Strategies for
Reducing Pollutants
from Irrigated Lands
in the Great Plains
M. L. Quinn, Editor
V.
Nebraska Water Resources Center /
Institute of Agriculture and ancj
Natural Resources /
University of Nebraska-Lincoln
U.S. Environmental
Protection Agency
Robert S. Kerr Laboratory
Ada, Oklahoma
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EPA 600/2-81-108
STRATEGIES FOR REDUCING POLLUTANTS FROM
IRRIGATED LANDS IN THE GREAT PLAINS
M.-L. Quinn, Editor
Authors: J.R. Gilley, D.G. Watts, R.J. Supalla, M.-L. Quinn,
M. Twersky, F.W. Roeth, R.R. Lansford, and K.D. Frank
Nebraska Water Resources Center
Institute of Agriculture and Natural Resources
University of Nebraska—Lincoln
Lincoln, Nebraska 68583
EPA Project Officer:
Alvin L. Wood
Source Management Branch
Robert S. Kerr Environmental Research Laboratory
Ada, Oklahoma 74820
EPA Grant No. R-805249
ROBERT S. KERR ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
ADA, OKLAHOMA 74820
July 1, 1982
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DISCLAIMER
Mention of trade names or commercial products in this report does not
constitute endorsement or recommendation for use.
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FOREWORD
Environmental protection efforts dealing with agricultural and nonpoint
sources have received increased emphasis with the passage of the Clean Water
Act of 1977 and the subsequent implementation of the Rural Clean Water Pro-
gram. As part of this Laboratory's research on the occurrence, movement,
transport, fate, impact, and control of environmental contaminants, data
and analytical methodologies are developed to assess the causes and possible
solutions of adverse environmental effects of irrigated agriculture.
Efforts to achieve water quality goals include the identification and
application of best management practices (BMPs) to control agriculturally
related water pollutants. This report examines irrigation management prac-
tices and how they contribute to water quality degradation arising from the
loss of sediments, nutrients and pesticides from irrigated cropland. Alter-
native irrigation practices are evaluated with respect to their effects upon
water quality and the economy of the agricultural producer. Strategies for
the development of pollution control programs are described which should be
useful in reaching technically sound and economically feasible environmental
management decisions. This report should especially benefit environmental
planners and managers as they attempt to identify water quality problems and
to implement control strategies to alleviate those problems in the irrigated
Great Plains.
Clinton W. Hall
Director
Robert S. Kerr Environmental Research
Laboratory
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ABSTRACT
A manual has been prepared which will serve as a planning guide for
determining alternative management practices to limit nonpoint source water
pollution from irrigated lands in the Great Plains. The manual has five
independent yet integrated sections, intended to assist agency personnel and
others in formulating areawide management plans for irrigation practices.
Section One contains a summary of federal water pollution legislation
as it relates to irrigated agriculture. The areal extent and intensity of
irrigation in the newly-defined Irrigated Great Plains is given, along with
a review of selected physical characteristics of the region. Five irrigated
crop production areas are broadly delineated in this introductory section.
Pollutants in irrigation return flows are identified, defined, and
described in Section Two. The authors then examine the effects of current
irrigation management practices on pollution in the return flow that results
from surface runoff and deep percolation. The most probable pollution
problems in the Irrigated Great Plains are discussed.
In Section Three, various alternative irrigation management options
to reduce pollution from irrigation return flows are considered. Specific
practices are rated according to their ability to reduce pollutants from
surface runoff and deep percolation.
In Section Four, the authors discuss the relative degree of economic
effects which selected alternative management options could have on both
central and southern plains agricultural producers. An analysis is given
of seven management options which planners would be likely to consider for
policy formulation. The economic effects associated with each of these
options are examined.
Control program strategies for implementing alternative irrigation
practices are presented in Section Five. Included is the problem-solving
sequence necessary to develop a management program for limiting potential
site-specific pollution problems. Examples of two different types of
return flow problems in the Great Plains are used to illustrate and assess
potential solutions to deep percolation and surface water runoff.
This report was submitted in fulfillment of Grant No. R-805249 to
the Nebraska Water Resources Center, University of Nebraska—Lincoln under
the sponsorship of the U.S. Environmental Protection Agency. This report
covers the period of October 1, 1977, to August 31, 1979, and work was
completed as of February 15, 1980.
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CONTENTS
Foreword
Abstract iv
Figures vii
Tables ix
Acknowledgments xiv
1. Irrigation in the Great Plains—An Overview 1
Introduction 1
Summary of Pertinent Legislation 3
The Great Plains: Its Water, Irrigation and Crops ... 5
2. Water Quality of Irrigation Return Flows in the
Great Plains 33
Definition and Description of Irrigation
Return Flows 33
Identification of Pollutants in Irrigation
Return Flows 39
Significance of Irrigation Return Flows in
the Great Plains 49
Effect of Current Irrigation Management
Practices on Irrigation Return Flows 55
3. Available On-Farm Irrigation Management Alternatives
for Reducing Pollution from Irrigation Return Flows 92
Irrigation System Management 93
On-Farm Water Management 101
Soil Management 104
Nutrient Management 106
Pesticide Management 110
A. Economic Feasibility of Farm Management Alternatives
to Reduce Pollution 117
Introduction 117
Management Options to be Evaluated 117
Analytical Procedures 118
Results-Water Management Options 119
Economics of Reduced Tillage 133
Fertilization Practices 134
Pesticide Usage 136
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5. Control Program Strategies for Irrigation Return Flows. . . . 137
Development of a Control Program 137
Recommendations for Two Site-Specific Cases 141
Summary 145
References 146
Appendices
A. County and Subarea Data 156
B. Commonly-Used Herbicides, Insecticides, and Miticides .... 173
C. Base Data and Procedures for the Crop Budgets and
Irrigation Cost Estimates 179
D. Metric Conversions 184
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FIGURES
Number Page
1 The traditional Great Plains and the newly-defined Irrigated
Great Plains, as the latter term is used in this Section 6
2 Subareas of the Irrigated Great Plains, based on river
drainage 8
3 Distribution of irrigation density throughout the Irrigated
Great Plains, based on irrigated hectares per total hectares
in each subarea 13
4 Location of the Ogallala formation 15
5 Average annual precipitation (in cm) in a region comprised
of climatic divisions within the ten Great Plains states.
Boundaries are similar to those of the Irrigated Great Plains. . . 16
6 Distribution, by season, of average annual precipitation in
select climatic divisions in the Irrigated Great Plains 17
7 For the United States, maximum 30-minute rainfall intensities
(in cm) which can be expected during any two-year period 20
8 Distribution of irrigated corn in the Irrigated Great Plains,
based upon irrigated hectares of corn (5,000 or more) in
the various subareas 23
9 Distribution of irrigated alfalfa in the Irrigated Great
Plains, based upon irrigated hectares of alfalfa (5,000 or
more) in the various subareas 24
10 Distribution of irrigated sorghum in the Irrigated Great
Plains, based upon irrigated hectares of sorghum (2,000 or
more) in the various subareas 25
11 Distribution of irrigated soybeans in the Irrigated Great
Plains, based upon irrigated hectares of soybeans (200 or
more) in the various subareas 26
12 Distribution of irrigated cotton in the Irrigated Great
Plains, based upon irrigated hectares of cotton (500 or
more) in the various subareas 27
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Number Page
13 Return flows from irrigated agriculture 35
14 Processes that influence water flow and agrochemical
balances in an irrigated agricultural production system 36
15 Representative magnitude of available water-holding
capacity of agricultural soils 38
16 Some of the irrigation systems used in the Great Plains 56
17 A summary of deep percolation water losses in 35 fields
irrigated by either center-pivot or surface irrigation
systems 60
18 Schematic relationship between irrigation management
levels and the degree of pollution in irrigation return
flows 64
19 Diagram of nitrogen cycle inputs into and outputs from
soil-plant-water system 70
20 Effect of nitrogen fertilizer additions on potential
leaching in irrigated corn 71
21 NO.-N leaching and water losses for three nitrogen
fertilizer application methods with different rainfall
patterns 73
22 Processes influencing the fate and behavior of
pesticides 84
23 Master flow chart: development of control program
strategies to reduce nonpoint source pollution from
irrigated lands in the Great Plains 138
24 Flow chart for assessing deep percolation problems 139
25 Flow chart for assessing surface water runoff problems 140
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TABLES
Number Page
1 Percent Irrigated of Each Subarea Which Lies Within the
Irrigated Great Plains, Along with Percent Irrigated of
Each Subarea's Total Cropland 10
2 Counties in the Irrigated Great Plains with 50 Percent
or More of their Total Hectares in Irrigation 12
3 Monthly and Annual Rainfall (in cm), Mitchell and
Scottsbluff, Nebraska for 1929 and 1930 19
4 Irrigated Lands Planted to the Five Major Crops as a
Percent of the Total Irrigated Hectares in Each Subarea 29
5 Irrigated Hectares by Source of Water and Irrigation
Method in the Irrigated Great Plains 31
6 Probable Changes in Water Quality as a Result
of Irrigation 40
7 Expected Soil Resource Losses Under Four Conservation
Irrigation Management Options with Different Cropland
Uses 42
8 Estimated Amounts of Pollution Potential of Sediments
and Nutrients in Sediments in the Nebraska Middle
Platte River Basin 43
9 Irrigation Return Flow in the Great Plains States by
System Type and Source of Water 50
10 Irrigation-Related Pollution Problems in the Great
Plains States 52
11 Increases in Nitrates in Ogallala Groundwater in West
Texas Counties Having Different Soil Types 55
12 Estimated Irrigation Application Efficiencies for
Irrigation Systems 59
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Number Page
13 General Magnitude of Irrigation Water Amounts (Seasonal)
as Practiced for Five Crops in the Great Plains 61
14 Effect of Surface Irrigation Application Efficiency on
Sediment Yield 62
15 Relation of Sediment Yields from Row Crops During the
Irrigation Season 63
16 Effect of Land Slope from Surface Irrigation Fields on
Gross Sediment Losses 64
17 Sediment Delivery to Streams from a Range of Soil Types
from Representative Farms for Three Crop Management
Systems in Iowa 65
18 Soil Loss from Irrigation Furrows after 7 and 24 Hours
of Irrigation, as Affected by Tillage Corn Residue Treatment. . . 66
19 Expected Reduction of Soil Sediment Loss with Selected
Irrigation Management Practices 67
20 Amount of Major Nutrient Fertilizers Added and Estimated
Nutrient Removal in Harvested Yields of Irrigated Crops
in the Great Plains States 69
21 Potential Nitrate-Nitrogen Losses for Combinations of
Deep Percolation and Nitrate-Nitrogen Concentrations 72
22 Residual Soil Nitrates After Four Years of Furrow Irrigation. . . 74
23 NO -N Leaching During Crop Season Under Sprinkler-Irrigated
Corn on a Maddock Fine Sandy Loam at Oakes, North Dakota 75
24 A Three-Year Summary of the Influence of Deep Percolation
Losses of NO.-N Under Center Pivot Irrigation Systems
in Northeast Colorado 76
25 Nitrogen Losses with Surface Water Runoff During May Through
September with Three Tillage Systems 78
26 Average Nitrogen Losses with Water Runoff and Sediments
from Various Management Systems 79
27 Surface Runoff, Soil Sediment Yields and Phosphorus
Losses from Watersheds Under Three Management Operations 80
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Number Page
28 Pesticides Used in North Dakota, South Dakota, Nebraska,
Kansas, Oklahoma and Texas in 1976 81
29 Major Herbicides Used in North Dakota, South Dakota,
Nebraska, Kansas, Oklahoma and Texas in 1976 82
30 Major Insecticides Used in North Dakota, South Dakota,
Nebraska, Kansas, Oklahoma and Texas in 1976 82
31 Rates of Agricultural Pesticides Applied to Crops in
the Great Plains 83
32 Concentrations of Pesticide Residues in Tailwater Pits
Serving Corn and Sorghum Fields in Haskell County, Kansas,
Averaged Over 2 years 85
33 Distribution of Pesticide Residues Occurring in Tailwater
Pits During Irrigation Season 86
34 Estimated Percentages of Applied Pesticides Delivered to
Surface Water Sources 88
35 Summary of Pesticides Found in Water in Nebraska (1971-1976)... 89
36 Atrazine and Nitrate-Nitrogen Concentrations in Water From
Irrigation Wells in Merrick County, Nebraska 90
37 Alternative Irrigation Management Options to Reduce
Pollution from Irrigation Return Flows 93
38 Irrigation Systems Discussed Under the Alternative
Management Options 94
39 Potential Pollution Rating of Surface and Trickle
Irrigation Systems 95
40 Potential Pollution Rating of Sprinkler Irrigation Systems. ... 96
41 Irrigation System Management Practices for Surface and
Trickle Irrigation Systems and Their Related Pollution
Reduction 98
42 Irrigation System Management Practices for Sprinkler
Irrigation Systems and Their Rated Pollution Reduction 100
43 On-Farm Irrigation Water Management Options and Their
Rated Pollution Reduction 102
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Number Page_
44 Soil Management Options for Controlling Irrigation
Return Flows and Their Rated Pollution Reduction 105
45 Evaluation of Application Methods for Optimum
(not Excessive) Amount of Nutrients 108
46 Management Practices to Reduce Pesticide Contributions
in Irrigation Return Flows and Their Rated Pollution
Reduction 112
47 Per Hectare Costs and Returns for Corn (Grain) by Type
of Irrigation System for the Southern High Plains, 1979 121
48 Per Hectare Costs and Returns for Cotton by Type of
Irrigation System for the Southern High Plains, 1979 122
49 Per Hectare Costs and Returns for Grain Sorghum by
Type of Irrigation System for the Southern High
Plains, 1979 123
50 Per Hectare Costs and Returns for Wheat by Type of
Irrigation System for the Southern High Plains, 1979 124
51 Per Hectare Costs and Returns for Alfalfa by Type of
Irrigation System Central Great Plains, 1979 126
52 Per Hectare Costs and Returns for Corn by Type of
Irrigation System Central Great Plains, 1979 127
53 Per Hectare Costs and Returns for Grain Sorghum by
Type of Irrigation System Central Great Plains, 1979 128
54 Per Hectare Costs and Returns for Corn Under Conventional
and Reduced Tillage Systems, Central Great Plains, 1979 135
A-l Estimated Percentage of County Area Irrigated and
Percentage of County Cropland Irrigated for all Counties
in the Irrigated Great Plains 156
A-2 Subareas, All or Part of Which are Included in the
Irrigated Great Plains, and the Drainage Basin
Each Represents 171
B-l Herbicides Commonly Used in Five Crops in the Great Plains. . . . 173
B-2 Insecticides and Miticides Commonly Used in Five Crops
in the Great Plains 176
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Number Page
C-l Machinery Inventories for the Central and Southern
Plains Farm Situations 180
C-2 Production Input Prices Used in Estimation of
Crop Budgets 181
C-3 Investment Costs for Selected Irrigation Systems,
Central Plains 182
C-4 Investment Costs for Selected Irrigation Systems,
Southern Plains 183
D-l Unit Conversions for Length 184
D-2 Unit Conversions for Area 184
D-3 Unit Conversions for Volume 185
D-4 Unit Conversions for Mass to Weight 186
D-5 Concentration in Water 186
D-6 Unit Conversions for Special Combinations 187
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ACKNOWLEDGMENTS
Authorship of the sections in this manual is as follows:
Section One. Irrigation in the Great Plains—An Overview
by M.-L. Quinn
Section Two. Water Quality of Irrigation Return Flows in the Great Plains
by J. R. Gilley, D. G. Watts, F. W. Roeth, and M. Twersky
Section Three. Available On-Farm Irrigation Management Alternatives for
Reducing Pollution From Irrigation Return Flows
by J. R. Gilley, D. G. Watts, F. W. Roeth, K. D. Frank,
and M. Twersky
Section Four. Economic Feasibility of Farm Management Alternatives to
Reduce Pollution
by R. J. Supalla and R. R. Lansford
Section Five. Control Program Strategies for Irrigation Return Flows
by J. R. Gilley and M. Twersky
M. Twersky was Coordinator during the first stage of the project, and
M.-L. Quinn served in this capacity, and also as editor, during the
concluding period.
At the time this project was begun, two committees were formed—the
Nebraska Committee and the Great Plains Committee—to provide the authors
with advice from irrigation and water quality specialists throughout the
region. Members of these Committees (listed below) supplied much useful
information and reviewed all or part of the manual material while it was
in draft form. Their continued support and guidance is sincerely appre-
ciated.
Constructive suggestions on manual content were received from state
agencies and state university personnel concerned with irrigation-related
activities in the Great Plains. Particularly helpful were views on water
quality problems associated with irrigated agriculture in the region.
The authors also have received the assistance of numerous individuals
here at the University of Nebraska. Susan Miller and Russell Fries compiled
data needed for tables and maps used in the manual. Graphics specialist
Sheila Smith prepared the figures in Sections 2 and 5. Ruth Dickinson aided
in library work and helped with the conversion tables. Lorraine Kruger did
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an excellent job in typing the final manuscript. Other support staff
included cartographer Dave Schuman and typists Kathy Thompson and Evon Meyer.
Project administration was handled by the Nebr. Water Resources Center,
with Acting Director Gary L. Lewis and former Director M. Wayne Hall serving
as managers of the project. This responsibility later passed to M.-L. Quinn.
A special note of thanks is extended to the EPA Project Officer, Alvin
L. Wood, for his valuable advice and endless cooperation in the pursuit of
the goals of this project. Other EPA personnel—particularly Arthur Hornsby-
also provided the authors with constructive suggestions on the content of the
manual while it was in draft form.
NEBRASKA COMMITTEE
V. W. Benson, Agr. Economist
U.S. Dept. of Agriculture
Econ. Stat. Coop. Service
Lincoln, Nebraska
K. D. Frank, Assoc. Professor
Department of Agronomy
South Central Station
University of Nebraska
Clay Center, Nebraska
J. R. Gilley, Professor
Dept. of Agr. Engineering
University of Nebraska
LincoIn, Nebraska
C. G. Haberman, Head
Water Quality Section
Nebr. Dept. of Envir. Control
Lincoln, Nebraska
M. W. Hall, Chairman -
Missouri River Basin Commission
Omaha, Nebraska
S. K. Hoppel, Head
Water Quality Planning Section
Nebr. Natural Resources Commission
Lincoln, Nebraska
G. L. Lewis, Acting Director
Nebr. Water Resources Center
University of Nebraska
Lincoln, Nebraska
M.-L. Quinn, Asst. Professor
Nebr. Water Resources Center
University of Nebraska
Lincoln, Nebraska
F. W. Roeth, Assoc. Professor
Dept. of Agronomy
South Central Station
University of Nebraska
Clay Center, Nebraska
R. J. Supalla, Assoc. Professor
Dept. of Agr. Economics
University of Nebraska
Lincoln, Nebraska
M. Twersky, Research Associate
Dept. of Agr. Engineering
University of Nebraska
Lincoln, Nebraska
D. G. Watts, Professor
Dept. of Agr. Engineering
University of Nebraska
Lincoln, Nebraska
— Former Director of the Nebr. Water Resources Center.
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GREAT PLAINS COMMITTEE
NORTH DAKOTA
J. W. Bauder —
Extension Soils Scientist
Montana State University
Bozeman, Montana 59717
WYOMING
G. L. Christopulos
Wyoming State Engineer
Cheyenne, Wyoming 82002
OKLAHOMA
J. E. Carton, Professor
Dept. of Agr. Engineering
Oklahoma State University
Stillwater, Oklahoma 74074
KANSAS
2/
D. R. Hay -
Ext. Spec., Water Res. & Irrig.
Dept. of Agr. Engineering
University of Nebraska
Lincoln, Nebraska 68583
COLORADO
E. G. Kruse, Agr. Engineer
U.S. Dept. of Agriculture
Sci. & Education Adm.—Agr. Research
Engineering Research Center
Colorado State University
Fort Collins, Colorado 80523
NEW MEXICO
R. R. Lansford, Professor
Agr. Economics and Agr. Business
New Mexico State University
Las Cruces, New Mexico 88003
TEXAS
L. New
Area Agr. Engineer - Irrigation
Texas Agr. Extension Service
Lubbock, Texas 79401
MONTANA
G. L. Westesen
Extension Agr. Engineer
Montana State University
Bozeman, Montana 59717
SOUTH DAKOTA
J. L. Wiersma, Director
Water Resources Institute
South Dakota State University
Brookings, South Dakota 57006
— Formerly Assistant Professor, Dept. of Soils, North Dakota University,
Fargo, North Dakota.
2/
— Formerly Extension Irrigation Engineer, Kansas State University,
Manhattan, Kansas.
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SECTION 1
IRRIGATION IN THE GREAT PLAINS—AN OVERVIEW
by
M.-L. Quinn
INTRODUCTION
This manual is to serve as a guide for determining alternative manage-
ment practices intended to reduce nonpoint source water pollution which may
result from irrigated agriculture in the Great Plains. While the personnel
of water planning agencies are considered the manual's principal audience,
it also may be of assistance to farmers, along with others whose work is
related to the impact irrigated agriculture may have on water quality.
