonal salt contribution from the farms. This analysis,
presented in Fig. 29, indicates that almost 60 percent of
the salt loading occurs in the April through May period.
Skogerboe and Walker(1972) have stated previously that an
increase in irrigation efficiency may not be accompanied by
an equal percentage reduction in salt pickup. These results
substantiate that this conclusion was correct. For example,
if the average irrigation efficiency is increased during the
April through May period, it is obvious that a great deal
more salinity damages can be avoided than a similar increase
later in the season when efficiencies are already high and
would be difficult to improve.
Over the years of study in the Grand Valley, several re-
searchers have determined the total pickup from the area in
terms of contribution per acre. The U. S. Environmental
Protection Agency (1971) stated this value to be about 8 tons
68
-------
tJI
vo
lOOr
03
O
H-
V(~
uu
80
60
c 40
o
a
o
"a.
0.
20
0
1971
1972
1973
1
1.
1
Apr
May
Jun Jul
Irrigation Season
Aug
Sept
Fig. 25. Comparison of application efficiencies for the Martin farm before and
after the implementation of irrigation scheduling.
-------
100
80
60
2
"o
E 40
20
0
May
Jun Jul
Irrigation Season
Aug
Sept
Fig. 26. Comparison of farm efficiencies for the Martin farm before and after the
implementation of irrigation scheduling.
-------
lOOr
80
1971
1972
US l973
• 60
o
c
-
-J y
M 2=
UJ
c
o
"5
o
"E.
Q.
40
20
0
Apr
May
Jun Jul
Irrigation Season
Aug
Sept
Fig. 27. Comparison of application efficiencies for the Bulla farm before and
after the implementation of irrigation scheduling.
-------
100
NJ
80
O
c
0>
O
60
Ill
£ 40
w.
O
LL
20
0
Apr
May
I
I
Jun Jul
Irrigation Season
Aug
Sept
Fig. 28. Comparison of farm efficiencies for the Bulla farm before and after the
implementation of irrigation scheduling.
-------
100
-J
00
Apr
May
Sept
Jun Jul
Irrigation Season
Fig. 29. Seasonal distribution of salt pickup from the farms in the test area.
-------
per acre, while Skogerboe and Walker (1972) listed a value
of approximately 12 tons per acre. The difference between
these two values is found in the number of irrigated acres
involved in the computations. Until 1971 when Walker and
Skogerboe (1971) completely mapped the valley, typical es-
timates of irrigated acreage were about 120,000 acres, while
the actual values were slightly over 70,000 acres. On the
basis of these facts, the existing estimates of salinity
contribution per acre are reconciled. In the data presented
in Tables 7-11, and using values for groundwater quality
listed in Table 5, the salt contribution per acre on the
Bulla and Martin farms ranged between 8.1 and 9.2 tons per
acre. However, other groundwater data taken from drains
and wells indicate the values in Table 5 are at the lower
limits of the variation encountered. Data from the upper
limit wells show that the per acre contribution ranges be-
tween 10.6 and 12.6 tons. Consequently, the values were
established again by this study as being in the range of
9-12 tons per acre. Since this study is based on more ex-
haustive data, than were the conclusions of earlier studies,
the reliability of these results are substantially reinfor-
ced.
IRRIGATION SCHEDULING IN GRAND VALLEY
The results discussed in the previous paragraphs demonstrate
that the potential effects of irrigation scheduling could
be most significant if both the timing and quantity of
irrigation applications can be controlled. In the Grand
Valley, as well as most irrigated regions, the effectiveness
of irrigation scheduling as a salinity control measure is
contingent upon the degree of service provided to the
farmer. An additional important factor is the irrigator's
ability to achieve the suggestions provided by scheduling
services. This study has focused the attention of the in-
vestigators on topics that are influential towards the suc-
cessful use of irrigation scheduling in Grand Valley: (1)
flow measurement; (2) water control; (3) communication of
data; and (4) local administration.
Water Measurement
Even though the quantity of water required by the root zone
conditions may be listed in the irrigation scheduling com-
puter printout, the reality of the Grand Valley situation
is that irrigators do not know how much water is entering
the root zone. Only a very few farms have a flow measuring
device at the farm inlet. The only farms where measurements
are actually recorded are those involved in research studies.