Farmers have been irrigating fields in the Great Plains since the 1860"s
(Borrelli, 1979) so it is reasonable to ask the question: why at this
particular time has a manual been written on alternative irrigation manage-
ment practices? Following are the three principal reasons for the creation
of this volume:
(1) Currently, more and more people throughout the Great Plains
are having to make planning decisions and establish guidelines pertaining to
water quality. This surge of activity stems largely from programs begun as
a result of Public Law (P.L.) 92-500, the "Federal Water Pollution Control
Act Amendments of 1972," passed by Congress in October 1972.
(2) The dominant economic activity in the Great Plains is agri-
culture. In 1974, the U.S. Department of Agriculture classified 62 million
hectares in the region's ten states as cropland used for crops (U.S. Dept.
of Agr., 1978). Of this amount, an estimated 12 million hectares, or
roughly 19.5 percent, were irrigated as of 1978 (Irrigation Journal, 1978).
Thus, irrigation agriculture is one of the avenues for human impact on water
quality in the Great Plains, being more significant in some areas than
others.
(3) In many instances, personnel from local, state, and regional
resource agencies do not have the expertise in irrigation practices which is
now needed. Thus, these agencies are at a disadvantage as they strive to
comply with the requirements of federal legislation concerning nonpoint
source pollution as related to irrigated agriculture.
The objective of this project was to "produce a manual providing
technical guidance on the best available practices for controlling nonpoint
pollution associated with irrigation agriculture in the Central Plains"
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(Hall, 1977). — In addition, there were three results which this under-
taking was expected to produce:
"(1) a state-of-the-art analysis and evaluation of current irriga-
tion practices in light of their effects on water pollution, showing the
extent and magnitude of the pollution problem from irrigation;
(2) considerations and evaluation of management alternatives to
these current practices, as well as resulting improvements in water quality;
(3) development of a proposed strategy for implementation of these
alternatives" (Hall, 1977).
The manual contains five sections, plus appendices. Section One
presents a summary of pertinent legislation and important physical charac-
teristics of the Great Plains region. The next three sections elaborate on:
— irrigation return flow and how it is affected by agricul-
tural management practices (Section 2)
— available on-farm irrigation management alternatives
(Section 3)
— economic feasibility of farm management alternatives
(Section 4).
Then in Section 5, guidance is given on the selection of appropriate manage-
ment systems. The appendices at the end of the manual contain further
detailed information for the reader's reference.
Statement of Philosophy
As one moves from the regional to the state and county levels, and then
to the individual farm, the seeming homogeneity of the Great Plains quickly
disappears. There is, in fact, a wide range of soil types, subsurface
geology, rainfall conditions, water chemistry, and water availability, all
of which have helped create a truly heterogeneous physical system. Upon
this physical system, man has superimposed his own pattern of crops and
agricultural practices.
This complexity makes it impractical to suggest a single agricultural
management procedure—for the entire region, for a state within the region,
or even for one county—that could be expected to lessen any negative effects
which irrigation might have on water quality. (Even a management directive
— Subsequent to the writing of the proposal by Dr. Hall, it was decided to
focus the study on the Great Plains, rather than the Central Plains.
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to stop all irrigated agriculture in the Great Plains—an unlikely extreme—
probably would not eliminate the problem.) Thus, most management practices
intended to reduce nonpoint pollution from irrigation will be site-specific.
Assuming, however, that irrigated agriculture is going to continue in the
Great Plains, there are some management tools which, when modified to reflect
particular local conditions, can ease the impact in the existing problem
areas or reduce the development of future problem areas.
Introduced in the manual is a procedure for developing control programs
where there is the potential for nonpoint source pollution from irrigated
agriculture. While every site will differ, there are a number of common
components within the natural and man-made systems which, working together
in various combinations, will contribute to a site being more (or less)
prone to irrigation-induced nonpoint pollution.
This manual should become the standard guide in the development of
management alternatives to limit nonpoint source pollution from irrigated
agriculture within the Great Plains. The specific manner in which these
alternatives are modified and used, however, will vary from state to state,
agency to agency, and individual to individual.
SUMMARY OF PERTINENT LEGISLATION
Concern for water pollution is not new, nor is legislation to deal with
the problem. Section 13 of the Rivers and Harbors Act of 1899 (The Refuse
Act) is often cited as one of the first federal laws to address the pollution
of waters. This law, however, was seldom enforced (Warnick, 1977).
The Federal Water Pollution Control Act of 1948 (P.L. 80-845) was a
major step forward. In keeping with past policy, this law recognized "the
primary responsibilities and rights of the States in controlling water
pollution,..." Public Law 80-845 has been amended and expanded by the
following acts:
Water Pollution Control Act Amendments of 1956
Federal Water Pollution Control Act Amendments of 1961
Water Quality Act of 1965, P.L. 89-234
Clean Waters Restoration Act of 1966, P.L. 89-753
Water Pollution Control Amendments of 1972, P.L. 92-500
Clean Water Act of 1977, P.L. 95-217.
The creation of the U.S. Environmental Protection Agency (EPA) in 1970 must
also be included as an important, related event because that agency was then
placed in charge of the federal government's water pollution programs
(Warnick, 1977).
As far as nonpoint source pollution from irrigated agriculture is con-
cerned, P.L. 92-500 (passed in 1972) and P.L. 95-217 (passed in 1977), are
particularly significant. The 1972 law, which is administered by the EPA,
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contains the following definition under Section 502:
"The term 'pollution1 means the man-made or man-induced
alteration of the chemical, physical, biological, and
radiological integrity of water."
This law also contains the frequently-referenced Section 208 entitled,
"Areawide Waste Treatment Management." Section 208 specifies:
— that the states prepare plans for areawide waste treatment
management;
— that these plans contain alternatives for such management;
— that the plans be applicable to all wastes generated
within a designated area.
Regarding nonpoint source pollution from irrigated agriculture, this same
section of P.L. 92-500 goes on to say:
"Any plan prepared under such process shall include, but not
be limited to — [(A) through (E)]
(F) a process to (i) identify, if appropriate, agricul-
turally and silviculturally related nonpoint sources of
pollution, including runoff from manure disposal areas, and
from land used for livestock and crop production, and
(ii) set forth procedures and methods (including land use
requirements) to control to the extent feasible such sources;..."
In December, 1977, Congress passed the Clean Water Act (P.L. 95-217) which
amends item (F) from Section 208 quoted above to read as follows:
"(F) a process to (i) identify, if appropriate, agricul-
turally and silviculturally related nonpoint sources of
pollution, including return flows from irrigated agriculture,
and their cumulative effects, runoff from manure disposal
areas, and from land used for livestock and crop production,..."
The new wording is underscored.
In addition, the 1977 law adds to Section 402 of P.L. 92-500 a paragraph
stating that no permit, neither federal or state, shall be required for
"discharges composed entirely of return flows from irrigated agriculture."
By making it clear that return flows from irrigated agriculture were to be
regarded as nonpoint sources of pollution and by stating specifically that
such flows did not require permits (as did point sources), the way was then
open for more defined efforts to deal with this particular problem. Each
state's plan must now recommend those regulatory programs considered neces-
sary to reduce or prevent pollution from irrigated agriculture.
There is another important distinction regarding nonpoint sources of
pollution (hence, return flows from irrigated agriculture). Management
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practices to curtail nonpoint source pollution are to be tailored to the
uniqueness of the area where the problem exists (Minton, et al., 1978).
In other words, the EPA, in its enforcement of these laws, recognizes
geographical differences and suggests a site-specific approach. In contrast,
point sources of pollution must adhere to a fixed set of effluent standards
which is applied nationwide.
Section 35 of the Clean Water Act of 1977 made one further important
addition to Section 208 in the 1972 law. It authorized the Soil Conservation
Service to "administer a program to enter into contracts...of not less than
five years nor more than ten years with owners and operators having control
of rural land for the purpose of installing and maintaining measures incor-
porating best management practices to control nonpoint source pollution for
improved water quality..." In addition, the federal government agreed to
provide technical assistance in carrying out these management practices and
also to share up to 50 percent of their total cost.
As a result of these various pieces of legislation, nonpoint source
pollution from irrigated agriculture is the focus of much attention at the
state level and has been for several years. This manual is intended to
assist the personnel of the numerous state agencies in the Great Plains in
making decisions on this subject, for inclusion in their management plans.
THE GREAT PLAINS: ITS WATER, IRRIGATION, AND CROPS
Definition
Traditionally, the Great Plains of the United States has been defined
as that region which lies between the Rocky Mountains on the west and the
prairies on the east, reaching from Texas to the Canadian border
(Thornthwaite, 1936). It includes portions of ten states: North Dakota,
South Dakota, Nebraska, Kansas, Oklahoma, Texas, New Mexico, Colorado,
Wyoming, and Montana (Figure 1). Low rainfall, relatively flat terrain,
few trees, and a gradual increase in elevation from east to west are four
characteristics which make the Great Plains a distinct geographic unit and
which set it apart from the rest of the United States.
For the purposes of this manual, however, a further refinement of
these physiographic and political boundaries has been necessary. Thus,
within the ten states of the traditional Great Plains, there has been
delineated the 'Irrigated Great Plains' (Figure 1). As will be explained
in a moment, this large expanse of land (some 1,500,000 square kilometers)
includes a few areas generally not regarded as plains.
Determination of the size and extent of this particular region was
based primarily on the number of irrigated hectares per county in the ten
states. The number of irrigated hectares in Montana, Wyoming, Colorado,
Nebraska, Kansas, and New Mexico were obtained from the 1976 Agricultural
Statistics compiled by the Agriculture Department Crop and Livestock Report-
ing Service in each state. For North Dakota, 1977 data from the state's
Extension Service were used. Information for Oklahoma was derived from a
-------
Outline of traditional
Great Plains
0 60 160 240 320 400 480 Km.
Figure 1. The traditional Great Plains and the larger Irrigated Great
Plains, as the latter term is used in this manual.
-------
1977 Irrigation Survey prepared by the Oklahoma State University Extension
Service. In the case of Texas, it was decided to include in this study
only the northern part of the state. County data were obtained from the
1976 Agricultural Statistics, in conjunction with the 1976 High Plains
Irrigation Survey. The state of South Dakota no longer compiles figures
for irrigated hectares on a county basis. Thus, it was necessary to use
the county data from the U.S. Agricultural Census for the year 1974. As a
result, what is thought to have been a considerable increase in irrigation
in South Dakota during the 1976-1977 drought (R. Beyer, per. com.) is not
reflected here. Information on irrigated hectares for all ten states was
cross-checked with other sources wherever possible.
County size in the ten Great Plains states varies widely, ranging from
105,000 hectares (Clay County, South Dakota) to 2,358,500 hectares (Fremont
County, Wyoming). For this reason, a minimum number of irrigated hectares
per county could not be used to determine what counties should be included
in the Irrigated Great Plains. As much as possible, the major criterion used
was the number of irrigated hectares as a percent of total hectares in each
county—the minimum percent being a matter of judgment. Also considered was
the amount of total cropland under irrigation in the individual counties.
Based on computations developed for this manual, 28 counties in the
Irrigated Great Plains have less than 0.1 percent of their areas under
irrigation. The bulk of the lightly-irrigated counties lies in North and
South Dakota. In large part, these counties were included for physiographic
reasons and for areal consistency. All 416 counties in the Irrigated Great
Plains and their respective percentages are listed in Table A-l in Appendix A.
(While the figures listed in Table A-l were compiled with much care, they are
subject to some error and should be used accordingly.)
Also listed in Table A-l are figures showing total hectares of cropland
in each county, along with the percent of that cropland which is irrigated.
(The definition of 'cropland1 is the same as that used by the U.S. Department
of Agriculture in the 1974 Agricultural Census, which defines cropland as:
"land from which crops were harvested or hay was cut; land in orchards,
citrus groves, vineyards, and nursery and greenhouse products; land used only
for pasture or grazing; land in cover crops, legumes, and soil improvement
grasses; land on which all crops failed; and land in cultivated summer
fallow. It also includes cropland that is idle.") In some counties such as
Beaverhead County, Montana and Park County, Wyoming, a high percentage of
total cropland is irrigated. Yet, it may represent only a small percent of
the county's total area. Instances of this nature were evaluated on a case-
by-case basis.
The county information was then organized into larger geographic divi-
sions known as subareas, defined by the U.S. Water Resources Council of the
Department of Interior (U.S. Water Resources Council, 1970). While based on
river basins, the boundaries of these subareas follow county lines but, in
some cases, cross over state lines (Figure 2). For example, Subarea 1025
represents the drainage basin of the Republican River and is composed of
12 counties in Nebraska, 3 counties in Colorado, and 10 counties in Kansas.
-------
0 60 160 240 320 400 480 Km. \
\
/
Figure 2. Subareas of the Irrigated Great Plains, based on river
drainage. Dashed lines show excluded parts of subareas.
-------
Data organization focusing on drainage basins provides a useful geographic
component to this manual on alternatives for managing nonpoint source
pollution from irrigated agriculture. Appendix Table A-2 is a list of all
37 subareas used, and the drainage basin each represents.
It must be emphasized that sections of several states included within
the Irrigated Great Plains are a marked departure from the traditional inter-
pretation of the Great Plains. Western Montana is one example. The amount
of irrigation in a number of western Montana counties warranted their
inclusion, even those in Subarea 1002, which is quite mountainous
(G. Westesen, per. com.).
The Bighorn Basin, nestled on the west side of the Big Horn Mountains in
northcentral Wyoming, represents the other major departure. As in the case
of western Montana, the number of irrigated hectares in the counties of
Park, Big Horn, Hot Springs, Washakie, and Fremont warranted the inclusion
of this section of Wyoming. Furthermore, the Bighorn Basin is regarded as
a potential troublespot for irrigation-related water quality problems
(Bill Long, per. com.).
When a subarea extended into a sector where there were comparatively
few irrigated hectares, the counties in that part of the subarea were
excluded from the study. (The exception of major parts of the Dakotas has
already been mentioned.) This was the case for Subareas 1011, 1018, 1019,
1027, 1102, 1103, 1105, 1106, 1110, 1113 and 1206. The excluded portions
of these subareas are shown by dashed lines in Figure 2. The Iowa and
Minnesota portions of Subarea 1017 also were excluded, as was Yellowstone
National Park from Subarea 1008.
In the Irrigated Great Plains as a whole, there are roughly nine million
hectares under irrigation (see Tables 4 and 5). Figure 3 gives an overview
of where this irrigation is located in the region. The distribution used in
this map was determined by calculating the percent of irrigated hectares per
total hectares in each subarea. For example, there are 5,922,000 hectares
in Subarea 1025, of which 628,000 hectares are irrigated—that is, 10.6 per-
cent. (These 628,000 irrigated hectares represent roughly 19 percent of the
total cropland in that subarea.) The largest percentage of irrigated land
is in Subarea 1205 (Brazos headwaters), where 38 percent of the subarea's
3,381,000 total hectares are under irrigation. The irrigation densities
for all the subareas are listed in Table 1, along with figures for total
cropland in each subarea and the percentage of that cropland which is
irrigated.
It is not surprising that within Subarea 1205 is located the most
intensively irrigated county in all the Irrigated Great Plains. This is
Hale County, Texas, with 77 percent of its total area irrigated. Those
counties having 50 percent or more of their total area under irrigation
are listed in Table 2.
North and South Dakota stand out in Figure 3 as having little irriga-
tion. Only a handful of counties in the two states have over 1 percent of
their total area irrigated, and many are under 0.5 percent. It is important
-------
TABLE 1. PERCENT IRRIGATED OF EACH SUBAREA WHICH LIES WITHIN THE
IRRIGATED GREAT PLAINS, ALONG WITH PERCENT IRRIGATED OF
EACH SUBAREA'S TOTAL CROPLAND
Subarea
1205
1021
1112
1027
1104
1109
1022
1025
1208
1103
1020
1019
1018
1015
1002
1113
1102
1007
1008
1026
Percent
of subarea
irrigated
38 %
29
28.7
26.7
16.6
16.6
11.5
10.6
10.5
9.7
8.2
7.6
6.1
4.4
4.0
3.5
3.3
3.0
3.0
2.6
Total
hectares ,
in subarea —
(1,000)
3,381
1,955
1,092
2,192
2,455
4,788
1,675
5,922
3,747
5,316
3,465
5,036
7,935
4,890
3,431
6,259
4,898
2,157
7,389
4,480
Cropland in
Total 2/
hectares —
(1,000)
2,022
962
504
1,681
1,351
1,508
1,290
3,275
1,053
3,679
999
2,059
676
1,070
277
2,384
762
245
310
2,402
subarea
Percent ,,
irrigated —
64.0%
60.0
62.0
34.8
30.0
59.4
15.0
19.0
37.3
14.0
28.5
18.7
71.0
20.0
49.0
9.2
21.0
26.4
71.0
5.0
(Continued)
10
-------
TABLE 1. (Continued)
Subarea
1003
1110
1105
1009
1010
1108
1105
LI
1017 -'
1206
1004
LI
1012 -
1011
1006
1106
4/
1016 -'
LI
1014 -'
4/
1013 -'
Percent
of subarea
irrigated
2.3%
2.0
1.7
1.6
1.6
1.6
1.2
1.2
1.1
0.9
0.7
0.6
0.5
0.4
0.3
0.2
0.2
Total
hectares 1 ,
in subarea —
(1,000)
6,191
2,089
820
3,820
5,471
2,775
4,503
3,228
1,891
4,329
5,309
4,297
2,100
936
5,540
4,245
9,911
Cropland in
Total 2/
hectares —
(1,000)
1,872
645
386
175
764
155
1,034
2,399
619
500
456
1,566
1,008
470
3,804
1,244
3,896
subarea
Percent , ,
irrigated —
7.7%
6.7
4.0
34.4
11.4
28.5
5.3
3.2
3.4
8.0
8.3
1.5
1.0
0.8
0.5
0.7
0.5
— When the entire subarea was not included in the Irrigated Great Plains,
the size given here is for the included portion only.
2 /
— Based on figures from the 1974 Census of Agr., (for each of the ten states
3/
in this study) U.S. Dept. of Commerce, Bureau of the Census.
— This percent was obtained by taking 1976-1977 figures for irrigated
hectares (1974 figures for So. Dak.) and dividing them by 1974 U.S. Dept.
of Commerce Census of Agr. figures for total cropland. As total cropland
is fairly stable from year to year, any error introduced by this procedure
11
-------
TABLE 1. (Continued)
would be slight. As noted in the text, the one exception would be
subareas involving South Dakota counties.
4/
— These subareas include counties in South Dakota. Thus, the figures
related to irrigated hectares are based on 1974 U.S. Census of Agriculture
data and thereby thought to be somewhat low.
TABLE 2. COUNTIES IN THE IRRIGATED GREAT PLAINS WITH 50 PERCENT
OR MORE OF THEIR TOTAL HECTARES IN IRRIGATION
County
Hale
Palmer
Castro
Lamb
Hamilton
Hansford
Haskell
Phelps
Merrick
Hall
York
Lubbock
Swisher
State
Texas
Texas
Texas
Texas
Nebraska
Texas
Kansas
Nebraska
Nebraska
Nebraska
Nebraska
Texas
Texas
Percent
of county
irrigated
77.6 %
66.5
66.3
66.0
64.5
60.9
56.6
55.0
53.4
53.0
52.5
52.3
50.6
Total
hectares
in county
(1,000)
253.7
222.6
227.8
264.7
139.2
234.7
150.1
140.8
124.2
139.2
149.3
231.5
231.9
Percent of
county's total
cropland which
is irrigated
84.8 %
82.0
84.6
95.0
74.0
100.0
67.0
71.0
79.0
78.0
63.3
63.8
73.0
Subarea
No.
1205
1205
1205
1205
1027
1109
1104
1021
1021
1021
1027
1205
1112
12
-------
I
Percent Irrigated
Less than 1%
1% to 5%
5% to 15%
15% to 30%
30% or more
0 80 160 240 320 400 480 Km.\
Figure 3. Distribution of irrigation density throughout the Irrigated
Great Plains, based on irrigated hectares per total hectares in each subarea.
13
-------
to point out, however, that within any of the lightly-irrigated subareas,
there may be small sections where irrigation is quite concentrated. The
23,000 irrigated hectares of the Belle Fourche Project in South Dakota's
Butte County, located in Subarea 1012 ''Cheyenne River drainage), is an
example (J. Wiersma and D. Wilde, per. com.)- Thus, the densities of
irrigation shown in Figure 3 do not imply, a priori, that nonpoint source
pollution does or does not occur in certain parts of the Irrigated Great
Plains. A water quality problem related to irrigated agriculture can
develop at any given location if there exists a particular combination of
soil conditions, climatic factors, cropping patterns, irrigation systems,
and agricultural management practices.
The subareas of most intensive irrigation shown in Figure 3 largely
overlie the Ogallala aquifer (Figure 4). The major exception is the
included portion of Subarea 1027, which is the drainage basin of the Big
Blue River in Nebraska.
Precipitation and Evapotranspiration
Figure 5 shows the average annual precipitation, based on state climatic
divisions, across a region which closely approximates the Irrigated Great
Plains. The range is from 74 centimeters (cm) in southeastern Nebraska to
33 cm in northern Montana and southeastern New Mexico to 26 cm in Wyoming's
Bighorn Basin. A distinct east-to-west decrease in annual precipitation is
apparent, along with a south-to-north decrease in the eastern half of the
region. Along the region's western boundary, there is less change in pre-
cipitation from south to north and it follows no discernible pattern. Agri-
culturalists and others have long cited 51 cm as being the average annual
precipitation needed for a stable economy based on dryland crop production
(Powell, 1878; Webb, 1931). Of the 53 climatic divisions included in
Figure 5, 39 receive an average yearly precipitation at or below this amount.