74
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Consequently, the utilization of irrigation scheduling in
Grand Valley is unlikely to produce any substantial re-
ductions in the valley salt loadings, because there will be
no reductions in water passing through the root zone unless
measuring devices are employed and the amount of water ap-
plied is controlled. The results presented earlier re-
inforce this conclusion.
The practices which have yielded these conditions are pri-
marily twofold. First, the irrigation companies relinquish
their responsibility for the water immediately upon diver-
sion from the canal turnout gate into the lateral subsystem.
Some lateral diversions have as many as ten or twenty indi-
vidual irrigators subscribing to the flow. Since an under-
standable inequity exists in the allocation of the water
among the several interests, a tendency exists to supply
enough water to meet each demand. Previous studies by pro-
ject personnel have shown that most turnout gates discharge
significantly more flow than is listed as the water right.
In fact, Skogerboe and Walker (1972) report that the water
duty for the valley often ranges between seven and ten acre-
feet per acre of irrigated land annually. The second di-
mension relative to local water management is related to the
first in that the cost of water is so low as to render eco-
nomic incentives for more efficient use nonexistent.
The first requirement to overcome laxity in water management
is standardized flow measurement. If ditch companies insist
on controlling only canal or ditch flows, then they should
deliver only the prescribed flow to each lateral. Legal
history and experiences are replete with examples indicating
that diverting water in excess of the required supply does
not insure the validity of the right. In examining histori-
cal land use changes, the irrigable acreage has been marked-
ly decreased by roads, ditches, homes, schools, etc. Con-
sequently, flow measurement at the canal turnout is needed
to reduce wasteful and salt-loading water use practices.
Water measurement is also needed between the canal turnout
and the field head ditches to indicate to the farmer how
much water he is applying.
Water Control
The judicious use of water on agricultural lands requires
careful consideration of the factors which affect the ir-
rigator's ability to control and regulate his water supply.
Some of the more important of these factors are structural
items like check structures, division boxes, and drops, as
well as nonstructural factors such as farmer attitudes, ir-
rigation practices, and system flexibility.
75
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The condition of the water delivery network which operates
between the canal turnout and the field head ditch is gen-
erally in a state of disrepair for most areas in the Grand
Valley. Laterals that are not eroding deeply into the soils
are heavily infested and obstructed with weeds and debris.
At locations where water is divided among two or more ir-
rigators, the typical wood or concrete checks and dividers
are in need of both rehabilitation and redesign.
The farmer is the first link in the food and fiber produc-
tion chain and he takes numerous risks, requiring conserva-
tive judgment in most decisions. Subsequently, innovations
to the agricultural industry must be adequately demonstrated
before wide acceptance is achieved. Under the objectives of
salinity control, the incentives which would encourage an
irrigator to participate in an irrigation scheduling program
also include convenience, increased productivity, as well as
decreasing downstream damages. As a result, improved control
of water resources should be presented as an approach to
minimizing the detrimental effects of inefficiency in crop
production.
Farming practices have a pronounced effect on water use.
For example, in the Grand Valley the tendency is to irrigate
the crops the same way during each irrigation. In the early
part of the growing season, when plant water needs are low
and the topsoil is kept moist to facilitate good plant
emergence, excessive water is applied resulting in high deep
percolation losses.
Different irrigation practices have different labor, capital,
and time requirements. The labor required for a sprinkler
system would, for example, be generally less than the flood
types and thus facilitate water use improvements by the
irrigator, but at an increase in capital investment. During
the 1973 irrigation season, the labor requirements for
making an irrigation set on a furrow system using plastic
dams to raise the water surface and siphon tubes to get the
water to the furrows were determined as a means of evalua-
ting the labor necessary to improve irrigation efficiencies.
The study was conducted in the head ditch of the Martin farm
using plastic dams to control water levels in the ditch.
With the water supply available, it was possible to run on
the average 47 of the one-inch diameter siphon tubes or 25
of the 1-1/4-inch tubes. The 1-inch size was used mostly to
provide better control of the water (field tailwater was
reduced), as well as using more tubes simultaneously in
order to reduce the total time to irrigate the field. A set
was changed by pulling the dam and moving it 47 furrows
across the field and then re-installing and sealing the
portable dam. All the tubes must be picked up, moved and
76
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restarted. In this study, it was found that one inexperi-
enced man could set 47 tubes in less than twenty minutes,
or at a rate of about 2.5 tubes per minute. In a previous
paragraph, the effect of reducing the time water is applied
to the Martin farm was shown to have a substantial impact
on decreasing deep percolation losses. Recalling that
during the first irrigation in 1972, deep percolation losses
could have potentially been reduced from 45.05 acre-inches
to 18.09 acre-inches by irrigating in 12-hour sets rather
than the usual 24-hour sets. This improvement, which would
decrease salt loading by an estimated 27 tons, could have
been accomplished with only one hour more labor being
invested.