Seasonality—
Distribution of average annual precipitation, by season, for select
climatic divisions is shown in Figure 6. This information adds an important
dimension to the rainfall picture and partially explains why dryland agricul-
ture (though precarious) is possible in this region. Rainfall in the Plains
displays a marked seasonality, with most of the moisture arriving in the
spring and summer. Except in the western portion of the region, the..percen-
tage which is received during the growing season (May to September) —
— The actual length of growing season in the Irrigated Great Plains
decreases from south to north. The period May to September is used as
an average.
14
-------
108°
44°
106°
~T~
104"
102°
—I—
100°
1
SOUTH DAKOTA
98°
96"
94"
MINNESOTA
WYOMING
42"
IOWA •
40°
COLORADO
38°
36"
34"
NEW MEXICO
32°
\
\
0 50 100 150 KILOMETERS
1 L
Figure 4. Location of the Ogallala formation, a multi-state geologic unit
and an important aquifer.
(Weeks, 1978)
15
-------
Figure 5. Average annual precipitation (cm) in a region composed of
climatic divisions within the ten Great Plains states. Boundaries are
similar to those of the Irrigated Great Plains. (Modified from: U.S. Dept.
of Commerce, Climatic Atlas of the United States, 1968.)
16
-------
Seasonal Precip./Ave. Annual Precip.
Growing
Winter Spring Summer Fall Season
Seasonal Precip./Ave. Annual Precip.
Growing
Winter Spring Summer Fall Season
10
percent
27
10 37 31 22 57
36 38 17 63
11 32 40 17 63
11
20
Definitions:
Winter - Dec, Jan, Feb
Spring - Mar, Apr, May
Summer - June, July, Aug
Fall - Sept, Oct, Nov
Growing Season - May to Sept—
The actual length of growing
season in the Irrigated Great
Plains decreases from south to
north. The period May to Sept
is used as an average.
29
17 66
18 69
21 65
25 58
Figure 6. Distribution, by season, of average annual precipitation in select climatic
divisions in the Irrigated Great Plains.
-------
increases from south to north. This occurrence, along with a south-to-north
decrease in evaporation, permits more crop production in the drier northern
plains than would otherwise be the case.
Despite the rainfall received during the growing season, supplemental
water is desirable (and in many places, necessary) in most years to assure a
good crop. Thus, rainfall seasonality affects irrigation needs and practices
throughout the Irrigated Great Plains.
Variability—
Another characteristic of precipitation in this region is its variabil-
ity. Generally speaking, the lower the average annual precipitation in an
area, the greater the variability—that is, greater variation in the amount
of precipitation received from year to year. In the Irrigated Great Plains,
the expected deviation from mean annual precipitation ranges from 15 percent
to 25 percent (Biel in Strahler, 1969). In other words, if the mean annual
precipitation is 40 cm and the expected deviation is 25 percent, then 30 cm
might fall in one year and 50 cm the next year.
Not only does precipitation in the region vary a good deal from year to
year but also from place to place within any given year. Consider, for
example, the 1929 and 1930 precipitation records for Mitchell and
Scottsbluff, Nebraska—towns only eight miles apart (Table 3). In 1929 and
1930, Mitchell received a two-year total of 88.6 cm and Scottsbluff received
a similar two-year total of 88.7 cm of precipitation. Yet, for each of the
individual January-to-December periods, Mitchell's annual precipitation was
significantly different from that of Scottsbluff; 9.5 cm lower in 1929 and
9.4 cm higher in 1930.
Corn, an important crop in the Irrigated Great Plains, has acute water
needs during particular periods of its growth cycle. When grown in the area
around Mitchell and Scottsbluff, Nebraska, for example, the period of late
July and much of August is crucial. In August, 1929, Mitchell received
1.3 cm of rainfall. During the same month in the following year, it
received 10.5 times that amount, or 14.4 cm.
Such yearly and monthly variability in precipitation has a significant
impact on Great Plains agriculture. It means that many farmers are generally
going to need a supplemental supply of water in order to maintain an econom-
ically-viable level of crop production over an extended period of years.
Intensity—
Rainfall intensity is a third factor which must be mentioned in a
discussion of agriculture and its relationship to water quality in the
Irrigated Great Plains. Figure 7 is a rainfall intensity map which shows
the maximum 30-minute rainfall which could be expected during any two-year
period. The 6 cm isopluvial line, for example, connects points where 6 cm
of rain could be expected to fall within a 30-minute period (considered a
quite intense rainfall) during any two consecutive years. The 6 cm
18
-------
TABLE 3. MONTHLY AND ANNUAL RAINFALL (IN CM), MITCHELL AND SCOTTSBLUFF, NEBRASKA
FOR 1929 AND 1930 -
1929:
Mitchell
1930:
Mitchell
Total Jan. Feb. Mar. Apr.
35.32 0.18 0.48 2.08 6.2
53.23 0.40 0.99 0.28 7.0
May
4.16
9.83
June
_ Cm
7.72
3.88
July
wing sec
1.88
0.66
Aug.
1.37
14.48
Sept. Oct.
7.82 2.64
8.76 5.92
Nov. Dec.
0.79 0.00
0.76 0.20
1929 + 1930
88.55
1929:
Scottsbluff 44.88 0.46 1.3
1930:
Scottsbluff 43.81 1.7
1929 + 1930 88.69
Mitchell:
30-year ave. 35.54
(1941-70)
Scottsbluff:
30-year ave. 37.01
(1941-70)
21
4.3 7.92 3.05 6.6 4.55 1.95 8.25 4.14 2.34 T-
0.56 0.56 4.4 10.26 3.25 1.19 10.0 4.6 4.8 2.06 0.43
- From Thornthwaite (1936)
7.1
- Trace = less than 0.025 cm
-------
C«AB- 2, RAINFA^ fREOU£l\.C' OTL4S Of TM£ UMTfC STATES
NT Of COMMERCE,*£«TMfB 6J«E»U, TECHNICAL PAPER NO «o'
ro
o
Figure 7. For the United States, maximum 30-minute rainfall intensities in centimeters (cm)
which can be expected during any two-year period. (Isopluvial interval = 1 cm).
(Modified from: Hershfield, 1961)
-------
isopluvial line arches northward over the Great Plains, reflecting the
influence of the Gulf of Mexico as a moisture source. Thus, intense rain-
fall is a characteristic common to much of the Irrigated Great Plains, but
particularly in the southern portion of the region.
When such large quantities of water fall on the ground during short
periods of time, potential pollutants such as fertilizers, pesticides, and
sediments can be washed from cultivated fields and into receiving waters.
Fields planted to row crops are more vulnerable in this regard than are
those with cover crops, especially on the steeper slopes. If the heavy rains
occur early in the growing season, when the plants are still small and much
of the ground is exposed, the impact is likely to be even greater.
Most of the Irrigated Great Plains has recorded its maximum 24-hour
rainfall (that is, the largest amount of rain to fall in a 24-hour period)
during the summer months (Weather Bureau, 1963). The importance of this
precipitation occurrence lies in the fact that the summer season is when the
impact on agriculturally-related water quality problems is apt to be the
greatest. In fact, in parts of the region, prolonged heavy rains have a
greater impact than do applications of irrigation water because the farmer
has no control over rainfall timing and amount (D. Watts, per. com.).
Evapotranspiration—
Transpiration is the process by which plants transpire and release water
to the atmosphere. When combined with the evaporation of water from the soil
surface, the collective term of evapotranspiration (ET) is used. Evapotran-
spiration can be thought of as an 'invisible river1 which carries water away
from an area just as assuredly as a regular river. Average evapotranspira-
tion rates for the Great Plains during the summer may range from 0.4 cm a
dav in the northern plains to 0.6 cm a day in the southern plains (J. Stone,
per. com.). Perhaps more important, however, are the deviations from average.
Corn, for example, can experience a peak ET rate of 0.8 to 0.9 cm a day in
the northern plains and 1.1 to 1.2 cm a day in the southern plains
(D. Watts, per. com.).
Evapotranspiration continually transports water from the soil through
the plant to the atmosphere, and in the process, performs such vital
functions as supplying nutrients to the plant and regulating its temperature.
Over the course of a growing season, large quantities of water are required
to accomplish these ends. As a case in point, total ET for corn during the
growing season may vary from approximately 56 cm in the northern plains to
as much as 80 cm in the southern plains. Expressed another way, one corn
plant in the Great Plains can evapotranspire from 87 to 125 liters of water
during the growing season (D. Watts, per. com.).
Theoretically, it is when evapotranspiration exceeds rainfall that
irrigation is needed. The larger the difference between these two para-
meters, the more irrigation water will be required. This is one of the
reasons, then, that evapotranspiration must be considered when discussing
management alterantives to help reduce nonpoint source pollution from
irrigated agriculture. In addition, as water evapotranspires into the
21
-------
atmosphere, it leaves behind the salts which it contained. On occasion,
this salt residue can contribute to an irrigation-related water quality
problem.
In summary: rainfall seasonality, variability, and intensity, along
with evapotranspiration have had, and will continue to have, an influence
on agriculture, on irrigation, and on water quality in the Great Plains.
Irrigated Crops and Water Sources
Crops—
The United States has about 24.5 million hectares of farmland under
irrigation (Irrigation Journal, 1978). Of this amount, the Irrigated Great
Plains accounts for about 9 million or 37 percent (see Tables 4 and 5).
Data on the region's five major irrigated crops—corn, alfalfa, sorghum-,
soybeans, and cotton—make possible further evaluation and comparison. —
The distribution of these crops within the Irrigated Great Plains is shown
in Figures 8 through 12. This distribution is based simply on the number of
irrigated hectares of each crop in the various subareas.
Figure 12, for example, shows that irrigated cotton is grown mostly in
eastern New Mexico, northern Texas, and southwestern Oklahoma. On the other
hand, alfalfa is spread across all ten states, as shown in Figure 9. Such in-
formation is useful when considering potential nonpoint source pollution from
irrigated agriculture in the entire Irrigated Great Plains. First, it shows
in broad perspective where row crops are located, as opposed to cover crops.
Second, it suggests where crop-specific farm practices would most likely be
found. Third, it makes clear that for a widely-dispersed crop like corn,
the range of irrigation management practices is going to be greater than for
a crop with a more limited geographic distribution such as cotton. This is
because the widely-dispersed crop will encounter a greater variety of
physical conditions such as different lengths of growing season, soils,
topography, and precipitation.
The concentration of a particular crop within a portion of a subarea
does not appear on these maps. Rather, such irrigated hectares would be
included in the overall figure for the entire subarea in which that crop
concentration is located. The maps should be examined with this fact in
mind.
— While the amount of cropland planted to irrigated soybeans is not large
in comparison to the other four crops, it is expected to increase in the
future.
22
-------
I
Irrigated Hectares
5,000 to 50,000
50,000 to 150,000
150,000 to 300,000
300,000 to 500,000
Figure 8. Distribution of irrigated corn in the Irrigated Great Plains,
based upon irrigated hectares of corn (5,000 or more) in the
various subareas.
23
-------
1
.Irrigated Hectare^
5,000 to 20,000
20,000 to 40,000
40,000 to 60,000
60,000 to 80,000
0 80 160 240 320 400 460 Km.
Figure 9. Distribution of irrigated alfalfa in the Irrigated Great Plains,
based upon irrigated hectares of alfalfa (5,000 or more) in the
various subareas.
24
-------
Irrigated Hectares
fxTT]
IS:;:;! 2,000 to 10,000
10,000 to 50,000
50,000 to 100,000
100,000 to 400,000
\
s
0 80 160 2K3 320 400 480 Km. ~"
Figure 10. Distribution of irrigated sorghum in the Irrigated Great Plains,
based upon irrigated hectares of sorghum (2,000 or more) in the
various subareas.
25
-------
Irrigated Hectares
320*10 480 Km-v
5,000 to 12,000
12,000 to 15,000
Figure 11. Distribution of irrigated soybeans in the Irrigated Great Plains,
based upon irrigated hectares of soybeans (200 or more) in the
various subareas.
26
-------
Irrigated Hectares
500 to 1,000
1,000 to 50,000
50,000 to 200,000
200,000 to 400,000
I
0 80 160 240 320)0 460 Km.
Figure 12. Distribution of irrigated cotton in the Irrigated Great Plains,
based upon irrigated hectares of cotton (500 or more) in the
various subareas.
27
-------
Further crop information is displayed in Table 4. In this table the
estimated number of irrigated hectares for each of the five crops in all the
subareas is listed as a percentage of the total irrigated hectares. For
example, Subarea 1010 (the Lower Yellowstone drainage) has 21 percent of its
87,000 irrigated hectares in corn, while in Subarea 1109 (part of the
Canadian River drainage), 26 percent of 793,000 irrigated hectares are
planted to corn.
The most intensively irrigated Subarea—Subarea 1205—shows the most
diversification among these five crops. Of its total irrigated hectares,
25 percent is in corn, 25 percent is in sorghum, and 29 percent is in cotton.
In contrast, Subarea 1021 which is also intensively irrigated, has 71 percent
of its total irrigated hectares in corn. Thus, within Subarea 1205 (Brazos
headwaters) one might expect a variety of management practices reflecting
the needs of the different crops (and also the longer growing season). On
the other hand, in Subarea 1021 (Platte River drainage) the management
practices would be those commonly associated with the cultivation of
irrigated corn (and a shorter growing season). Such information should be
considered when alternatives for managing nonpoint source pollution from
irrigated agriculture are being examined, particularly in a region as large
and diverse as the Irrigated Great Plains.
Water Sources—
Available figures show that for all ten Great Plains states combined,
the ratio between the number of hectares irrigated with groundwater and the
number of hectares irrigated with surface water is approximately 4 to 1. A
state-by-state listing is presented in Table 5. Groundwater is indeed the
major water source for irrigation in the region.
Table 5 also shows estimates for the number of hectares under the two
major methods of irrigation—surface and sprinkler. These figures required
extensive interpretation and thus are estimates which should be used with
care.
Groundwater studies—Concerns have arisen over the effects of continued
intensive groundwater irrigation on the resources and economy of the Great
Plains. Serious questions are being asked, such as: How long will the
groundwater last? What will happen when it is no longer economical to pump
groundwater? How will irrigated agriculture fare in the face of increased
urban and energy uses of water in the Great Plains?
Three major studies extending over several years have recently begun
to address these and other questions. The U.S. Geological Survey's five-
year "High-Plains Regional Aquifer-System Analysis" is the most extensive
of these investigations. Another is a three and one-half year study of the
impact on the nation's agribusiness of declining groundwater supplies in
the High Plains. This work is being done under the auspices of the
U.S. Department of Commerce's Economic Development Administration which,
in turn, has contracted the services of a private consulting firm,
Camp Dresser and McKee, Inc., of Austin, Texas. The U.S. Bureau of
28
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TABLE 4. IRRIGATED LANDS PLANTED TO THE FIVE MAJOR CROPS AS A PERCENT OF
THE TOTAL IRRIGATED HECTARES IN EACH SUBAREA
Subarea
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
Total irrig.
hectares
(1,000)
136
144
40
54
10
65
220
60
87
24
44
20
9
213
17
34
482
385
285
567
Corn
0.6
3
7
10
3
2
0.2
21
10
2
5
22
60
59
82
8
28
72
71
Alfalfa Sorghum Soybeans Cotton
38
42
56
42
39
40
28
37
28
41
21 0.2
30 2
22 8
11 0.2
24 3
11 23
12
15
8 11
5 1 0.7
(Continued)
29
-------
TABLE 4. (Continued)
Subarea
1022
1025
1026
1027
1102
1103
1104
1105
1106
1108
1109
1110
1112
1113
1205
1206
1208
Total
Total irrig.
hectares
(1,000)
193
628
118
586
160
521
405
15
4
44
793
43
313
220
1,292
21
393
8,645
Corn
72
64
45
81
10
40
38
3
2
25
26
6
23
4
25
Alfalfa
4
8
38
2
42
14
4
20
45
17
2
44
1
14
2
5
5
Sorghum Soybeans Cotton
1 5
2
12
9 1
11
14
18
4
3
5 2
32
20 0.3
32 1 6
16 21
25 1 29
32 46
23 50
Note: Figures for irrigated hectares of corn, alfalfa, sorghum, soybeans,
and cotton—upon which the above crop percentages are based—were secured
from the sources discussed on pages 5 and 6 of this manual.
30
-------
u>
TABLE 5. IRRIGATED HECTARES BY SOURCE OF WATER AND IRRIGATION METHOD
IN THE IRRIGATED GREAT PLAINS -
Colorado
Kansas
Montana
Nebraska
New Mexico
North Dakota
Oklahoma
South Dakota
Texas
Wyoming
Total
Total
irrigated
hectares
(1,000)
753^
1,384
558
2,902
240
56
385
151^
2,681
263
9,373
Groundwater
Irrigation method
Total
300
1,330
41
2,367
221
36
337
75
2,681
30
7,418
Sprinkler Surface
- (1,000 hectares)
158 142
415
34
1,032
134
34
191
60
654
17
2,729
915
7
1,335
87
2
146
15
2,027
13
4,689
Total
311
54
517
535
19
20
48
71
—
233
1,808
Surface Water
Irrigation method
Sprinkler Surface
(1,000 hectares) -
17
25
89
78
10
5
7
61
—
9
301 1
294
29
428
457
9
15
41
10
—
224
,507
— Table prepared by M. Twersky. The information was compiled from and cross-checked with
Irrigation Journal's 1977 Irrigation Survey, the latest available state Agricultural
Statistics
21
— These two
, and irrigation extension
states have a small number
experts in
of hectares
the various states.
where both surface
and groundwater are used.
The numbers are not included in this table.
-------
Reclamation is conducting the third investigation—a four-year effort
which will focus on the High Plains south of the Arkansas River.
As a result of these and perhaps other studies, it is possible that
recommendations may eventually be made which, if implemented, could affect
agriculturally-related water quality programs in the Great Plains. For
this reason, planners and others, who are developing management alternatives
for lessening the negative effects of irrigated agriculture on the quality
of water, might find it useful to follow the progress of these investigations.
32
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SECTION 2
WATER QUALITY OF IRRIGATION RETURN FLOWS IN THE GREAT PLAINS
by
J. R. Gilley, D. G. Watts, F. W. Roeth, and M. Twersky
Irrigation is an important tool in the stabilization of crop production
and farm income in the Great Plains. It provides the means to overcome the
adverse effects of highly variable and often inadequate precipitation.
Irrigated crop production requires many of the same management practices as
dryland agriculture. However, some practices, particularly the application
of nutrients and pesticides, are likely to be more intensive in areas where
crops are irrigated. While this generally leads to increased crop yields, it
also increases the possibility in some situations for negative impacts on the
environment. For example, excessive amounts or mismanagement of water,
nutrients, or pesticides or the use of improperly designed or managed irriga-
tion systems can create nonpoint source pollution problems.
One of the major problems facing modern agriculture is that of develop-
ing management practices for the maintenance of high production levels while
minimizing hazards to the environment. Such practices must be tailored to
fit the varying conditions imposed by soil, climate and crop. An optimum set
of practices at one location may be disastrous at another where conditions
are entirely different. Appropriate management practices for a given set of
conditions can be much better defined and implemented when both the people
who manage agricultural production and those responsible for environmental
protection have a clearer understanding of the general interrelationships
that exist between soil, plant, water and management practices. A grasp of
a few basic concepts is extremely helpful in understanding how current
irrigation practices affect the quality of surface runoff and subsurface
drainage water from irrigated lands. Improved understanding enhances our
ability to alter these practices in order to improve the environment and
maintain an economic level of production.
This chapter presents some important agronomic concepts and practices
related to irrigation management. Soil-water-plant relationships, surface
and subsurface return flows, nutrients and pesticides, irrigation methods and
systems, and related management practices are considered as parts of the
entire agricultural production system. These parts are all interrelated such
that modifications of one may change the other with subsequent effects on the
extent and magnitude of the irrigation water quality problem.
DEFINITION AND DESCRIPTION OF IRRIGATION RETURN FLOWS
Irrigation return flow (IRF) is that part of the water supply
33
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(precipitation plus irrigation) which is not used to supply the crop evapo-
transpiration demand or the necessary leaching requirement and which eventu-
ally travels back to surface or groundwater sources. The two broad catego-
ries of irrigation return flow are surface water runoff and deep percolation.
The quantity and quality of IRF are influenced by many natural phenomena as
well as man-controlled management practices (Figure 13). The management of
the irrigated agricultural production system affects both the amount and
quality of the water that flows back to surface or groundwater sources. In
this manual, the discussion of return flow is limited to flow from irrigated
lands. Over 200 references describe the complex nature of IRF (Walker, 1977)
Soil-Plant-Water Concepts
The important factors affecting the balance of water and agrochemicals
in an irrigated agricultural production system are shown in Figure 14. The
water balance shown in Figure 14 can be summarized as follows:
Precipitation
Irrigation
Transpiration
- < Evaporation
Surface Water
Runoff
Deep
Percolation
Irrigation Water
Return Flow
Increase in
Stored Soil
Water
^
Water starts infiltrating into the soil as soon as irrigation water is
applied. The rate of water infiltration decreases from relatively high rates
during the early phase of an application to a nearly constant rate. When the
rate of water application exceeds the soil infiltration rate, water starts
collecting on the soil surface. The time at which this happens depends on
the type of soil, irrigation system, and water delivery rate. Most of the
accumulated surface water eventually becomes surface water runoff, although
some of the water will stay on the soil surface in small depressions and be
infiltrated after the irrigation period.