One of the important nonstructural improvements which could
be undertaken involves the flexibility of the system to ad-
just irrigation and cropping practices to meet the seasonal
changes which occur in water supply, soil conditions, cli-
matological events, and economic demands. Farming practices
are particularly affected by changes in these factors.
During some years, the land may not have been properly pre-
pared due to late precipitation or difficulty in scheduling
the use of farm equipment. The water requirements, length
of irrigations, and cropping rotations may then require
alteration to maximize economic return while minimizing salt
pickup. Occasionally, periods of water or nutrient shortages,
as well as energy shortages, may necessitate changes in
farming practices. A great deal of adjustment in this area
is needed, not only by the farmer, but also by those who
serve his material and service needs—and good water control
is especially important in meeting necessary adjustments.
Communication of Data
Probably the most critical need for the irrigation sched-
uling program itself is an effective line of communication
between the irrigator and the computer. It is apparent in
the Grand Valley, judging from recent experiences, that such
lines have not become properly functional. A farmer may be
reluctant to accept scheduling when the schedule repeatedly
calls for an irrigation while his crop of alfalfa has just
been cut and is still on the ground. Apparently, one farmer
in Grand Valley withdrew from the program because of this
very problem. Yet, the program really has no means of know-
ing the crop has been cut if the information that was pro-
vided says only that the root zone has been depleted past
its limit and an irrigation is needed. In this case, the
field representative has failed to determine the field con-
ditions, and the farmer has not voluntarily supplied the
necessary information.
77
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Since the scheduling depends to a large extent on the soil
moisture in the root zone, some decision must be made as to
where the samples should be taken in the field. This loca-
tion can determine the time between irrigations because the
time depends on the available moisture in the root zone. By
looking at Fig. 30, which was taken from data collected on
the Martin farm during 1972, the moisture variation across
the furrow at various depths can be seen. By taking the
samples at different locations, different moisture contents
will be obtained which in turn would offset the computed
water depletions, thereby resulting in different dates to
begin irrigation. If the moisture sample was taken in the
furrow rather than in the crop row, there may be some ad-
ditional time between irrigations which would correct the
problem of poor irrigation timing being experienced on some
fields. It may be advisable to sample soil moisture at
various locations in order to select one which is represen-
tative of the typical field conditions.
Local Administration
It has previously been noted that existing concerns for
salinity control and local farming attitudes have not been
congruent. This can also be generalized for the local in-
stitutions which may be available to promote more efficient
use of water. One of these is the linkage between irrigator
and canal company in which responsibilities do not overlap,
which hinders water control along the laterals. Another is
the possibility associated with consolidating various levels
of water management to achieve a system which is more re-
sponsive to the political realities of salinity in the Colo-
rado River Basin. In addition, the solicitation of funding
to effect salinity related improvements by local groups de-
serves further attention. In these areas and others, addi-
tional study should be given to their opportune use.
EFFECTIVE SCIENTIFIC IRRIGATION SCHEDULING
A schematic representation of an effective irrigation
scheduling program is shown in Fig. 31. Irrigation sched-
uling consists of two primary components; namely, evapo-
transpiration and available root zone soil moisture. The
climatic station portrayed in Fig. 31 represents some of
the climatic data that must be collected in any irrigated
area in order to calculate daily evapotranspiration.
The other major category of required data pertains to soil
characteristics. First of all, field capacity and wilting
point for the particular soils in any field must be deter-
mined. However, the most critical data are the infiltration
(intake) characteristics of the soil. Although there are
many techniques for measuring infiltration, the data for
78
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c
O
o
2 Days Before Irrigation
2 Days After Irrigation
30
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O
0>
25
a>
I 20
o
\5
10
0
\-2 ft.
o
h.