The soil acts as a reservoir holding the infiltrated water until it
evaporates from the soil surface, is used by plants or percolates downward
past the plant root zone. Water not retained in the root zone continues to
percolate (drain) downward into the soil subsurface and acts as a solvent,
transporting mineral salts, soluble fertilizers and pesticides. If this
percolating water encounters relatively impermeable strata, it may form a
temporary or "perched" water table and move laterally above the main ground-
water level to a stream. In many cases it percolates directly to the general
groundwater aquifer, becoming part of the existing groundwater supply which
then may move laterally to a stream. Thus, deep percolation is part of IRF
and is a carrier of soluble nutrients and pesticides.
The changes that take place in irrigation water runoff as it flows
34
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SU&5UKFAC6 DRAINAGE
=&
•saHs
•sedimenis
. pMospHate
• pesiic'idea
Figure 13. Return flows from irrigated agriculture.
35
-------
OJ
Figure 14. Processes that influence water flow and agrochemical balances in an irrigated
agricultural production system.
-------
directly over the soil surface are quite different from those that occur when
water moves as deep percolation. The quality of water flowing directly over
the soil surface is affected by soil particles which are detached by both
droplet impact and flowing water. Any water that travels over the soil
surface to reach a stream is called surface runoff. However, the surface
water runoff occurring at the lower end of an irrigation field is known as
tailwater. Tailwater may evaporate, percolate, be consumed by other plants
or flow into surface streams. It can also be collected and pumped back into
the irrigation delivery system or used by irrigators further downstream.
Crop Water Use—
Net crop water use or evapotranspiration (ET) is a sum of the evapora-
tion from the soil surface and the transpiration from the crop leaf surface.
Early in the growing season almost all of the water use represents direct
evaporation. As the crop grows, however, the proportion of ET that goes into
transpiration may increase to about 90 to 95 percent for crops that fully
shade the ground.
If there is adequate moisture in the soil the actual daily rate of water
use by a crop depends on both the stage of growth of the crop and the
potential ET, which is primarily determined by daily weather conditions.
Accurate estimates of crop water use are essential for irrigation scheduling,
an important water management tool. Annual, monthly, weekly and even daily
crop water requirements can be determined by several methods (Jensen, et al.,
1970; Jensen (ed.), 1974; and Stegman, et al., in prep.).
Normally the amount of water delivered and applied to an irrigated
field must be larger than the net crop water use requirement in order to
compensate for unavoidable losses to surface water runoff, deep percolation,
nonuniformity of application and, in case of sprinkler systems, evaporation
and wind drift losses during application. Soil, climate, crop and available
water resources all influence the method used to establish the gross
irrigation requirement. The complexity of these factors makes it difficult
in some cases to identify the amount of water actually needed for successful
irrigation. Thus, while the net seasonal water requirements of various crops
are approximately known, both the actual net water use and the gross amount
to be applied depend upon particular areas and situations.
Soil-Water Balance—
Although only part of the water in the root zone is used for crop growth,
either excessive or deficient amounts of soil water can reduce crop produc-
tion. When excess water is applied, most is lost from the root zone by deep
percolation before it can be used by the growing crop. From the standpoint
of production, this is not necessarily a negative impact. In some cases,
deep percolation maintains a satisfactory salt balance in the root zone by
removing salts which are retained in the soil as crops transpire or as water
evaporates from the soil surface. Without a mechanism for salt removal, the
soil would eventually become unproductive.
Effective soil-water management requires knowledge of how much water a
37
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soil can hold. The water-holding capacity of a soil is dependent on its
texture, structure, organic matter content and apparent bulk density. Soil
moisture and water-holding capacity may be expressed as (1) a percentage of
dry weight of soil, (2) percentage of soil volume, or (3) depth of water per
unit depth of soil. The latter is a convenient way of evaluating stored soil
moisture and is the commonly-used means of expression.
The available soil moisture for plant use is the amount of soil moisture
that can be held in the root zone between field capacity and wilting point.
Field capacity is the water content of the soil after excess water has
drained away and the rate of downward movement has normally ceased. (The
time of drainage varies with soil texture and structure.) The permanent
wilting point is the soil water content at which plants can no longer extract
soil moisture at a sufficient rate to overcome moisture stress. Another term
often used to describe soil moisture status is the available soil moisture
deficit, the difference between field capacity and the existing soil moisture
in the root zone.
Generally, the larger the percentage of fine particles in a soil, the
higher the water-holding capacity. Thus, coarser-textured soils have less
water available for plant use than finer-textured soils. The total soil
water available for plant growth can be estimated by multiplying the avail-
able soil moisture by the rooting depth of the crop. Estimates of the
available soil moisture of different soils can be found in many sources
including Christensen and Westesen, 1978; Fischbach (ed.), 1977; Stegman,
in prep.; Harmon and Duncan, 1978. The approximate range of available soil.
water for given soil textures is shown in Figure 15. As an example, the
figure shows a median value of 0.16 cm of available water per cm of soil
depth for a silt loam soil. Thus, in a 120 cm deep root zone, there would
be .16 x 120 = 19.2 cm of water available for plant use when the soil was
"full" or at the field capacity water content.
COARSE TEXTURED
SAND-LOAMY SAND
MEDIUM TEXTURED
L(
)AM-SILT LOAM
FINE-TEXTURE
SILTY CLAY-CLA1
f
0.04 0.08 0.12 0.16 0.20
CM OF WATER PER CM OF SOIL DEPTH
0.24
Figure 15. Representative magnitude of available water-holding capacity
of agricultural soils.
38
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IDENTIFICATION OF POLLUTANTS IN IRRIGATION RETURN FLOWS
Pollution is an undesirable change in the physical, chemical or
biological characteristics of air, land and water. These changes may harm-
fully affect human life, animal species, industrial processes, living con-
ditions or culture. They also may waste or cause the deterioration of raw
material resources. Considerable literature exists on the evaluation of
specific irrigation-related pollutants (Ayers and Westcot, 1976; Boone, 1976;
Exner and Spalding, 1979; Wendt, et al., 1976). The common water quality
factors in IRF and the changes in water quality likely to occur as water
flows over the land surface and through the soil are summarized in Table 6.
Other factors must also be considered when determining impairments of
water quality. Among them are: (a) the complexity of pollutant loading
processes; (b) the large number of localized conditions affecting pollutant
loading; and (c) the contribution of the same kind of pollutants from various
sources, both natural and man-induced. Natural processes supply some known
pollutants. The effects of these can be separated in some cases from those
of man-induced processes through the use of site-specific information.
Erosion and Sediments
Soil erosion and sediment transport occur through a two-step process
which involves: (a) detachment of soil particles by water droplet impact,
splash, and flowing water; and (b) the transport of detached soil particles
by flowing water and splash. Precipitation, irrigation and land slope are
the primary sources for the energy required to accomplish this process.
In the Great Plains, most erosion is caused by precipitation, although
improperly designed and operated irrigation systems can also cause erosion.
The Universal Soil Loss Equation (USLE) is useful for predicting erosion from
rainfall. The equation reflects how certain factors influence erosion due to
rainfall. However, it has extremely limited applicability for estimating
erosion caused by irrigation. These factors are discussed in great detail
in other publications (Stewart, B. A., et al., 1975, 1976; Nelson, 1978;
Harmon and Duncan, 1978).
Some soils are more susceptible to erosion losses than others. In
general, soils that are high in silt, low in clay, and low in organic matter
are the most susceptible to erosion. Thus, medium-textured soils erode more
easily than the other soil types. The soil erodibility factor of the USLE
is the best indicator of the degree of soil loss for various soil types.
Degree and length of land slopes also affect soil erodibility. These
factors affect the transport portion of the erosion process through their
influence on runoff velocity. As the slope increases, the velocity of runoff
increases, and the capability of runoff to both detach and transport soil
materials increases. In general, as the length of the slope is doubled,
erosion losses can increase 2.5 times.
The USLE has been primarily used to predict soil erosion losses under
39
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I/
TABLE 6. PROBABLE CHANGES IN WATER QUALITY AS A RESULT OF IRRIGATION -
Quality
factors
Surface water runoff
Irrigation return flow source
Deep percolation
Sediments and
Colloids
Nitrate
Phosphate
Pesticides
Salts
(Total
Dissolved
Solids)
Sodium and
Chloride Ions
Organics
Often more than in water source but may
be less; highly variable.
More likely a slight increase than a
decrease; highly variable.
Content may increase, but closely
correlated with erosion of fertile
topsoil.
Highly variable content. Likely
associated with amount of erosion.
Not greatly different from water
sources.
Relatively unchanged.
Manures, debris, etc., likely to
increase.
Little or no sediment or colloidal
materials in the flow.
Greatest hazard from heavily fertilized,
over-irrigated, coarse-textured soils.
Decrease if considerable in source. Not
likely to greatly increase.
A reduction in many instances. Concen-
trations likely to be low.
Concentration increased usually 2-7 times.
Depends on amount in the supply, number of
times reused and the amount of residual
salts being removed.
Both proportions and concentration likely
to increase.
Most oxidizable and degradable materials
to decrease.
- Modified from Boone (1976).
-------
limited, but defined, dryland conditons. However, factors of the equation
have been applied to irrigated situations (Nelson, 1978). Soil losses
expected for irrigated croplands are given in Table 7, which shows that the
use of improper irrigation systems on row crops creates the greatest possible
chance for soil losses. Much of the total soil loss shown in Table 7 is
caused by rainfall, even under ideal management conditions.
A distinction exists between soil loss and sediment yield. Soil loss
is the amount of soil set in motion without regard to distance or direction
of movement. Sediment yield is that portion of the soil loss which is
actually delivered to the edge of the field. The impact of sediments on
water quality can be considerable and highly variable. Sediments can reduce
the quality of IRF by carrying plant nutrients, pesticides and other mater-
ials adsorbed on soil particles.
Chemicals
Although nutrients and pesticides increase crop yields and improve
plant quality, they can become nonpoint source pollutants. Those nutrients
considered to be the greatest pollution hazards are nitrogen (N) and phos-
phorus (P). Primary sources of these nutrients are commercial fertilizers,
animal wastes, plant residues and soils. The increased use of pesticides
also has increased the potential of pesticide pollutants being carried by
IRF. Much has been written about N and P and pesticide materials, their
forms, characteristics, chemical make-up, and methods of transport (Frere,
1976; Stewart, B. A., et al., 1975, 1976; Int. Garrison Diversion Study
Board, 1976; Harmon and Duncan, 1978).
Research indicates that under most management systems negligible
quantities of soluble nitrates (N0_) exist in surface runoff as the runoff
leaves the field. A notable exception is when nitrogen is applied by
injection directly into the irrigation water (fertigation). In this case,
the N0_ concentration in the runoff will be essentially the same as in the
applied water. Because of this, it is essential to collect and reuse runoff
or tailwater when fertigation is practiced. This will be discussed in
detail in later sections.
Except for the case of fertigation, the concentrations of nitrates in
surface runoff water decrease during an irrigation or rainfall event on
medium- and coarser-textured soils. This occurs because nitrates readily
enter the soil and are unavailable for transport in surface water runoff.
In contrast, phosphorus (P) as phosphate is usually immobile in soil
because the ionic form readily adsorbs to soil particles and reacts to form
insoluble salts; therefore, the amount of P moving in the soil solution is
low when compared to the total amount of P in the soil. Levels of P in
groundwater are not generally increased by the application of fertilizers
containing P, and the movement of P in surface runoff water and deep per-
colation is small. Most of the P contained in surface runoff is adsorbed
on the sediment.
Al
-------
TABLE 7. EXPECTED SOIL RESOURCE LOSSES UNDER FOUR CONSERVATION IRRIGATION MANAGEMENT
OPTIONS WITH DIFFERENT CROPLAND USES -
Conservation
management option
21
Cropland use —
Row crops Broadcast crops Forages (in rotation)
ISJ
Use of most tolerable and
3/
best management practices. —'
Use of irrigation systems
and land practices which , ,
enhance soil erosion. —
Mismanagement of water
applications with irrigation
system.
Use of improper cultural
or fertility practices.
2.7
22.4
8.3
6.7
metric tons/ha-yr - - - - -
2.2 1.1
8.3
5.6
5.6
5.6
2.2
1.1
- Modified from Nelson, 1978.
2/
— Soil resource loss based on a medium-textured soil in Nebraska with an erodibility
K = 0.32, on a land slope of 3 percent, and an irrigation field run of 92 meters.
— Most likely soil losses under ideal management systems, primarily due to rainfall.
A/
— Lack of terracing, lack of contouring, etc.
-------
Potential losses of both N and P in surface IRF are directly related to
the quantities of sediments carried in surface runoff water (Nebraska Natural
Resources Commission, 1975a and 1975b; Regional Planning Office for Big Horn
Basin 208 Policy Board, 1978). That is, most N and P losses in runoff result
from the loss of sediments to which these elements are attached, not from N
and P being directly in solution in the water. The amounts of N and P lost
through erosion vary among different soils and land uses. As shown in
Table 8, nutrient losses from row crops are higher than with other cropland
uses because higher fertilizer applications and increased tillage (resulting
in more erosion) are used with row crops.
TABLE 8. ESTIMATED AMOUNTS OF POLLUTION POTENTIAL OF SEDIMENTS AND NUTRIENTS
IN SEDIMENTS IN THE NEBRASKA MIDDLE PLATTE RIVER BASIN -
Land use
Soil loss
Nutrients in sediments
N P
Row crops
(corn, sorghum,
soybeans)
Broadcast crops
(wheat, small
grains)
Forages
(alfalfa, grasses)
metric tons/ha-yr
22
kg/metric tons of sediments
46
45
16
2.1
1.6
0.3
— Summarized from Neb. Nat. Res. Com., Middle Platte River Basin Water
Quality Management Plan, Table 6-11,12. Figures include both irrigated
and nonirrigated areas.
Adsorption of pesticides to soil particles is the single most important
process affecting the quantity of pesticides in IRF. While pesticide losses
are low with respect to the amount of material actually applied, losses of
the more soluble pesticides can reach nearly five percent of the amounts
applied.
43
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Salinity
In the arid zones of the western United States, control of salinity in
the crop root zone is in many cases the primary factor determining long-term
viability of irrigation agriculture. Similarly, in many parts of the north-
ern Great Plains, salinity is a potentially serious barrier to the expansion
of irrigation. This sharply contrasts with the situation in the central and
southern plains where salinity presents a moderate to serious problem in
only a small percentage of the soils and waters. In any area where the
problem exists, however, salinity can have a very negative impact on crop
production and/or on the quality of irrigation return flow. Furthermore,
there is a potential for increased return flow problems in the future as
lower quality water is used to develop irrigation or as percolation from new
irrigation projects enters saline soil formations and flushes saline water
into rivers and streams.
Salinity problems can be grouped into two broad categories of
(1) natural occurrences, and (2) those resulting directly or indirectly from
man's activity. The paragraphs which follow outline the causes and the
general approach to the solution of some salinity problems.
Various salts (of importance to irrigation agriculture) are formed from
combinations of calcium, magnesium, sodium, or potassium, with sulfate,
carbonate, biocarbonate, chloride and other elements which are found in the
earth's groundwater, surface streams and lakes. Salts are brought into
solution as water percolates through or runs over soils and rock materials.
In some cases, these sources contribute relatively high concentrations of
salt into the surface water system. Water may flow through formations of
very saline materials producing "mineral" springs and groundwater outflow
having an extremely high salt concentration. An example of this is the
series of salt flows or springs along part of the Red River and a few of its
tributaries in western Texas and Oklahoma. At medium to high stream flow
rates the saline spring outflow is diluted by the river water. However,
when streamflow is low, the concentrated salinity from these springs dras-
tically reduces the overall water quality in that reach of the river where
they are located. Similar examples can be enumerated throughout the plains
region. Saline seeps occur in agricultural zones of Montana and Wyoming;
small saline stream valleys are found in the higher rainfall zone in east-
ern Nebraska; unuseable saline groundwater underlies many parts of the
Dakotas. These are all the result of the re-entry into the water cycle of
salts contained in sediments deposited millions of years ago.
When water tables are very near ground level (approximately 90 cm or
less) evaporation at the soil surface can cause an upward flow of water from
the water table to the top of the ground. As the water evaporates, the
dissolved salts are left behind in the upper soil horizons. Over a period
of time this process may cause large concentrations of salt in the soil,
making it unuseable or, at best, marginal for agriculture.
Such conditions exist to a varying extent in a number of river valleys
in the Great Plains. For example, parts of the Arkansas, Platte, and
Republican River valleys, particularly west of the 100th Meridian, have a
44
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few areas that support only salt-tolerant vegetation. Similar areas exist
in Wyoming, Montana and the Dakotas. While these saline soils may be in
irrigated areas, they existed long before irrigation agriculture came on the
scene. This is not to say that human activity does not contribute to the
expansion (or reduction) of such saline areas. The point here is that
saline soils and waters exist naturally in the Great Plains. In many parts
of the plains states they may constitute a sizeable proportion of the
salinity problems.
Salts in Agricultural Soils—
When irrigation water that contains various dissolved salts is applied
to the land, crops remove a part or all of the water and leave the salt in
the soil. If nothing is done to remove it, salinity levels will gradually
increase, resulting in a continual decline in crop yield. A shift to salt
tolerant crops allows continued production for a time. Eventually, however,
total abandonment of production may be necessary in extreme cases.
In arid and semi-arid zones, this problem is usually resolved in two
ways. First, careful attention is paid to the quality of the water applied.
Irrigation waters that are high in total salts are avoided whenever possible.
This reduces the amount of salt that must be contended with and delays the
buildup of salinity to levels that can drastically affect production.
Secondly, additional water is applied beyond the amount required to meet
evapotranspiration by the crop. The extra water (leaching fraction) perco-
lates downward through the root zone and leaches out a portion of the salts
left from the water transpired by the crop. The required leaching fraction
is defined as that portion of the total water application needed for leach-
ing in order to maintain a favorable salt balance in the root zone. It may
vary from only 1 or 2 percent to more than 10 or even 20 percent of the
applied water, depending on the quality of the irrigation water and the salt
tolerance of the crops being grown.
In many cases, particularly where surface irrigation is practiced, deep
percolation due to excess water application may frequently exceed the re-
quired leaching fraction. However, in certain instances where the infiltra-
tion rate of water is very low, it may be extremely difficult to achieve the
necessary leaching fraction. Examples would include very fine-textured soils
having naturally low water infiltration rates, severely compacted soils
(with the same result) and soils affected by excessive amounts of exchange-
able sodium. Excess sodium greatly reduces the infiltration capacity and
rate of movement of water through the soil. Where compaction is a problem,
changes in cropping patterns and careful management may improve soil struc-
ture sufficiently to maintain an adequate leaching capability. Where sodium
is a problem, reclamation through the addition of soil amendments that either
contain or form calcium sulfate is almost essential. Reclamation of sodic
soils is generally a several-year process.
Reclamation of saline soils is usually possible. This includes both
soils that have salinized due to poor management or lack of drainage and
45
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those that have salinized under "natural" conditions. Application of large
volumes of water to leach the salts from the soil is normally required. It
is essential, therefore, that there be either natural or artificial drainage
to remove the saline percolate or leachate that flows from the lower part of
the root zone. Additionally, it should be noted that salts thus removed can
be expected to emerge in the groundwater or surface water system at some
other point.
Fortunately, in eastern Wyoming, in Nebraska and other Great Plains
states to the south, the irrigation water is generally of good quality
(low salinity levels). Furthermore, there is sufficient precipitation in
at least the eastern two thirds of the region to leach accumulated salts
from the crop root zone. For this reason, there are large expanses of
irrigated land where no special management practices are either being used
or are required to control salinity. This is particularly true in areas
served by pump irrigation from high quality groundwater supplies. Where
rainfall is insufficient to provide the needed leaching, excess irrigation
normally solves the problem. The leachate containing the dissolved salts
percolates through the root zone to the groundwater system. Over long
periods of time this process results in increased salt concentration in the
groundwater of such areas. However, this is normally a slow process because
of the large amount of groundwater involved. Groundwater mining will
probably result in the reduction of irrigation pumping from some major
aquifer systems, such as the Ogallala, long before salinity concentrations
become a problem. Where water is pumped from relatively shallow river valley
aquifers, replenishment and dilution by recharge from the river may keep salt
concentrations in the same range as that of the river water.
Problem Areas—
There are always important exceptions to general statements. For
example, in western Kansas a number of salinity problems have resulted from
groundwater pumping (Balsters and Anderson, 1978). Declines in groundwater
levels in several areas have reduced the hydraulic head sufficiently in the
freshwater aquifers to permit the inflow of very saline water from adjacent
formations not tapped directly by irrigation wells. In another part of the
state, recharge from the Republican River normally maintains a layer of
fresh groundwater overlying the saline groundwater of the Dakota Formation.
Wells located along the Republican River have intercepted a part of the
river's groundwater recharge. As a result, brackish groundwater has
intruded into the freshwater zone more distant from the river, greatly
reducing the quality of irrigation water from wells at those locations.