W
3
u.
o
30
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-*-
| 20
o
0
a> 15
3
1 io
* 5
n
P
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r-
^
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~ c
o
o
1^
;— -^ ^ |
vP
o^
i i i I
30
25
20
15
10
5
O
•-
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-
-
-
•-•^^e^^^-^C^- -^ ^
: ^^^
till
c
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o
o
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O
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o
O
-------
00
o
Diversion
Structure
Flow Measurement
Structures
Field Evaluation
of Soil Moisture
Flow Measurement
Weather
Station
Tailwater
Runoff
MM
Deep Percolation
Losses
Fig. 31. Schematic of requirements for effective irrigation scheduling.
-------
furrow irrigation can be obtained from advance-recession
tests and infiltrometer rings. Only by knowing how intake
rates change with time during a single irrigation, as well
as throughout the irrigation season, can meaningful pre-
dictions be made as to: (a) the quantity of water that
should be delivered at the farm inlet for each irrigation;
and (b) the effect of modifying or changing irrigation prac-
tices upon deep percolation losses.
With good climatic data and meaningful soils data, accurate
predictions as to the next irrigation date and the quantity
of irrigation water to be applied can be made. In order to
insure that the proper quantity of water is applied, a flow
measurement structure is absolutely required at the farm
inlet.
81
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SECTION VII
REFERENCES
1. Briggs, L. J. and Shantz, H. L. 1913. The Water Re-
quirements of Plants. Investigation in the Great
Plains in 1910 and 1911. U. S. Department of Agri-
culture, Bureau of Plant Industries Bulletin 284.
2. Buckman, H. 0. and Brady, N. C. 1969. The Nature and
Properties of Soils. 7th Edition. The MacMillan
Company, Inc., London.
3. Buffam, B. C. 1900. The Use of Water in Irrigation in
Wyoming and Its Relation to the Ownership and Distri-
bution of Natural Supply. U. S. Department of Agri-
culture, Office of Experiment Station Bulletin No. 81.
4. Cowan, I. R. 1965. Transport of Water in the Soil-
Plant-Atmosphere System. Journal of Applied Ecology,
Vol. 2, pp 22-239.
5. Hagan, R. M., Raise, H. R., and Edminster, T. W. 1967.
Irrigation of Agricultural Lands. Agronomy Series No.
11, American Society of Agronomy. Madison, Wisconsin.
6. Harris, P. S. 1920. The Duty of Water in the Lache
Valley, Utah. Utah Agricultural Experiment Station
Bulletin No. 173.
7. Hemphill, R. G. 1922. Irrigation in Northern Colorado.
U. S. Department of Agriculture Bulletin No. 1026.
8. Israelsen, 0. W. and Hansen, V. E. 1962. Irrigation
Practices and Principles. John Wiley and Sons, Inc.,
New York.
9. Jensen, M. E. 1969. Scheduling Irrigations with Com-
puters. Journal of Soil and Water Conservation, Vol.
24, No. 5, pp 193-195.
10. Jensen, M. E. 1972. Programming Irrigations for Great-
er Efficiency. Reprinted from, Optimizing the Soil
Physical Environment Toward Greater Crop Yields.
Academic Press, Inc., New York, New York.
11. Jensen, M. E. 1972. Irrigation Water Requirements and
Management Classnotes from a summer course. Agricul-
tural Engineering Department, Colorado State Univer-
sity, Fort Collins, Colorado. July.
82
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12. Jensen, M. E. and Haise, H. R. 1963. Estimating Eva-
potranspiration from Solar Radiation. Journal of the
Irrigation and Drainage Division, American Society of
Civil Engineers, Proc. 89, No. IR4, pp 15-41.
13. Kruse, E. G. and Haise, H. R. 1974. Water Use by Na-
tive Grasses in High Altitude Colorado Meadows. Ag-
ricultural Research Service, U. S. Department of
Agriculture. Report ARS-W-6. February.
14. Lewis, M. R. 1919. Experiments on the Proper Time and
Amount of Irrigation, Twin Falls Experiment Station,
1914, 1915, 1916. U. S. Department of Agriculture.
15. Mead, E. 1887. Report of Experiments in Irrigation
and Meteorology. Colorado Agricultural Experiment
Station Bulletin No. 1.
16. Mills, A. A. 1985. Farm Irrigation. Utah Agricultural
Experiment Station Bulletin No. 39.
17. Pair, C. H., Hinz, W. W. , Reid, C., and Frost, K. R.
Ed. 1969. Sprinkler Irrigation. Sprinkler Irrigation
Association, Washington, D. C.
18. Penman, H. L. 1948. Natural Evaporation from Open
Water, Bare Soil, and Grass. Proceedings of the
Royal Society of London (A), 193: 120-145.