The quality of surface water in the Arkansas River entering Kansas may,
at times, be marginal for irrigation. Several factors contribute to this
situation. First is the concentrations of salts by irrigation return flow
from districts in southeast Colorado. A second important reason is that
deep percolation from the irrigated lands passes through underlying saline
formations. This increases the salinity of the groundwater return flow
which feeds the Arkansas River. Similar examples can be cited in other
states.
-------
In other small areas of the central and southern plains and in greater
parts of North and South Dakota, problems either have developed or may
develop. These may be located either where individual farmers pump directly
from relatively saline groundwater or where large areas are irrigated with
water transported into an irrigation district from stream diversions and/or
reservoir systems. In the former case, the origin of the problem is obvious;
direct loading of soils with salts contained in applied waters. In the
latter, the problems are more complex in origin and end result. Here, deep
percolation, including the necessary leaching fraction, continually adds to
groundwater storage beneath the irrigated lands. Because no large volume
of groundwater is being removed by pumping, water tables are raised. Ground-
water outflow to streams may increase sufficiently to bring the additional
recharge into balance with the subsurface outflow system. However, the
increased groundwater outflow (which constitutes a part of the irrigation
return flow) will contain the more saline leachate from the crop root zone.
Under some circumstances (a large groundwater reservoir, relatively small
amount of percolation and/or low salinity irrigation water) the quality of
the water in the groundwater system may not be much affected. In other
cases, there may be a substantial decrease in groundwater quality with a
corresponding increase in salinity of groundwater-fed stream flow. Further-
more, in areas that are underlain by marine deposits and/or saline waters at
shallow depths (particularly in parts of the Dakotas), increased percolation
from the root zone may pick up additional salts, further adding to the salt
load of both the groundwater and the surface waters which are fed by that
groundwater further downstream.
Requirements for Drainage—
Where additional input to groundwater percolation from irrigated lands
is not balanced by additional groundwater outflow to surface streams, the
water table may rise into the crop root zone. Artificial subdrainage in the
form of ditches or buried drain pipes must then be installed to prevent
waterlogging and rapid salinization of the upper soil profile. Here the
same mechanism that leads to "natural" salinization by way of evaporation
and salt deposition would come into play. The drains must maintain the
water table at a depth below the surface (usually 150 cm or more) sufficient
to control upper root zone salinity and also provide the outlet for both the
necessary leaching fraction as well as any excess percolation.
In many cases, 20 to 40 years may elapse after project initiation
before water tables rise enough so that artificial drainage may be required.
An example is the W. C. Austin irrigation project in southwest Oklahoma
which began full-scale operation in 1948. Currently, after 32 years of
operation, subdrains are being installed on some project lands because
excess percolation has finally brought the water table into the crop root
zone. In some locations of the Tri-County irrigation project in south
central Nebraska, the water table has risen over 100 feet during the 40-year
period since development. In the next few years, additional groundwater
pumping in this region may be necessary to offset continued percolation from
the irrigation water applied to the project lands from stored surface
supplies. Otherwise, some drain installations may be needed. Similar
47
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problems are found on the lands of the Riverton Project and Bighorn Basin
Project in Wyoming where drain lines are being installed on a field-by-field
basis to control the water table level and root zone salinity.
In these and similar cases, the resulting drainage waters will have
increased salt content as compared to the irrigation water and will add to
the salt loading of the receiving surface waters. In some instances, (the
Tri-County case, for example) the increased salt concentration of the ground-
water outflow is so small that it does not create any downstream problems.
In others, adjustments may be necessary to deal with reduced water quality.
Improved irrigation management can minimize the volume of drainage water to
be dealt with and, to some extent, can reduce total salt emission from the
root zone. It must be emphasized, however, that where drainage is required
and is not installed, abandonment of the land for agricultural use will
ultimately occur.
Conclusion—
In most of the Great Plains region, the salinity of irrigation return
flow is neither a problem nor an issue. Indeed, cases can be cited where
increased groundwater outflow resulting from increased percolation on
project lands has created a benefit by way of increased streamflow. How-
ever, in other locations, particularly in the northern plains, concern is
created by the potential problems associated with salinity in the groundwater
component of irrigation return flow. A recent case in point is a large
surface water impoundment project where uncertainty about the overall impact
of increased salinity in downstream return flow resulted in a halt in
project construction.
A favorable root zone salt balance is absolutely essential to insure
the permanence of irrigation agriculture. Where supplemental irrigation
is practiced with high quality water, that balance is easily achieved with
little or no adverse environmental impact. At the other extreme—meeting
all crop water needs with water of marginal quality—the quality of ground-
water or downstream surface water is certain to be reduced by the net salt
outflow from irrigated lands. The potential seriousness of salinity
problems must be assessed on a case-by-case basis, based on the integrated
effects of irrigation water source, water quality, rainfall amount, soil
and subsoil conditions including natural salinity, groundwater systems, and
crop types to be grown.
Concentration and Mass Emission
Two distinctly different but related terms are useful in evaluating and
assessing water pollution from IRF. Concentration is the amount of a
component or components that are dissolved or suspended in a unit volume
of solution. Concentration can be expressed by weight or volume (ppm, mg/1,
g/g). Mass emission is the total amount of material that is transported
over some defined period of time. The mass emission term is obtained by
multiplying the concentration of a material by the total volume or weight
of the material in which it is dissolved or suspended. These two terms may
48
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provide different interpretations of water quality problems.
The significance of concentration or mass emission depends on the
context in which the information is used. Concentration is important when
defining the limits of acceptable water quality, because many systems usually
respond to the degree of concentration. Mass emission is a more significant
parameter when total material losses are considered. A critical limit of
materials in 1RF might not be reached until a certain capacity for assimilat-
ing the materials is exceeded, regardless of the concentration of materials
dissolved or suspended. For example, relatively small volumes of surface
runoff or deep percolation can contribute relatively low mass emissions of
pollutant materials to receiving waters even if pollutant concentrations are
high. However, high volumes of surface runoff and deep percolation have a
considerably greater capacity for losses of pollutant materials. In this
respect, mass emission is a better parameter for assessing pollution than
concentration (Letey, et al., 1978).
SIGNIFICANCE OF IRRIGATION RETURN FLOWS IN THE GREAT PLAINS
The amount of water that moves as surface runoff or deep percolation
must be known before the transport of sediments and chemicals from an
irrigated field can be determined. Runoff return flow estimates are based
on water losses from plot studies (Interagency Task Force, 1978; Boone, 1976;
Irrigation Extension Personnel, per.com., 1978; Ribbens and Shaffer, 1976),
and relative irrigation water application rates and efficiencies (Kruse and
Heermann, 1977; Kruse, 1978). Estimates of return flows from,surface runoff
and deep percolation are affected by assumptions regarding the amount of
water lost from either surface or sprinkler irrigation systems. The return
flow factor (that portion of the gross irrigation application that is return
flow) for surface irrigation systems ranges between 0.16 to 0.72, while for
sprinkler irrigation systems the return flow factor ranges from 0~15 to 0.25.
For a number of reasons, including amount of water supply available and
irrigation methods used, this factor varies from state to state and is given
in Table 9.
Some fields may have no irrigation return flow. This could occur with
sprinkler systems on level land with soils having medium to fine textures
(or even coarse textures if management is adequate). However, when surface
irrigation methods are used, deep percolation return flows are potentially
greater in coarse-textured soils because of the smaller water-holding
capacities and greater hydraulic conductivities of these soils.
Extent of Irrigation Flow in the Great Plains
Irrigation return flows may be estimated by the following method:
IRF = A x D x R (1)
where IRF is the irrigation return flow, expressed in hectare-centimeters;
A is the irrigated area, in hectares: D is the gross irrigation application,
-------
TABLE 9. IRRIGATION RETURN FLOW IN THE GREAT PLAINS STATES BY SYSTEM TYPE AND SOURCE OF WATER -
I/
Gross irrigation
application
State
Colorado
Kansas
Montana
Nebraska
in New Mexico
O
North Dakota
Oklahoma
South Dakota
Texas
Wyoming
AVERAGE
TOTAL
Sprinkler
51
46
61
51
67
30
48
41
46
_53
49.4
Surface
76
56
122
61
84
38
58
51
56
58
66.0
Irrigation return
flow factor -1
Sprinkler
0.25
0.25
0.25
0.25
0.20
0.20
0.20
0.20
0.20
0.25
0.22
Surface
0.43
0.38
0.65
0.36
0.44
0.50
0.21
0.25
0.16
0.72
0.41
Irrigation return flow
Groundwater
Sprinkler
2,014
4,772
519
13,158
1,796
204
1,833
492
6,017
225
31,030
Surface
4,641
19,471
555
29,317
3,216
38
1,778
191
18,161
543
77,911
Surface water Both sources
Sprinkler
217
288
1,357
995
134
30
67
500
119
3,707
Surface Both systems
9,608 4,641
617
33,940
10,035
333
285
499
128 64
___
9,354
64,799 4,705
Total
return flow
21,121
25,148
36,371
53,505
5,479
557
4,177
1,375
24,179
10.241
182,153
— Irrigation Journal 1975-77; per. com. with state irrigation personnel.
— Portion of gross irrigation requirement that becomes irrigation return flow.
-------
in centimeters; and R is the return flow factor. On an individual farm
basis, the return flow factor is a variable depending upon a host of manage-
ment factors, irrigation application rates, soil properties and topography.
However, for purposes of comparison, a constant factor has been used for each
individual state within the Great Plains. The estimates of irrigation return
flows from the individual Great Plains states are given in Table 9. Other
estimates of irrigation return flows have been made for the Great Plains
(Boone, 1976; Interagency Task Force, 1979).
The IRF estimates shown in Table 9 are less than those previously
calculated, even though irrigated areas have substantially increased. The
primary reason for the differences in return flow estimates is the irrigation
return flow factor. On a statewide basis this reflects the sum effects of
irrigation method, water management, predominant soil types, crops and a
host of other factors.
Irrigation Return Flow Water Quality Problems in the Great Plains
The Water Resources Council (U.S. Water Resources Council, 1978)
recently assessed water problems and the significance of these problems in
various areas of the United States. Erosion was shown to be the prevalent
problem in only limited areas of the Great Plains, with the most serious
erosion existing in the southwest portion of the region. Other areas of
serious erosion exist in both North Dakota and South Dakota. The Council
report did not determine how much of the erosion is related to irrigation
activities.
A survey conducted by Great Plains' 208 planners and irrigation experts
helped determine the extent of irrigation-related pollution problems.
Results of this survey are summarized in Table 10. Disagreements exist
regarding the influence of irrigation activities on the identification of
pollution problems associated with IRF, although such problems are in
evidence.
The scale of irrigation related nonpoint source pollution of surface
water supplies by sediments and nutrients, and groundwater pollution by
nutrients within the irrigated areas of the Great Plains, has not yet been
fully determined. Irrigation in this region is extensive, with more
agricultural chemicals being applied as irrigated areas expand. There
exists the probability of significant degradation of the water quality of
IRF through over-fertilization and misuse of irrigation water. Expanded-
irrigation development on coarse-textured soils represents a special IRF
pollution hazard because of the low water-holding capacity of sandy soils.
Over-irrigation or rainfall immediately following an irrigation can result
in leaching of soluble pollutants.
An estimate of the impact of current practices, showing the extent and
magnitude of pollution problems from irrigation by regional case histories,
is presented in the following paragraphs. Information in these histories
may or may not apply to specific situations confronting users of this
manual.
51
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TABLE 10. IRRIGATION-RELATED POLLUTION PROBLEMS IN THE GREAT PLAINS STATES -
I/
Scate
Potential pollution problems from irrigation return flows
Host probable pollution problems
208 Planners
Colorado High priority in the South Platte
and Arkansas river basins.
Yes, but unsure of priorities and areas
Montana Medium priority in all areas but
mountainous.
Irrigation Experts
Groundwater for irrigation is important.
Low water-holding capacity of sandy soils
and shallow water table along the Platte
are problem areas.
Local groundwater quality problems especi-
ally in parts of Harvey, Reno, Stafford,
and Pratt Counties. Water quality degrad-
ation due to irrigation is limited.
Irrigation is not a major polluter Some
minor problems in Lower Yellowstone Valley
with return flows.
Sediments and nutrients in Larimer and Weld
Counties, 66 percent of sediment loads and 55
percent of nitrate-nitrogen contribution to
surface and groundwater attributable to
irrigation.
Salts and sediments In western Kansas. Salinity
problems along the southern border of southwest
Kansas. Potential problems with agrochemlcals.
Sediments. Excess nitrogen leached from
Irrigated sugar beets and corn.
Nebraska
in
t
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Central High Plains—
Colorado—A survey of 270 irrigated farms in central and eastern
Colorado showed that residual nitrate-nitrogen (NO_-N) can accumulate in
the soil profile (Ludwick, et al., 1976). This nitrogen can then be leached
by precipitation or the initial irrigations of the following season and
moved to receiving waters. A water quality management study, authorized
under Section 208 of P. L. 92-500, found that irrigated agriculture was the
major contributor to water pollution in Larimer and Weld Counties, Colorado
(Larimer-Weld Regional Council of Governments, 1977). Irrigation contributed
66 percent of the sediment loads, 95 percent of the total dissolved solids
(TDS), and 55 percent of the nitrogen found in both surface water and ground-
water. Irrigation discharges to streams were eight times larger than dis-
charges from other water sources. However, sediments in receiving streams
did not present a serious problem.
Kansas—A Kansas water quality study (supported in part with Section 208
funds) concluded that IRF is a small part of the possible water quality
effects associated with irrigation (Balsters and Anderson, 1978). Surface
return flows from irrigation may be a problem in areas along the Arkansas
River and in the organized irrigation districts of northcentral Kansas. On
the state's upland irrigated areas, in most cases, little irrigation runoff
reaches the water courses. The authors go on to say, however, that nitrogen
contamination of shallow aquifers may become a problem because of the
increased use of tailwater pits. Problems of brackish groundwater contami-
nating adjacent heavily-pumped aquifers have occurred in parts of the
Republican River basin, the Equus Beds north of Wichita, and the Great Bend
Prairie region south of Great Bend.
Montana—Limited research data on irrigation-related nonpoint source
pollution are available from Montana. Sediment is the major pollutant when
a problem does occur. Irrigation return flows have better quality than the
applied water, due to the filtering effect of the soil. Although it has
not been quantified, excess nitrogen is believed to be leached from lands
planted to sugar beets and corn. The leached nutrients, however, do not
constitute a pollution problem in the river waters (G. Westesen, per. com.).
North Dakota—Irrigation is not considered a major contributor to non-
point source pollution due to: (1) the small percentage of land under
irrigation; (2) the lack of concentration of irrigated areas; and (3) the
water quantities applied are often equal to, or less than, seasonal ET.
Little opportunity exists for mass emission of sediments and nutrients.
Surface irrigation is practiced along the lower reaches of the Yellowstone
River and sediment loads occasionally are recognized as a pollution problem.
Soils in this area are fine-textured and readily erodable (J. Bauder,
per. com.).
Nebraska—Considerable investigation has been done on determining the
extent of groundwater contamination of the Ogallala Aquifer. For example,
nitrate-nitrogen concentration increased from 2.5 to 3.2 ppm during 1976,
a 29 percent increase in the state average (Olson, 1976).
53
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Nitrate leaching losses from irrigated sandy soils appear to be the
prime contributor to continued nitrate build-up in two areas of Nebraska.
Studies have shown nitrate distributed throughout the soil profile, from
the soil surface to the water table, after 10 years of irrigated corn on
sands (Muir, et al., 1976; Boyce, et al., 1976).
The growing of irrigated corn on coarse-textured soils and the wide-
spread use of N-fertilizers can result in the leaching of NO to the ground-
water. It has been estimated that 50 percent of the commercial N-fertilizers
used in Holt County (located in the eastern part of Subarea 1015) leach to
groundwater (Exner and Spalding, 1979). Exner and Spalding (1979) estimated
an average increase of 1.1 ppm of NO.-N per year as representative of the
irrigated region which they studied. This average could well represent other
areas of the Sandhills that are intensively irrigated.
Southern High Plains—
New Mexico—Irrigation in the High Plains of New Mexico constitutes
only about 10 percent of the total land use. Limited research indicates
that irrigated agriculture is a minor water polluter when compared to other
pollution sources, with nitrates being the primary problem. Saline ground-
water is a serious problem in the Mesilla Valley of the Rio Grande River,
which lies outside the Irrigated Great Plains.
A study by Taylor and Bigbee (1973) indicated a relationship between
irrigation season and nitrate content in the groundwater in the High Plains.
Soils in the High Plains are fine sandy loams to loams. Agricultural areas
where little or no N-fertilizer was used had low NO--N in the groundwater,
regardless of water use. Differences in the peak or NO_-N concentrations
during different times of the growing season were related to the presence of
coarse-textured soils. The poor water quality of the groundwater in this
region was attributed in part to the large use of nitrogen fertilizers on
the coarser soils.
Texas—A situation similar to that found in the High Plains of New
Mexico exists in the irrigated High Plains of Texas. The groundwater supply
in specific areas of the rolling plains where the soil is sandy and the
water table is shallow are susceptible to nitrate increases. This will most
likely occur when high N-fertilizer applications are immediately followed by
heavy rainfalls or irrigations.
Reeves and Miller (1978) examined the distribution of NO -N, Cl~, and
Total Dissolved Solids (TDS) in the groundwater of west Texas. Both coarse-
textured and fine-textured soils exist over the groundwater table in this
area. High nitrate values caused primarily by deep percolation of nitrogen
fertilizers occur in the groundwater in intensively-cultivated areas having
sandy soils (Table 11). The coarse-textured soils were identified with the
regional pattern of poor quality groundwater. Note that Hale County, the
most intensively-irrigated county in the entire Irrigated Great Plains as of
1976, showed a decrease in NO -N over the period 1951 to 1970.
54
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TABLE 11. INCREASES IN NITRATES IN OGALLALA GROUNDWATER IN WEST TEXAS
COUNTIES HAVING DIFFERENT SOIL TYPES -
_ . ., NO_-N in ppm
Dominant soil 3 KK
County type in county 1951 1970 Increase
Daws on
Hockley
Terry
Crosby
Hale
Farmer
Swisher
Sandy
Sandy
Sandy
Fine-textured
Fine-textured
Fine-textured
Fine-textured
2.9 9.9
0.5 1.6
1.8 5.2
1.1 1.1
0.7 0
0.5 0
0 0
7.0
1.1
3.4
0
-0.7
-0.5
0
- Modified from Reeves and Miller, 1978.
EFFECT OF CURRENT IRRIGATION
MANAGEMENT PRACTICES ON IRRIGATION RETURN FLOWS
Irrigation System Management
Irrigation Methods—
Irrigation water is applied by three basic methods—surface, sprinkler,
and trickle-drip. The systems most commonly used in the Great Plains are
illustrated in Figure 16. Each irrigation method has its own characteristics
which make it more desirable or less desirable for a given location. Tech-
nical information describing the design, operation and use of irrigation
systems is available from a variety of publications (Merriam, 1968, 1977;
Stegman, et al., in prep.; Fischbach, 1975; Westesen, 1977). These publica-
tions should be consulted for the advantages and limitations of each irriga-
tion system when used in particular locations.
Surface systems—The most widely-used irrigation systems in the Great
Plains are surface systems. In surface irrigation, water flows by gravity
from the upper end to the lower end of a field. The simplest and cheapest
method is wild flooding, when water is allowed to flow out of the irrigation
ditch and over the field. When row crops with furrows are present, better
control is obtained with siphon tubes to transfer water in the open-ditch
system to either furrow or border irrigated fields. The main characteristic
55
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IRRIGATION SYSTEMS
SPRINKLER SURFACE
CENTER PIVOT
SIPHON
TAIL WATER(PUMPBACK)
TRICKLE'DRIP
Figure 16. Some of the irrigation systems us"ed in the Great Plains,
56
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of these systems is that the rate of applied water entering the soil profile
is primarily determined by the physical properties of the soil.
Gated-pipe systems, a form of surface irrigation, move water to the
field more efficiently than open ditches. These systems require only small
pressure heads in the pipe system to distribute water. Gated-pipe is adapted
to field slopes of one percent or less for soils with medium-to-low infiltra-
tion rates. These systems can be used on land slopes up to two percent in
areas where soil erosion from summer rainfall is not a problem. Gated-pipe
irrigation systems can be adapted to apply water automatically. This reduces
the pipe handling, decreases labor cost and time, and improves the efficiency
of water application.
The water which runs off and collects at the lower end of a surface
irrigated field (tailwater) can be recaptured and reused by way of tailwater
reuse systems. Only a small amount of pumping is needed to return and reuse
tailwater in the irrigation system.
Sprinkler systems—Sprinkler irrigation systems have proven to be versa-
tile methods of applying water, especially where surface methods cannot be
used due to excess slope or very sandy soils. Sprinkler systems substitute
capital and energy for the more intensive labor required with surface methods.
Sprinklers are well adapted to many soils and to fields with steeper slopes
or irregular topography. However, one type of sprinkler system may work well
on a particular field while another type of sprinkler system may not. Sprink-
ler systems, as do other irrigation methods, require careful matching with
soil type, climate and crop conditions.
Sprinkler systems can be classified as moved or moving types of systems.
Moved sprinklers—handmove, solid set, skid-tow and side-roll—apply and
distribute water from a fixed point during the irrigation cycle. Moving
sprinklers—center pivots and travelers (big guns and booms)—move as they
apply and distribute water during the irrigation cycle. Whether a system is
moved or moving affects the irrigation design for the field, as well as
system capability to control the amount of water application.
The irrigation pipes of hand-moved irrigation systems are hand-assembled
for a single irrigation cycle. After the irrigation, the pipes are dis-
assembled and moved to the next fixed irrigation location. The area to be
irrigated with solid-set systems is covered with a grid of pipes and sprink-
lers which are not moved until the end of the irrigation season. Permanent
sets are solid sets with buried pipes which are not moved at all.
The skid-tow sprinkler systems consist of rigidly coupled laterals
connected by a flexible joint to a main line, which is usually positioned
in the center of the field. The laterals are towed end first over the main
line from one side of the field to the other by a tractor. Outriggers keep
the lateral upright.
Center pivot sprinkler systems are self-propelled moving lateral pipes
which pivot around a central point. The pipes are suspended on towers
57
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supported by wheels, which are automatically propelled by pneumatic,
mechanical, hydraulic, or electric power. These systems irrigate circular
areas of about a 400-meter diameter, or about 52-57 hectares. A simple large
sprinkler mounted on a trailer constitutes the traveler irrigation system.
This sprinkler covers 3-4 hectares during a set. A motor connected to a
cable pulls the sprinkler across the field. A flexible hose is required to
supply water to the sprinkler from the pumping point.
Trickle-drip systems— Trickle (drip) irrigation allows greater control
than most other methods. It provides a very slow application of water
directly to the soil surface through pipes lying on or beneath the soil.
Normally, water is released through orifices or emitters, porous tubing, or
perforated plastic tubing operating under low pressures. A small but
constant amount of water can be applied frequently to a limited area of the
soil surface.
Water Management—
Irrigation water management is the timing and regulating of water
applications to satisfy both the crop water-use requirement and the leaching
required to maintain the salt balance without causing excess nutrient leach-
ing or erosion. Information on when to irrigate a given soil and crop is
available from a number of sources (Christensen and Westesen, 1978;
Eisenhauer and Fischbach, 1978; Merriam, 1977, 1978; Schneider, et al., 1976;
Stegman, et al., in prep.). However, basic water management procedures must
be understood before management schemes which minimize pollutants in IRF can
be created.
Water management encompasses the timing of the irrigation, the amount
of water applied, the uniformity of application, and the rate of application.
Efficient application and uniform distribution of water are extremely
important in minimizing the problems addressed by this manual. Water
application efficiency [the ratio of: (1) the water beneficially used to
supply the crop requirement or maintain a favorable salt balance to, (2) the
gross water application] depends on the type of irrigation system and its
ability to uniformly and timely apply water. Water application efficiencies
have been discussed by several authors (e.g. Kruse and Heermann, 1977;
Kruse, 1978; and Merriam, 1977). Typical values are given in Table 12.
The relative irrigation efficiencies of sprinkler and surface systems
can be misleading. If a sprinkler system is compared with a surface system,
(gated-pipe, for example) on a shallow coarse-textured soil or on a steep
land slope without a tailwater reuse system, the sprinkler system would be
much more efficient. However, if the sprinkler system is compared with a
surface system (gated-pipe with reuse) on a medium-textured soil with a one
percent slope or less, the opposite could be true.
58
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TABLE 12. ESTIMATED IRRIGATION APPLICATION EFFICIENCIES
FOR IRRIGATION SYSTEMS -^
Irrigation system
Suggested
efficiency
Range of
efficiencies
- - - - percent - - - -
Surface
Open ditch with no reuse
Open ditch with tailwater reuse
Gated pipe with tailwater reuse
Autogated with tailwater reuse
Sprinkler
Handmoved
Solid set
Skid tow
Side roll
Center pivot
Travelers (Big Gun)
Trickle-drip
50
65
70
85
70
75
75
75
80
75
90
30-70
45-80
60-80
80-90
70-75
65-80
65-80
65-80
70-85
65-80
85-95
- Fischbach (ed.), 1977.
The efficiencies are based on proper irrigation management and design.
One of the best ways of improving water management is through the use
of irrigation scheduling techniques (Jensen, 1975). These techniques
stress the application of the correct amount of water at the right time to
obtain maximum crop production, regardless of the application method.
Irrigation scheduling also can consider the probability of rainfall and
allow for proper soil-water storage to reduce deep percolation losses.
The effect of current water management practices on soil-water losses
in southwest Nebraska was surveyed by Watts, et al., (1974). The data on
applied water and losses for 35 corn fields with surface or sprinkler systems
are summarized in Figure 17. Total water application ranged from 16 to
67 cm, while water losses ranged from 0 to 38 cm. Most irrigation losses
occurred when the total applied water (irrigation + rainfall) exceeded
57 cm. Rainfall immediately after an irrigation or excessive irrigation
produced losses, even when seasonal water application was less than seasonal
crop water use. Although most of these losses were through deep percolation,
some can be attributed to surface runoff. Losses from surface irrigation
averaged 16 cm out of the average 42 cm applied while losses from the center
pivot system averaged 5 cm out of the 35 cm applied.
59
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I
78-
66-
TOTAL WATER
APPLIED
54
I ,
cc
I42
ETFrom 6/1-9/20
6
0-
-6
o -18-
(0
Id
CO
§-so-
Figure 17.
D RAINFALL
M PIVOT IRRIGATION SYSTEMS
M SURFACE IRRIGATION SYSTEMS
I WATER LOSSES
INDIVIDUAL IRRIGATED FIELDS
A summary of deep percolation water losses in 35 fields
irrigated by either center-pivot or surface irrigation systems,
(Watts, et al., 1974).
60
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Crop water requirement—Crop water requirement depends on the climate
and the crops grown. Different crops need different amounts of water,
depending on the length of the growing season and which part of the plant
is harvested as a crop (Table 13). The amount of water applied in any
irrigation management program should ultimately depend on how much water a
particular crop removes from the soil.
TABLE 13. GENERAL MAGNITUDE OF IRRIGATION WATER AMOUNTS (SEASONAL)
AS PRACTICED FOR FIVE CROPS IN THE GREAT PLAINS -
Seasonal water applications
Crop Average Low High
Alfalfa
Corn
Cotton
Sorghum
Soybeans
62
57
52
49
35
25
27
32
30
27
114
82
82
89
40
— Based on Fed. Energy Adm. and U.S. Dept. of Agr., Energy and U.S.
Agriculture: 1974 Data Base, 1977.
In Nebraska, Wilson, et al., (1978) limited the amount of applied water
below ET demands during some stages of corn growth without substantial yield
reductions during an irrigation season having 12.5 cm of rainfall. The
plants were under a small but continuous stress during the vegetative stages
of growth in a climatic region where daily water-use demand was not extreme.
In terms of yield per amount of water available (water-use efficiency), corn
produced most efficiently when irrigated during the pollination and grain-
filling (maturation) stages. As much as 12 cm of water was saved with no
yield reduction. However, limited irrigation applications which result in
reduced crop evapotranspiration especially during critical growth stages
(such as the pollination period of corn) as well as in high water demand
periods, will result in reduced crop yields.
Management schemes for limited irrigation in varying climatic situations
may also be used for other crops (Garrity, 1979). Grain sorghum showed less
overall sensitivity to the critical timing of water applications as compared
to corn. Use of limiting water management schemes for other crops (New,
1977; Stewart, J. I., et al., 1975) can mean less surface runoff and deep
percolation losses. Such schemes must be carefully applied, however, to
avoid economic losses for the producer.
61
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Water application rates — The rate of water application affects surface
water runoff; the greater the intensity exceeds the soil infiltration rate,
the greater the water runoff. High intensity irrigation rates can destroy
aggregates at the soil surface, sealing the surface and reducing the
infiltration rate.
It is generally easier to control application rates using sprinkler
systems as compared to surface irrigation systems. With the latter, water
must be applied at rates greater than the soil intake rate in order to
advance down field. For example, the stream size at the head of a furrow
must be sufficient to meet the irrigation requirements along the entire
furrow length. Yet, the stream size should be kept as small as possible to
keep the soil losses at a minimum. The maximum stream size depends on the
size, shape, and slope of the furrow. From a soil-loss standpoint, the
stream size on medium- textured soils should not exceed the value given by:
where q is the maximum nonerosive stream size, liters per second; and S is
the field slope, in percent.
Sediment losses — Normally, 20 to 40 percent of the water applied by
surface irrigation systems becomes surface runoff. Sediment loss decreases
as the irrigation efficiency increases (Table 14) .
TABLE 14. EFFECT OF SURFACE IRRIGATION APPLICATION EFFICIENCY
ON SEDIMENT YIELD-
__
Irrigation application efficiency — Sediment losses
percent metric tons/ha
30 1.30
40 0.44
50 0.25
70 0.10
80 0.07
— Fitzsimmons, et al., 1977. Portneuf silt loam soil'with slopes varying
from 0.8-1.2 percent. Table is a summary of irrigations performed in
1975 and 1976.
21
— Percentage of applied water which is retained on the field.
Sediment losses vary not only during the irrigation event, but also
during the irrigation season. They tend to be higher at the start of the
62
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irrigation season when the soil surface has recently been tilled and the
crop cover is sparse (Table 15).
TABLE 15. RELATION OF SEDIMENT YIELDS FROM ROW CROPS
DURING THE IRRIGATION SEASON -_
__-
Sediment levels —
Irrigation number In applied water In runoff water
mg/1
1st 41 2,020
2nd 76 946
Last 124 142
— Regional Planning office for Big Horn Basin 208 Policy Board, 1978.
2/
— Concentrations after a 400-meter length of run.
Impact of Water Management
on Pollution from Irrigation Return Flows—
The impact of current water management practices on water quality
problems resulting from irrigation return flows is difficult to determine
because of the lack of specific data. A schematic relationship of the
influence between level of water application management and the expected
level of IRF pollutants is shown in Figure 18. The amount of pollutants
in IRF is dependent on the quantity of water applied and the level of water
management skills used in operation of the irrigation system.
Soil Management—Although most erosion from irrigated lands in the
Great Plains is caused by precipitation, irrigation can contribute to
erosion, especially on steeper slopes. When water is applied directly to
the surface, as with surface systems, irrigation can generate large quanti-
ties of sediment within the system. Practices which (1) control surface
water runoff by reduction of runoff velocity; (2) increase water storage at
the soil surface; or, (3) increase soil surface infiltration rates will
reduce the amount of soil loss. These practices are discussed in the
following publications: Stewart, B. A., et al., 1975; Lane and Gaddis,
1976; Walter, et al., 1977; McDowell, et al., 1978; Harmon and Duncan, 1978.
63
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M
2
W
2
O
M
H
Pi
LOW MANAGEMENT
LEVELS
HIGH MANAGEMENT
LEVELS
QUANTITY OF APPLIED WATER
Figure 18. Schematic relationship between irrigation management levels
and the degree of pollution in irrigation return flows.
(Modified from Vlachos, et al., 1978).
Land Slopes—Land slope directly influences the amount of soil loss
which takes place under irrigation systems (Table 16). Considerable sediment
TABLE 16. EFFECT OF LAND SLOPE FROM SURFACE IRRIGATION FIELDS
ON GROSS SEDIMENT LOSSES -1
Land slope
percent
0.8
1.0
1.2
21
Soil losses —
metric tons/ha
-0.3 SI
1.6
1.6
— Fitzsimmons, et al., 1977.
Portneuf silt loam soil with slopes varying from 0.8-1.2 percent.
Table is a summary of irrigations performed in 1975 and 1976.
— Sediment losses during the initial irrigation.
—' Deposited on lower end of irrigation field, no sediment removed from
the field itself.
64
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losses can occur when surface systems are used on lands having slopes exceed-
ing one percent (Carter and Bondurant, 1976). Medium-textured soils on
steeper slopes have twice the potential for sediment loss as coarse-textured
soils. The greatest danger of erosion resulting directly from irrigation
application exists where surface irrigation systems are used on steep slopes.
Land leveling and land forming operations can successfully decrease erosion
from the steeper, surface-irrigated fields. Where land slopes are too steep
and land leveling is impractical, the use of sprinkler systems may be
required as a means of reducing erosion.
Conservation practices—Crop management systems can greatly affect the
magnitude of soil erosion and sediment losses (Table 17). Although these
TABLE 17. SEDIMENT DELIVERY TO STREAMS FROM A RANGE OF SOIL TYPES FROM
REPRESENTATIVE FARMS FOR THREE CROP MANAGEMENT SYSTEMS IN IOWA -f
Crop management Soil erosion Soil erosion Actual sediment
system on farm site which is delivered delivered from
to stream farm site
metric tons/ha percent metric tons/ha
Conventional crop 28 38 10.7
management with no
crop residue
Contouring with 2.8 15.5 38 5.9
metric tons of residue
Terracing with 2.8 7.4 7 0.5
metric tons of residue
— Harmon and Duncan, 1978.
data are a summary of calculated soil losses from nonirrigated crop lands,
the magnitude of these losses will most likely be representative of rainfall-
caused losses from irrigated fields in higher rainfall areas of the Great
Plains.
Tillage practices can have a large effect on soil losses (Table 18).
Estimated soil losses may range three-fold from no-till to maximum tillage
systems (Lane and Caddis, 1976). Aarstad and Miller (1978) emphasized that
the runoff water passing through the residues in the disked and till-planted
furrows was often less turbid than the incoming irrigation water. Even with
large irrigation water streams, the residue reduced soil losses when compared
with clean tillage furrows.
65
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TABLE 18. SOIL LOSS FROM IRRIGATION FURROWS AFTER 7 AND 24 HOURS
OF IRRIGATION, AS AFFECTED BY TILLAGE CORN RESIDUE TREATMENT - -^
Soil loss
Tillage treatment —
1975 -1 1976 -
- - - metric tons/ha-day - - -
After 7 hours:
Clean tillage 1.53 0.4
Disked - 0.09 0.04
Till-plant - 0.02 0.02
After 24 hours:
Clean tillage
5/
Disked -'
5/
Till-plant -
0.89
0.06
<0.01
1.3
0.2
0.1
- Aarstad and Miller, 1978.
21
— Fine sandy loam soil with 3 percent slope, second irrigation of the
season.
— Average of 2 replications.
4/
— Average of 8 replications.
— 3.5 metric tons/ha corn residue on surface before disking or planting.
In general, soil losses vary inversely with crop residue cover. Tillage
systems which leave crop residue on the soil surface, especially at the
beginning of the irrigation season, reduce erosion and water runoff. Proper
conservation tillage systems, especially with surface irrigation methods, can
considerably reduce sediment losses in tailwater during the early part of the
irrigation season.
Sediment losses from irrigated fields can be reduced by utilizing one or
more of the following procedures: irrigation systems can be modified or
changed, fields can be leveled, tillage operations can be reduced, or vegeta-
tive filter strips and sediment ponds can be installed (Fitzsimmons, et al.,
1977; Lindeborg, et al., 1977; Stewart, B. A., et al., 1975; Harmon and
Duncan, 1978). Examples of the effectiveness of these methods to reduce
sediment losses is shown in Table 19. Sediment in the tailwater of surface
systems can be filtered out as it passes through a grass strip or other
close-growing crop or through the use of sediment ponds. Runoff and sedi-
ments from steeper slopes or undulating topography can be reduced through the
66
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installation of properly designed sprinkler systems.
TABLE 19. EXPECTED REDUCTION OF SOIL SEDIMENT LOSS
WITH SELECTED IRRIGATION MANAGEMENT PRACTICES -
"Typical" sediment loss
Management practice . ,. 2/
retained on farm —
percent
3/
Sprinkler irrigation — 100
Mini-basin - 90
Sediment pond — 67
Grass or grain strip — 50
Flow cut-back — 30
- Lindeborg, et al., 1977.
21
— In this study conducted in the Magic and Boise Valleys of Idaho,
"typical" assumes current management practices on silt loam soils
with slopes from one-half to four percent.
— The 100 percent retention of sediment loss occurs when runoff is
reduced to zero. This would be true only if the water application
rate is less than or equal to the soil intake rate.
4/
— Mini-basins are shallow ponds constructed at the end of the field
to retain tailwater runoff.
— Sediment ponds are installed into the return waterway to decrease
flow velocity and retain sediment.
— Grass or grain strips are planted at the lower end of the field
to slow down tailwater and retain sediment.
— Flow cut-back means to reduce the streamsize of the surface irrigation
set when the water reaches the end of the field. The reduced flow
results in decreased erosion and soil transport.
Fertilizer Management
Much attention has recently been focused on the quality of irrigation
return flow as influenced by fertilization practices in irrigated agriculture
(Fried, et al., 1976; Pfeiffer, et al., 1978; Int. Garrison Diversion Study
Board, 1976; Duke, et al., 1978; Hay and Black, 1978; Whitney, 1978; Balsters
and Anderson, 1978; Regional Planning Office for Big Horn Basin 208 Policy
Board, 1978). It is very difficult to isolate any single practice as being
the major cause of water quality degradation. Water pollution associated
67
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with applications of nitrogen and phosphorus can be related in some degree to
application rates. Excessive rates can increase the residual buildup of both
nitrogen and phosphorus in the soil. The method of application, which is
influenced by whether or not the fertilizer material is incorporated, as well
as the time of application in relation to crop demand, also influence the
occurrence of these nutrients in irrigation return flows. Additionally, the
nitrogen source used (anhydrous ammonia, ammonium nitrate, etc.) is important.
To further complicate the matter, there may be interaction between certain
water and fertilizer management practices which may result in additional
losses of nitrogen. These interactions are affected by yearly rainfall
variation, root zone depths and soil texture. In spite of the complexity of
the problem, it is clear that there is room for improvement in fertilizer
management. Under certain conditions, these improvements can have a very
positive impact on the quality of return flow.
Use of Fertilizers in the Great Plains—
Nitrogen (N) and phosphorus (P) are the major nutrients required in
large amounts by the crops grown in the Great Plains. Potassium occurs
naturally in high quantities in the majority of the Great Plains soils and
does not normally constitute a fertilization problem. It may, however, be
required on the more sandy soils. Micro-nutrient fertilizers, used in small
amounts, do not appear to be a potential pollution source.
The average nitrogen and phosphorus fertilizer applications and the
amounts removed by common crops in the Great Plains are shown in Table 20.
Nitrogen applications usually are greater than the amount of nitrogen
removed in the harvested portion of the crop. However, the amount removed
from the soil varies greatly by type of crop or forage grown.
Numerous discussions exist about the behavior of nitrogen. This is
because nitrogen is the most common and yet the most limiting nutrient in
agriculture. An understanding of the cyclic behavior of nitrogen and its
use for irrigated crop production is essential for evaluating nitrogen/water
management practices. This cyclic nature of nitrogen in the soil-plant-water
system is shown in Figure 19. Nitrogen movement, especially that of nitrate-
nitrogen, through the soil is an important concern in irrigated agriculture.
Differences in nitrate losses closely reflect the fertilizer and irrigation
management practices used during the season. The amount of nitrate lost
depends on: (1) the amount of nitrogen applied; (2) timing of individual
applications; (3) amount and timing of water applications; (A) distribution
of seasonal rainfall; and (5) soil texture. Discussions of the individual
parts of the nitrogen cycle can be found in other publications (Frere, 1976;
Porter, 1975).
The amount of phosphorus removed by crops is usually greater than that
of the applied fertilizer. From the standpoint of crop production or water
quality, the irrigation management of phosphorus is not as critical as that
for nitrogen because most of the phosphorus is rapidly converted to an
insoluble form which remains very close to the point of application.
68
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ON
TABLE 20. AMOUNT OF MAJOR NUTRIENT FERTILIZERS ADDED AND ESTIMATED NUTRIENT REMOVAL
IN HARVESTED YIELDS OF IRRIGATED CROPS IN THE GREAT PLAINS STATES -
Crop
Corn
(grain)
Sorghum
(grain)
Cotton
Soybeans
Alfalfa
Harvested
Yield -
6.9
4.7
1.1
2.4
9.0
Nutrients applied
Nitrogen (N) Phosphorus (P 0 '
_ 1,0 /Via — — — —
190 45
160 34
60 38
12 22
5 33
3/
Nutrients removed —
) Nitrogen (N) Phosphorus (P2°-;^
— Iro /Via _
160 42
126 36
53 28
130 - 26 -
202 - 45 -
— Stewart, B.A., et al., 1975: Council for Agr. Science and Technology, 1975;
and per. com.
— Average yields over Great Plains region.
3/
— Estimated values for Great Plains.
4/
— Nutrients removed are greater than the nutrients applied.
-------
IMMOBILISATION W IMMOBILIZATION
SOIL ORGANIC MATTER"^
Figure 19. Diagram of nitrogen cycle inputs into and outputs from
soil-plant-water system. (Modified from Frere, 1976).
Nitrogen Losses and Fertilizer Management Practices—
The current existence of a few severe, localized NO -N pollution prob-
lems (especially in groundwaters) and the continued expansion of irrigation
on sandy soils (where N moves more rapidly) indicate that an evaluation is
needed of the impact different water and nitrogen management practices have
on nitrate leaching losses (Smika, et al., 1977).