19. Skogerboe, G. V. and Walker, W. R. 1972. Evaluation
of Canal Lining for Salinity Control in Grand Valley.
Report 13030 DOA/1-72. U. S. Environmental Protec-
tion Agency, Washington, D. C. October.
20. Slayter, R. 0. 1967. Plant Water Relationships.
Academic Press, Inc., New York.
21. U. S. Department of the Interior, Bureau of Reclamation.
1971. Irrigation Management Services Program 1970-71
Report. Region 7, Denver, Colorado. June.
22. U. S. Environmental Protection Agency. 1971. The
Mineral Quality Problem in the Colorado River Basin.
Summary Report and Appendices A, B, C, and D. Re-
gions VIII and IX. Denver, Colorado.
23. U. S. Environmental Protection Agency. 1972. Pro-
ceedings: Conference in the Matter of Pollution of
the Interstate Waters of the Colorado River and Its
Tributaries - Colorado, New Mexico, Arizona, Cali-
fornia, Nevada, Wyoming, and Utah. Regions VIII and
IX, Denver, Colorado. April.
83
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24 Walker, W. R. and Skogerboe, G. V. 1971. Agricultural
Land Use in the Grand Valley. Report AER71-72WRW-GVS1
Agricultural Engineering Department, College of En-
gineering, Colorado State University, Fort Collins,
Colorado. July.
25. Widstoe, J. A. 1912. The Production of Dry Matter
with Different Quantities of Water. Utah Agricultur-
al Experiment Station Bulletin No. 116.
26. Willardson, L. S. 1972. Attainable Irrigation Effi-
ciencies. Journal of the Irrigation and Drainage Di-
vision, American Society of Civil Engineers, Vol. 98,
No. IR 2. June.
84
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SECTION VIII
LIST OF PUBLICATIONS
U. S. Environmental Protection Agency and Colorado State
University. 1972. Managing Irrigated Agriculture to
Improve Water Quality. Proceedings of National Conference
on Managing Irrigated Agriculture to Improve Water Quality.
May 16-18. Grand Junction, Colorado.
Skogerboe, G. V. , Walker, W. R. , Bennett, R. S., and Taylor,
J. H. 1973. Irrigation Scheduling for Reducing Salinity
from Grand Valley. Paper 73-2532. Presented at the 1973
Winter Meeting of the American Society of Agricultural
Engineers, December 11-14, 1973. Chicago, Illinois.
Bargsten, G., Skogerboe, G. V., and Walker, W. R. 1974.
The Grand Valley: An Environmental Challenge. Colored
brochure and taped slide presentation prepared by Colorado
State University.
85
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SECTION IX
LIST OF SYMBOLS
Symbol Definition
a, b, ar, b: regression coefficients
E^ farm efficiency, %
Et expected evapotranspiration, inches
Efca potential evapotranspiration of well-
watered alfalfa, cal cm"2 day"1
Efc potential evapotranspiration of well-
watered grass, cal cm"2 day"1
e vapor pressure, mb
G sensible heat flux, cal cm"2 day"1
M maximum allowable soil moisture deple-
tion , %
M^ existing soil moisture depletion, %
N time until next irrigation in days
Rj^ net outgoing thermal radiation,
cal cm day"1
Rn net radiation, cal cm"2 day"1
RS solar radiation, cal cm"2 day"1
T average daily temperature °F
T^ temperature, °K
TX, Cfc regional constants
u2 wind speed at a specified height of z,
miles per day
Wa water to be applied, inches
a albedo
A changes in saturation vapor pressure
with temperature, mb/deg
Y dry air property, mb/deg
86 «U& GOVERNMENT PHINTINGOFFICEUy/4 546-319/431 i.j
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SELECTED WATER
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
1. Report No.
3. Accession No.
w
4. Title EVALUATION OP IRRIGATION SCHEDULING FOR
SALINITY CONTROL IN GRAND VALLEY,
7. Authors skogerboe, G.V., Walker, W.R., Taylor, J.H.,
and Bennett, R.S.
Agricultural Engineering Department, Colorado
State University, Port Collins, Colorado.
5, Report 'Date
6. .
•8« Perforating Organization
Report No.
10. Project No.
11. Contract/Grant Nfo,
S-800278
t3 s
12.
_EP&,_3f£ice_of Research and Development
at' Report 4fl
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