An example of the amount of nitrogen that can be lost from fertilizer
applications is shown in Figure 20. Potentially leachable nitrogen increases
rapidly when nitrogen applications exceed that required for maximum crop
yields. Nitrogen will be recycled to the soil if the stover is left in the
field. However, this nitrogen must be converted to the nitrate form
(Figure 19) before it is subject to leaching. If nitrogen application does
70
-------
ertilizer N
Subject
toLfoching N
M in Groin a Stover \
Subject to Leaching x£
If Stover Not RemovedX
100 200 300 400 500
AMOUNT OF NITROGEN APPLIED, kg/ha
Figure 20. Effect of nitrogen fertilizer additions on potential
leaching in irrigated corn. (Modified from Fried, et al.,
1976).
not exceed crop needs, there will be little nitrate available for leaching.
An exception may occur in sandy soils, where nitrogen applied early in the
season may be leached below the root zone by early rain or excessive early
irrigation (Watts, et al., 1978).
In practice, larger amounts of nitrogen fertilizers than required to
meet crop needs are usually applied. This extra amount of nitrogen as
nitrate is subject to leaching losses either by excessive irrigation or by
rainfall. This, in turn, could increase the nitrate concentrations in
ground and surface waters. The potential NO_-N losses in deep percolation
water for different nitrate concentrations and amounts of drainage water
are given in Table 21. Because the concentration of NO.-N in the soil or
water alone is not always a good indicator of leaching losses, the total
amount of percolating water also must be considered. Potentially, leaching
of nitrates is greater in the coarse-textured soils than the fine-textured
soils because the coarse-textured soils have more excessive water movement
71
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TABLE 21. POTENTIAL NITRATE-NITROGEN LOSSES FOR COMBINATIONS OF DEEP
PERCOLATION AND NITRATE-NITROGEN CONCENTRATIONS
Amounts of Concentrations of NO -N_in drainage water
drainage water 10 ppm 20 ppm 50 ppm
5
10
20
30
5
10
20
30
10
20
40
60
Lrr /Via
Kg/na -
25
50
100
150
which results in greater total mass emission.
Proper timing of fertilizer application also is important in the
reduction of nitrate loss. In coarser-textured soils, smallest losses will
occur most years when nitrogen fertilizers are applied as close to the time
of use by the crop as possible. Studies show that excess water (rain and/or
irrigation) during the early part of the growing season can result in leach-
ing of preplant N-applications so that the total uptake by the crop is
reduced and N-uptake by the crop is delayed (Hergert, 1978; Smika and Watts,
1978; Watts, et al., 1978).
Nitrate leaching losses for three application methods on a sandy soil
are presented in Figure 21. When the rainfall was below normal during all
of the growing season or during the first one-third of the season, N losses
for all three application methods were about the same and resulted mainly
from early season leaching of residual N0_ in the lower profile. Application
method had an important affect only when the springtime rainfall was above
normal, because the amounts of irrigation water were controlled. Deep
percolation losses of some nitrates are unavoidable within the limits of the
type of water management studied. Nitrate-nitrogen percolation could occur
because rainfall occurred immediately after an irrigation or because
particular rainfall events considerably exceeded the soil moisture holding
capacity.
Similar conditions could apply to an early fall fertilizer application
of nitrogen in areas where water movement through the soil profile frequently
occurs, or in areas where water movement through the soil is infrequent and
residual nitrate increases because nitrogen rates are not adjusted for
residual nitrogen.
Residual nitrate buildup results from the gradual accumulation of annual
fertilizer applications (Duke, et al., 1978; Ludwick, et al., 1976).
Examples of residual nitrate are shown in Table 22. The accumulation of
nitrate in the soil can be leached with winter or spring rains or with
72
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u>
RAINFALL PATTERN i
MAY-JUNE JULY-AUG SEP-OCT
N03~N LEACHING LOSSES
ABOVE
ABOVE
1 Q C K
ABOVE
ABOVE
BELOW
NORMAL
BELOW
NORMAL
1 Q fi 7
NORMAL
NORMAL
. 1 Q C O
BELOW
i n 7 A
BELOW
ABOVE
NORMAL
BELOW
21
146
NITEOGEN APPLICATION -'
BROADCAST (PREPLANT)
SIDEDRESS (LATE JUNE)
APPLIED WITH IRRIGATION
SYSTEM
50 100
N03-N LOSSES, kg/ha
150
TOTAL
WATER
LEACHED
/26cm
)l8cm
)l2cm
)l3cm
I cm
Figure 21. NO -N leaching and water losses for three nitrogen fertilizer application methods
with different rainfall patterns. (Watts, et al., 1978).
— Rainfall patterns relative to normal for each one-third of growing season.
2/
— For all methods:
(1) N applied of 246 kg/ha.
(2) Irrigation frequency = 4 days.
(3) Irrigate to replace ET minus rain during period.
(4) No irrigation unless at least 1.9 cm is required.
-------
TABLE 22. RESIDUAL SOIL NITRATES AFTER FOUR YEARS OF
FURROW IRRIGATION - -
Annual N
applied
kg/ha
0
67
134
202
269
Total N 3/
accountable —
- - - percent of
—
75
75
63
51
Residual , ,
soil nitrate —
applied nitrogen -
—
0.7
3.0
3.4
4.9
Total nitrate in -
the soil profile
- - kg/ha
19
21
35
47
73
- Ludwick, et al., 1976.
21
— Four furrow irrigations of 13 cm per irrigation in central and eastern
Colorado on a clay loam soil.
4/
— From determination of the total nitrogen balance of soil-plant system.
Estimated NO,-N remaining in the soil after 4 years of cropping.
5/
— Total NO_-N content in the upper 3 meters of the soil profile.
over-irrigation at the beginning of the next season. The nitrogen losses
which occur during the winter or spring, especially on sandy soils, will be
related directly to the amount of precipitation. Results of a recent study
in the northern Great Plains suggest that nitrogen should not be applied in
the fall on well-drained sandy soils since significant leaching could result
(Bauder and Montgomery, 1979).
In general, nitrogen fertilizers should be applied as close to the
time of use by the plant as possible for maximum use by the crop and minimum
accumulations and losses. Light applications of nitrogen, managed to meet
the needs of the crop during the growing period, tend to maintain lower
nitrate concentrations in the soil solution within the root zone. Also,
applications should be planned so that residual nitrate is minimized at the
end of the irrigation season, providing reduced potential for leaching with
winter precipitation. Nitrogen applications and leaching of nitrates for
sprinkler-irrigated corn in North Dakota are shown in Table 23. When
nitrogen fertilizer was applied at a one-time application of 168 kilograms
(kg), the maximum amount of nitrate was lost. When this nitrogen application
was split into three equal parts, the nitrate losses were reduced.
74
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TABLE 23. NO -N LEACHING DURING CROP SEASON UNDER SPRINKLER-IRRIGATED CORN ON A MADDOCK FINE
SANDY LOAM AT OAKES. NORTH DAKOTA -1
Year
1977 -
1978 -
Nitrogen-,
applied -
kg/ha
22x1
67x1
168x1
56x3
22x1
67x1
168x1
56x3
Amount of
NO -N leached
kg/ha
2.2
5.9
40.1
8.8
4.0
5.7
25.2
21.3
concentration
of leachate
ppm
5.4
3.3
63.5
18.0
3.2
3.2
12.5
7.7
Applied N leached
as NO -N -
percent
9.9
8.7
23.9
5.2
18.0
8.5
15.0
12.7
Relative
Yield
percent
70
84
100
99
48
70
91
100
— Bauder, et al., 1978.
21
— All nitrogen applied as NH.NO . Single and first applications applied side-dress at planting.
Split applications were broadcast applied. All treatments were applied to optimum irrigation plots.
Amount of applied nitrogen leached below 1.5 meters.
4/
Irrigation totalled 42.2 and 33.7 cm for 1977 and 1978, respectively. The 1977 and 1978 seasons
were quite similar, with 26.9 and 33.7 cm of rainfall, respectively. Individual irrigation
applications were reduced from 6.4 cm in 1977 to 3.8 cm in 1978.
-------
Nitrogen Losses and Irrigation Management Practices—
The previously-discussed factors become even more important when
combined with irrigation water management practices. Because nitrate leach-
ing is highly correlated with excess water application, improved control of
water application is essential where nitrate leaching is a problem.
Wendt, et al., (1976) concluded that nitrogen remained in the root zone
if the water applied was based on the potential ET only, regardless of the
irrigation system or the criteria used to apply the water. When irrigation
water was applied at twice the potential ET and fertilizer application was
greater than 200 kg per hectare, leached water had excessive concentrations
of NO -N. In Texas, the effects of sprinkler, furrow, and subirrigation
systems on movement of band-applied fertilizer NO -N in a loamy, fine sand
were generalized as follows: (1) under sprinkler irrigation, banded
fertilizer moved downward from the point of application with some lateral
movement as depth increased from rainfall and irrigation; (2) under furrow
irrigation, the fertilizer tended to move toward the center of the bed and
then downward, resulting in NO--N remaining in the upper portion of the soil
profile longer than for the sprinkler system; and (3) under subirrigation,
fertilizer moved upward and outward towards the furrows and then downward,
resulting in higher concentrations of fertilizer in the upper profile for a
longer time (Onken, et al., 1979). The influence of deep percolation water
on NO -N losses under center-pivot irrigation systems on a coarse-textured
soil in eastern Colorado is given in Table 24. Nitrate-nitrogen losses were
TABLE 24. A THREE-YEAR SUMMARY OF THE INFLUENCE OF DEEP PERCOLATION LOSSES
OF NO -N UNDER CENTER PIVOT IRRIGATION SYSTEMS IN NORTHEAST COLORADQ--
Deep percolation water NO -N losses
cm
7.3
1.9
1.2
0.5
kg/ha
60
30
19
0.1
- Duke, et al., 1978.
proportional to the deep percolation of water. Duke, et al., (1978) state
that the restriction of deep percolation of water to less than 3 cm per year
holds N-losses to acceptable levels. However, such restrictions may be
impossible in the more moist areas of the Great Plains.
The practice of applying fertilizers with the irrigation water, called
fertigation, yields the lowest amount of deep percolation nitrate losses.
76
-------
Dylla, et al., (1976), showed an average increase in NO -N losses of 18 and
59 kg per hectare, respectively, for plots irrigated with 2.5 and 5 cm as
compared to nonirrigated fertilized plots. Nitrogen fertigation with the
2.5 and 5 cm irrigation treatments, however, decreased the NO -N losses by
35 and 52 percent as compared to a single preplant application of granular
N-fertilizer to the irrigated plots.
More efficient water management is required to minimize NO_-N leaching
losses on the coarse-textured soils. Studies in Nebraska (Watts, et al.,
1978; Hergert, 1978) show that NO -N losses can be minimized in sandy soils
through irrigation scheduling. Losses probably cannot, however, be reduced
to zero. In years of normal rainfall, about 30 to 35 kg per hectare of
NO_-N may be lost with deep percolation of water during the growing season.
When water percolation losses are 12 cm or less, NO,-N leaching losses are
essentially independent of the NO -N applied during the growing season and
represent mainly leaching of the previous season's residual nitrate (Watts,
et al., 1978).
Improved irrigation scheduling procedures are essential management tools
when there is a risk of NO -N percolation losses. Specific recommendations
for controlling both irrigation amounts and timing with proper N-fertilizer
applications will lessen the potential pollution of NO -N in deep
percolation.
Other Nitrogen Losses—
Nitrogen losses also are associated with sediments in surface water
runoff. Typical results for soluble nitrogen components obtained with
surface runoff studies are shown in Table 25. In terms of total mass
emission losses, the amounts of nitrogen lost with organic matter and
sediments suspended in the runoff water usually are much greater (depending
on the amount of erosion) than those dissolved in the water. The total
nitrogen loss and sediment loss are directly related. However, the amount
of soil erosion and the total N lost can vary markedly amoung different soils
and different tillage systems. Those tillage systems which reduce erosion
also reduce nitrogen losses in the surface runoff (Table 26). The concentra-
tion of inorganic nitrogen in surface runoff from fields in Iowa ranged
from 1-12 ppm, (0.1 to 11 kg per hectare) varying both among fields and
among years within the same fields (Harmon and Duncan, 1978).
Phosphorus Losses—
Phosphorus loss is very small in comparison to nitrogen loss. Most of
the phosphorus that appears in irrigation return flow is associated with
the sediments in surface runoff. Phosphorus concentrations also tend to
correlate with the available phosphorus in the surface soil. The loss is
dependent on both the method of application and the placement. For example,
losses of phosphorus from corn irrigated with gated-pipe systems on a silt
loam in Nebraska were reduced by the placement of phosphorus fertilizer
with chisels, which put it below the depth from which sediments would
normally be removed by runoff (Cihacek, et al., 1974).
77
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TABLE 25. NITROGEN LOSSES WITH SURFACE WATER RUNOFF DURING MAY THROUGH
SEPTEMBER WITH THREE TILLAGE SYSTEMS - -
Water
Soluble nitrogen Total nitrogen
Tillage system
3/
Conventional —
Till -
Ridge -
runoff
cm
5.4
3.5
3.1
Erosion
metric tons
per ha
32
12
3.5
NH.-N
4
kg/ha
0.1
0.05
0.05
N03-N
- - -
0.4
0.3
0.2
in sediments
kg/ha
29
16
6
— Harmon and Duncan, 1978.
2/
3/
4/
5/
Average of three growing seasons on 0.6-1.8 ha watersheds in continuous
corn with 168 kg N-fertilizer per ha annual applications.
Conventional tillage utilizes a moldboard plow that inverts the soil
and totally incorporates residue into the soil.
Till planting is a one-pass tillage and planting system that permits
row cropping with limited soil disturbance. The till planter clears
a shallow path through the row of the previous crop, moving plant
residues out of the planting area.
Ridge planting is a cropping system in which crops such as corn are
planted on top of the ridge of the previous year's row. Crop residue
accumulates in the furrow and helps delay runoff and control erosion.
Erosion primarily removed the top surface soil which contains the high-
est concentration of organic matter and which may be rich in phosphorus from
fertilization. An example of phosphorus transported in surface runoff from
precipitation on a nonirrigated field is shown in Table 27, which illustrates
that losses from heavy phosphorus fertilization are higher, although losses
were less for corn planted on level terraces primarily because of the
reduction in runoff. Increasing the fertilizer rate from 39 to 97 kg P per
hectare for contour-planted corn nearly doubled the phosphorus losses in the
sediments.
Available evidence indicates that deep percolation of phosphorus is
small and not a major source of pollution (Balsters and Anderson, 1978:
Harmon and Duncan, 1978; Int. Garrison Diversion Study Board, 1976). The
phosphorus concentration of soil solutions is low, ranging from 0.01 to 0.1
ppm. Consequently, fertilizer applications do not greatly increase the
levels of phosphorus in percolating waters.
78
-------
TABLE 26. AVERAGE NITROGEN LOSSES WITH WATER RUNOFF AND SEDIMENTS FROM
I/ 21
VARIOUS MANAGEMENT SYSTEMS - -
Tillage management
3/
operation —
Conventional —
Till -1
Chisel -
Disk -
Ridge -
- Modified from
and Kenyon.
21
'•' T>..~~ft £,.„_. n-i
Water
runoff
cm - -
14
12
14
12
11
Harmon and
Erosion
metric tons
- per ha - -
49
33
28
17
11
Duncan, 1978.
Soluble
nitrogen
NH.-N NO ,-N
4 3
kg/ha
0.3 1.4
1.7 1.1
1.5 1.3
2.2 1.5
2.7 1.1
Soil types were:
Total nitrogen
in sediments
kg/ha
75
51
50
30
20
Ida , Tama ,
and down the land slope. Rainfall was 20-21 cm.
o /
- Each had an N-fertilizer application of 168 kg N/ha.
4/
5/
6/
7/
8/
Conventional tillage utilizes a moldboard plow that inverts the soil
and totally incorporates residue into the soil.
Till planting is a one-pass tillage and planting system that permits
row cropping with limited soil disturbance. The till planter clears
a shallow path through the row of the previous crop, moving plant
residues out of planting area.
Chisel plows operate at depths equal to or slightly deeper than mold-
board plows. Chiseling loosens dry soils and leaves up to three-
fourths of the residue at or near the surface.
Disk harrows have been used as both primary and secondary tillage tools.
They incorporate at least half of the plant residues into the soil
with each pass of the disk.
Ridge planting is a cropping system in which crops such as corn are
planted on top of the ridge of the previous year's row. Crop residue
accumulates in the furrow and helps delay runoff and control erosion.
79
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TABLE 27. SURFACE RUNOFF, SOIL SEDIMENT YIELDS AND PHOSPHORUS LOSSES FROM
WATERSHEDS UNDER THREE MANAGEMENT OPERATIONS -^
Management operation . Sediment
(applied phosphorus) Water runoff — yield
Phosphorus losses
Solution Sediment
Contour planted corn
(97 kg P/ha)
Level terraced corn
(97 kg P/ha)
Contour planted corn
(39 kg P/ha)
cm
8.1
0.9
6.8
r.ietric tons/ha - - - kg/ha - - -
25.0 0.17 1.05
1.3
17.0
0.05
0.11
0.08
0.58
— Harmon and Duncan, 1978.
21
— Total precipitation ranged from 76-78 cm.
Pesticide Management
Pollution resulting from pesticide use poses three questions:
(1) What types and amounts of pesticides are lost from
agricultural fields?
(2) Can these losses be reduced by changes in management
practices?
(3) Given a particular pesticide loss from a field, what is the
most likely impact on the water quality of return flows and
receiving waters?
Losses from pests in the United States probably average 30 to 40 per-
cent of total production and would be even higher without pesticides. The
use of pesticides has increased rapidly in the last decade because of their
effectiveness and labor-saving features. Total farm pesticide use in the
United States has increased 40 percent from 1966 to 1971, and 38 percent
from 1971 to 1976 (Eichers, et al., 1978). In 1976, 70 percent of the
pesticides used on crops in the Great Plains were applied to the five crops
under consideration in this manual (Table 28). Of these, 58 percent were
applied to corn. Herbicides and insecticides are the pesticides of most
concern because these constituted 92 percent of total pesticide usage in
major crops in the United States.
Specific herbicides and insecticides of major importance in these five
crops in the Great Plains are listed in Tables 29 and 30. More
80
-------
oo
TABLE 28. PESTICIDES USED IN NORTH DAKOTA, SOUTH DAKOTA, NEBRASKA, KANSAS, OKLAHOMA
AND TEXAS IN 1976 -
Herbicides Insecticides
Percent Quantity Percent
Area planted of area applied of area
Crop ha (millions) treated kg (millions) treated
Corn
Sorghum
Cotton
Soybeans
Alfalfa
Total pesticides
Total pesticides
5.93 81 11.12 50
6.36 50 5.45 28
2.12 68 1.27 30
1.22 51 1.09 9
2.96 <1 0.05 11
used for five crops 18.98
used for all crops 26.2
Quantity
applied
kg (millions)
4.27
1.63
1.09
0.09
0.45
7.53
10.9
— Eichers, et al., 1978.
-------
TABLE 29. MAJOR HERBICIDES USED IN NORTH DAKOTA, SOUTH DAKOTA, NEBRASKA,
KANSAS, OKLAHOMA AND TEXAS IN 1976 -
Herbicide
Atrazine
2,4-D
Alachlor
Propachlor
Propazine
EPTC
Trifluralin
Butylate
Cyanazine
Dicamba
Total of above herbicides
Total herbicide used
Major crop(s)
Corn and sorghum
Wheat, corn, and sorghum
Corn and soybeans
Corn and sorghum
Sorghum
Corn
Cotton and soybeans
Corn
Corn
Wheat, corn, and sorghum
Quantity used
kg (millions)
7.1
7.0
1.9
1.7
1.6
1.3
1.2
1.0
0.5
0.4
23.7
26.0
- Eichers, et al., 1978.
TABLE 30. MAJOR INSECTICIDES USED IN NORTH DAKOTA, SOUTH
KANSAS
, OKLAHOMA AND TEXAS IN 1976 -
DAKOTA, NEBRASKA,
2/
Insecticide —
Parathion
Carbofuran
Carbaryl
Disulfoton
Dyfonate
Methyl Parathion
Phorate
Malathion
Toxaphene
Major crop(s)
Sorghum, cotton, and corn
Corn and alfalfa
Corn and soybeans
Cotton and sorghum
Corn
Cotton
Corn
Alfalfa and sorghum
Cotton
Total of above insecticides
Total insecticide used
Quantity used
kg (millions)
2.4
2.0
1.7
1.5
0.7
0.6
0.5
0.4
0.2
10.0
10.9
— Eichers, et al., 1978.
21
— See tables in Appendix B for trade names of insecticides.
82
-------
comprehensive lists with information on trade names, transport, toxicity,
mobility and persistence are found in Tables B-l and B-2 in Appendix B.
The rates of pesticides applied to a crop range widely (Table 31) be-
cause application rate depends on the specific pesticide, the pest, the soil,
and climatic factors. The specific application rate required for effective-
ness is listed on each pesticide label for each crop and pest. Seldom is it
advisable to depart from this rate and it is illegal to apply a dosage
greater than that specified on the label.
TABLE 31. RATES OF AGRICULTURAL PESTICIDES APPLIED
TO CROPS IN THE GREAT PLAINS - _
Range of pesticide application
Crop Herbicide Insecticide
kg/ha
Cotton
Sorghum
Soybeans
Corn
Alfalfa
0.6-4.7
0.9-1.8
0.9-1.4
1.2-1.9
<0.1
0.5-13.6
0.3- 0.5
0.1- 0.2
0.7
<0.1
— Fed. Energy Adm. and U.S. Dept. of Agr., Energy and U.S. Agriculture:
1974 Data Base, 1977.
Processes Affecting the Fate of Pesticides—
Many factors affect the fate of pesticides in the environment, as shown
in Figure 22. After application, adsorption to the soil largely influences
other processes, especially leaching. The extent of adsorption is primarily
determined by the properties of the pesticide and the soil. Pesticides
leach more readily in sandy soils than in loams or clays or where organic
matter content is low. Water-soluble pesticides leach more rapidly and
readily than insoluble ones.
Persistence is the time span required to degrade a pesticide. In soil,
this is largely a function of the pesticide, the rate of application, soil
texture and organic matter content, and climatic conditions of which tempera-
ture and moisture are vitally important. Pesticides persist longer where
organic matter is low and under cool, dry conditions. As persistence
increases, the chance of a pesticide entering the runoff water increases;
however, pesticide mobility and timing of the runoff event following the
pesticide application are more important considerations. These latter
factors, along with application rate, largely determine the pesticide
83
-------
Figure 22. Processes influencing the fate and behavior of pesticides.
-------
concentration in runoff water and sediment. Infiltrating water will carry
some of the pesticide into the soil before surface runoff begins, resulting
in a lower concentration in the runoff water.
Chemicals which are weakly adsorbed will move with deep percolation
water; those which are strongly adsorbed move mainly with sediments; and
those with intermediate adsorptivity will move with both water and sediment.
Even though the pesticide concentration may be higher in the sediment than in
runoff water, as is often the case (see Table 32), the total amount of pesti-
cide in the water may be greater because the amount of runoff water is
usually much greater than sediment loss.
TABLE 32. CONCENTRATIONS OF PESTICIDE RESIDUES IN TAILWATER PITS SERVING
CORN AND SORGHUM FIELDS IN HASKELL COUNTY, KANSAS. AVERAGED OVER 2 YEARS -
Samples containing
the pesticide
Mean concentration
of the pesticide in
positive samples
Pesticide
Sediment
Water
Sediment
Water
Atrazine
Cyanazine
EPTC
Fonofos
Parathion
Phorate
- - -
26
50
64
27
0
67
percent - - -
38
44
67
47
100
82
parts
76
42
41
90
0
47
per billion
41
29
1
1
2
1
- Kadoum and Mock, 1978.
Most pesticide losses occur in the first runoff event following
application, and the sooner this happens, the greater the losses. The same
is true on sandy soil where percolation can occur. Pesticide adsorption and
degradation in the soil reduces the amount of pesticide available for water
transport.
Pesticides in Runoff Water—
Runoff losses of pesticides from an individual field are vastly
different than from a large watershed because of the attenuation processes
between the field and water course; therefore, generalizations of the amounts
of pesticides lost to the edge of a field are presented in this section.
These generalizations are a summary of Wauchope's review (Wauchope, 1978).
85
-------
Wettable powder pesticides (all are herbicides applied to the soil) show
the highest long-term losses. Losses up to 5 percent can be expected from
slopes of 10-15 percent and losses up to 2 percent from slopes of 3 percent
or less. If a large runoff event occurs within two weeks after a pesticide
application, losses may be three times larger. Emulsions of water-insoluble
pesticides show long-term losses of 1 percent or less. Water soluble pesti-
cides incorporated into the soil show seasonal losses of 0.5 percent or less.
As with wettable powders, initial losses can increase three-fold with large
water runoff events.
Pesticide concentration also varies during the irrigation season,
depending on the timing of irrigation relative to the pesticide application,
as illustrated in Table 33. Herbicides, which are mostly applied during
planting or shortly after crop emergence, had the greatest concentrations in
both sediment and water during June in this study.
TABLE 33. DISTRIBUTION OF PESTICIDE RESIDUES OCCURRING IN TAILWATER
PITS DURING IRRIGATION SEASON -
Pesticide
Concentration in sediment
May
June
July Aug.
Concentration in water
May
June July Aug.
- - - parts per billion - - - - - - parts per billion - - -
Herbicides:
Alachlor 0 47 30 0
Atrazine 38 124 65 32
Cyanazine 0 32 21 2
EPTC 4 26 8 0
Propazine 43 90 81 20
Terbutryn 0 117 36 34
Insecticides:
38
47
18
0
25
0
9
87
30
2
60
12
0
19
21
0
24
3
0
7
1
0
5
2
Carbofuran
Disulfoton
Fonofos
0
0
9
5
0
305
7
0
101
0
36
14
6
0
0
3
0
2
1
0
0
0
0
0
- Modified from Kadoum and Mock, 1978.
Only limited data are available on the impairment of water quality of
surface or groundwaters by pesticides. Attenuation processes which occur as
the pesticides are transported from the site of application to the receiving
stream decrease pesticide concentrations and amounts. Levels of pesticides,
86
-------
when detected, are in the parts per billion (ppb) range or lower.
Wauchope (1978) made rudimentary estimates of pesticide runoff losses.
Pesticides were placed in three groups and "rules-of-thumb" were proposed
for predicting annual losses that can be useful in evaluating potential water
quality problems for large agricultural areas. These estimates are:
(1) Foliar-applied Organochlorine Insecticides—Although persist-
ent, these have largely been replaced by other insecticides
and are not widely used on cropland in the Great Plains.
Toxaphene is the only one listed in Table 30. An average
of 1 percent per year of the amounts applied can be used for
estimating runoff losses.
(2) Wettable Powder Formulations—Wettable powder herbicides,
although not all persistent, have consistently high losses
and are widely used in the Great Plains. Dissipation and
dilution processes probably occur after these herbicides
leave the field because they are seldom detected in receiving
waters. However, loss estimates of 2 percent for 10 percent
slopes or less, and 5 percent for areas of over 10 percent
slopes were suggested for these materials.
(3) Nonorganochlorine Insecticides, Incorporated Pesticides,
and All Other Herbicides—All remaining pesticides belong
in this group which contains 61 percent of the herbicides
and 98 percent of the insecticides from Tables 29 and 30.
Many of them are water soluble, are not strongly adsorbed
to soil sediments, and have short persistence. Although
variable, one-half percent loss for these pesticides has
been suggested as reasonable.
The amount of pesticides delivered to water sources was estimated in
the Iowa Areawide Wastewater Management Study (Harmon and Duncan, 1978).
The expected values, based on adsorption classes, are given in Table 34.
Attenuation processes between the edge of the field and surface waters were
not considered. It was suggested that these estimated values are probably
high in view of the actual amounts of pesticides normally found in surface
waters. These values basically agree with the percentages suggested by
Wauchope (1978).
87
-------
TABLE 34. ESTIMATED PERCENTAGES OF APPLIED PESTICIDES
DELIVERED TO SURFACE WATER SOURCES -
Estimated losses of applied pesticides
Adsorption —
class
Weak
Medium
Strong
Normal . ,
range Maximum —
- - - percent - - -
0-1 10
0-5 20
0-0.5 2
Normal _,
range Maximum —
- - - percent - - -
0-0.1 1
0-1 4
0-2 10
— Harmon and Duncan, 1978.
21
— Weak: weakly held by soil colloids, readily leached in sandy soils
low in organic matter (1 percent or less), some movement in other soils.
Medium: moderate attraction by soil colloids, moderate movement in
sandy soils low in organic matter but little or no movement in other
soils.
Strong: strongly held by soil colloids, slight leaching in sandy soils
low in organic matter but little or no movement in other soils.
— Numbers are estimates of maximum pesticide losses that could be
expected under very unusual pesticide-soil-water conditions.
The distance between the treated agricultural site and final water
course is an important factor influencing the amount of pesticide entering
that course. For example, runoff losses from a 6 hectare watershed for the
herbicides alachlor, propachlor, and cyanazine were 0.54, 0.26, and 1.05
percent of the amounts applied. However, of the total applied within the
5,055 hectare watershed, runoff losses of these same herbicides were only
0.10, 0.14, and 0.08 percent, respectively, equaling 17 percent of the
edge-of-field losses (Harmon and Duncan, 1978). During a five-year study
in Nebraska, pesticides were found in 14 percent of the water samples
(Table 35). One herbicide (2,4-D) was found at all seven locations.
Concentrations of the pesticides were usually less than 1 ppb.
88
-------
I/
TABLE 35. SUMMARY OF PESTICIDES FOUND IN WATER IN NEBRASKA (1971-1976) -
Number of
measurements
Stream location (approx.)
Platte River at North Bend 260
Logan Creek at Fender 90
Elkhorn River at Waterloo 400
Salt Creek above Seal Slough 340
Salt Creek below Stevens Creek 370
Big Blue River near Crete 280
Little Blue River at Hollenberg 270
Number of Pesticides
positive normally
measurements found
11 2,4-D
Diazinon
18 Aldrin
DDE
DDT
Dieldrin
2,4-D
37 Dieldrin
Diazinon
2,4-D
2,4,5-T
29 2,4-D
2,4,5-T
101 2,4-D
Diazinon
Lindane
2,4,5-T
Silvex
Dieldrin
53 2,4-D
2,4,5-T
Diazinon
Dieldrin
34 2,4-D
2,4,5-T
- U.S. Geological Survey, 1971-1976.
Groundwater from irrigation wells in corn-growing areas of Merrick
County, Nebraska (Subarea 1021, as discussed in Section 1), where NO_-N
concentrations exceed 5 ppm were analyzed for atrazine (Spalding, et al.,
1979). The NO -N and atrazine concentrations of the 18 samples are shown
in Table 36. The lowest concentrations were found beneath poorly-drained
(fine-textured) soils where less leaching, more adsorption, and faster
degradation are expected. Atrazine, although not a highly soluble pesticide,
leached into the shallow groundwater under sandy soils because soil adsorp-
tion was low and water percolation was high, conditions which are highly
conducive to mass flow. Atrazine has probably been applied annually in
this area for 15 to 20 years at rates of 1 to 3 kg per hectare per year.
89
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TABLE 36. ATRAZINE AND NITRATE-NITROGEN CONCENTRATIONS IN WATER
FROM IRRIGATION WELLS IN MERRICK COUNTY. NEBRASKA^
Soil characteristics Atrazine
Drainage
Somewhat poor
Moderate
Well
Surf ace- texture Ave. Max.
- - ppb - -
Loam-clay loam 0.25 0.44
(Fine- textured)
Sandy loam-loam 1.44 3.99
(Medium- textured)
Fine sandy loam-loam 1.52 6.96
(Coarse- textured)
NO_-N
j_ ..
Ave . Max .
ppm
13 26
24 30
20 31
- Modified from Spalding, et al., 1979.
The persistence of atrazine, especially in sandy soils, enhances its oppor-
tunity for leaching.
The Water Quality Report for the International Garrison Diversion Study
Board (Int. Garrison Diversion Study Board, 1976) indicated that the proba-
bility of pesticides in runoff from irrigated lands was very low during the
growing season in North Dakota. Herbicides rarely were found in the irri-
gated regions of the Souris River despite widespread use of pesticides.
However, it was conceded that pesticides could appear in IRF from direct
surface runoff in the springtime (Bauder, per. com.).
Influence of Management Practices on Pesticide Losses—
Pesticide losses can best be controlled through a combination of
runoff- and erosion-limiting devices. Erosion management alone will not
provide a total solution because pesticide losses are usually greater in
water runoff than in sediment.
Most extensive pesticide losses occur when precipitation closely
follows pesticide application, usually in the spring when pesticide applica-
tion is at its peak and thunderstorms are prevalent (Wauchope, 1978). Total
dosage applied is extremely important when applications are made during the
rainy or irrigation season. Frequent applications of less persistent
materials may cause more problems due to the higher probability of rainfall
immediately following these applications. Careful attention to correct
application rate, the substitution of alternative pesticides or control
methods (particularly if conditions are conducive to runoff or leaching) or
soil incorporation of the pesticide can decrease pesticide losses from
runoff.
90
-------
Other management practices for controlling pesticide losses have been
suggested; however, their relationship to other crop production management
practices is complex (Caro, 1976; Harmon and Duncan, 1978; Dean and Mulkey,
1978). Greater losses usually result when pesticides are used unnecessarily
or in excessive amounts.
Improved soil-water management can reduce pesticide losses by reducing
runoff. Reduced tillage systems, which leave more surface residue, reduce
losses of chemicals transported in sediment, but may only have a minor impact
on soluble chemical losses (Bondurant and Laflen, 1978; Harmon and Duncan,
1978).
Proper irrigation water management practices are effective in reducing
the movement and accumulation of pesticides. Runoff is most likely to occur
with the first irrigation after pesticide application, and if that irrigation
closely follows application. The danger of pesticide movement is essen-
tially eliminated if the irrigation is not sufficient to cause excessive
surface water runoff. Catching the irrigation tailwater will reduce the
likelihood of pesticide runoff entering a water course.
It is unlikely that potential pesticide losses can be eliminated on
irrigated lands, especially in runoff from slopes exceeding 12-18 percent
(Harmon and Duncan, 1978). On coarse-textured soils and at lower levels of
irrigation management, the probability of movement with deep percolation
increases.
91
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SECTION 3
AVAILABLE ON-FARM IRRIGATION MANAGEMENT ALTERNATIVES FOR
REDUCING POLLUTION FROM IRRIGATION RETURN FLOWS
by
J.R. Gilley, D.G. Watts, F.W. Roeth, K.D. Frank, and M. Twersky
The specific water quality problems resulting from nonpoint source
pollution from irrigation return flows were presented in the preceding
section. Section three deals with the various alternative measures which
can be taken to reduce or control pollution problems resulting from irriga-
tion return flows. Direct water runoff and deep percolation from irrigated
cropland resulting from applied irrigation water can rarely be eliminated.
However, it can be reduced and, in some cases, substantially reduced by
carefully-selected combinations of management practices. Specific practices
directed to the control of irrigation return flows under each of five alter-
native management options are discussed. These alternative irrigation
management options given in Table 37 are:
1. irrigation system management
2. on-farm water management
3. soil management
4. nutrient management
5. pesticide management.
It is important to recognize that these five management options are not
totally independent. Interrelationships exist among them which may affect
the choice of a particular practice in a given situation. Compromises may
be necessary. The choices must meet not only environmental requirements but
also meet the economic need of specific situations. For example, increasing
the stream size and reducing water application time for furrow irrigation
systems may decrease the amount of pollutants contributed by deep per-
colation, but may increase the amount of surface runoff. Thus, a reuse
system must be installed to capture the runoff water to reduce pollution by
surface return flows. Likewise, the introduction of conservation tillage
to control erosion may result in the use of greater amounts of chemicals to
control crop pests, so that the net benefit to the quality of the irrigation
return flows may not be as great as might be expected. Some of the more
important interrelationships are described in the discussions of individual
management options. Some specific control practices are listed and dis-
cussed that are of marginal value in many cases. However, in a particular
set of circumstances, one of these practices could be the best recommenda-
tion.
92
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TABLE 37. ALTERNATIVE IRRIGATION MANAGEMENT OPTIONS TO REDUCE
POLLUTION FROM IRRIGATION RETURN FLOWS
Table number in
Management option this section
1. Irrigation system management
—Types of irrigation systems 38
—Potential pollution rating of surface and
trickle systems 39
—Potential pollution rating of sprinkler systems 40
—Surface and trickle irrigation management 41
—Sprinkler irrigation management 42
2. On-farm water management 43
3. Soil management 44
4. Nutrient management 45
5. Pesticide management 46
The pollution reduction capability of specific control practices within
each management option is estimated by comparing the specific practice to
a particular standard. This rating scale is:
Low (L) 0-10 percent reduction
Moderate (M) 10-50 percent reduction
Substantial (S) 50-100 percent reduction.
It must be emphasized that these ratings are only estimates. Many of the
ratings given in the tables are based on limited quantitative data. The
judgment of the authors was used when no data existed. Furthermore, the
singular impact of any control action individually applied to a given
irrigation situation may be rated low. In contrast, when that same control
action is used as one of several supporting practices in a control program,
its impact may be higher than estimated. The impact of a given control
practice will vary depending on the individual site.
IRRIGATION SYSTEM MANAGEMENT
The general types of irrigation systems evaluated herein are listed in
Table 38. Descriptions of these systems were given in Section 2.
Evaluations of the potential pollution from surface and trickle
irrigation systems and sprinkler irrigation systems as a function of soil
type and slope are shown in Tables 39 and 40, respectively. These ratings
93
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TABLE 38. IRRIGATION SYSTEMS DISCUSSED UNDER THE
ALTERNATIVE MANAGEMENT OPTIONS
Irrigation system —
SURFACE:
Contour Ditch (Wild Flooding)
Graded Borders
Furrows without reuse system
Open ditch
Siphon tube
Gated pipe
Furrows with reuse system
Open ditch
Siphon tube
Gated pipe
Automated gated pipe with reuse system
SPRINKLER:
Solid Set
21
Center-Pivot —
Moved (hand move, side-roll, skid-tow)
Moving (travelers, boom)
TRICKLE:
Trickle/Drip
— The individual systems are described in greater detail in Section 2.
2/
— Including low pressure center-pivot systems.
-------
TABLE 39. POTENTIAL POLLUTION RATING OF SURFACE AND TRICKLE IRRIGATION SYSTEMS -
I/
VO
Ul
Types of surface
and trickle
irrigation- ,
systems —
SURFACE :
Contour ditch
Graded borders
Furrows without
reuse
Furrows with reuse
Automated gated pipe
with reuse
TRICKLE:
Trickle/Drip
Coarse- textured
(Sand to loamy
Land
0-2
H
H
H
M
M
L
slope
2-4
H
H
H
H
M
L
soil y
sand)
(percent)
4-6
H
H
H
H
M
L
>6
H
H
H
H
H
L
4/
Medium-textured soil — Fine-textured
(Loam to silt loam) (Silty clay to
Land
0-2
M
M
M
L
L
L
slope
2-4
H
H
H
M
L
L
(percent)
4-6
H
H
H
M
L
L
>6
H
H
H
M
M
L
Land
0-2
M
L
M
L
L
L
slope
2-4
H
M
H
L
L
L
soil 1'
clay)
(percent)
4-6
H
M
H
M
L
L
>6
H
M
H
M
M
L
— Rating Scale (Relative):
H - High potential pollution hazard
M - Moderate potential pollution hazard
L - Low potential pollution hazard
21
— Irrigation systems described more fully in Table 38.
3/
— Primarily deep percolation problems.
4/
— Both deep percolation and surface water runoff problems.
— Primarily surface water runoff problems.
-------
TABLE 40. POTENTIAL POLLUTION RATING OF SPRINKLER IRRIGATION SYSTEMS -
I/
Types of
Sprinkler
irrigation^
systems —
SPRINKLER:
Solid Set
Center-Pivot —
Moved -
Moving -
Coarse-textured soil
(Sand to loamy sand)
Land slope (percent)
0-2
L
L
M
L
2-4
L
L
M
L
4-6
L
L
M
M
6-10
M
M
H
H
>10
M
M
H
H
Medium- textured soil
(Loam to silt loam)
Land slope (percent)
0-2
L
L
L
L
2-4
L
L
L
L
4-6
L
M
M
M
6-10
M
H
H
H
>10
M
H
H
H
Fine-textured soil
(Silty clay to clay)
Land slope (percent)
0-2
L
M
L
M
2-4
L
M
L
M
4-6
L
H
M
H
6-10
L
H
H
H
>10
M
H
H
H
— Rating Scale (Relative):
H - High potential pollution hazard
M - Moderate potential pollution hazard
L - Low potential pollution hazard
21
— Irrigation systems described more fully in Table 38.
3/
— On fine-textured soils, primarily surface water runoff problems.
4/
— On coarse-textured soils, primarily deep percolation problems.
— On fine-textured soils, primarily surface water runoff problems.
-------
assume good (reasonable attainable) irrigation management. They are only
general estimates indicating how pollution problems will increase or decrease
depending on soil type and increasing land slope. The specific pollution
problem on irrigated lands will have to be evaluated on an individual site
basis.
Regardless of the degree of management, soil type, or irrigation system,
steeper slopes increase the potential for pollution problems. Generally,
slopes greater than 2 percent are not recommended for most surface irrigation
systems, particularly on coarse-textured soils. Sprinkler systems can be
used on steeper land slopes; however, they are not generally recommended for
slopes over 6 percent on medium- and fine-textured soils because of the
increased potential for runoff.
Even if properly selected and designed, a particular type of irrigation
system is not always more efficient than another method. Changing from one
irrigation system to another only constitutes a change in the method of
water application. Such a change should not be made based on the premise
that one system is always more efficient than another. Replacing a surface
irrigation system with a sprinkler irrigation system can reduce pollution
hazards in some situations. However, pollution hazards are dependent on
both the type of system in use for the given soil type and the individual
irrigator's management skills. A well-managed surface irrigation system may
have less pollution problems than a mismanaged sprinkler system. We
emphasize that the ratings of Tables 39 and 40 are only guides which have
been developed to help the users of the manual better understand the response
of irrigation systems under a given set of conditions.
The evaluation of irrigation systems depends on a number of variables
considered for particular conditions. There are many situations where
conversion from one irrigation system to another would not be feasible due
to soils, topography, acreage, crop, economics, or energy requirements.
Surface Systems
The primary pollution hazards from surface irrigation systems are deep
percolation losses from coarse-textured soils and the surface runoff on
fine-textured soils. Medium-textured soils can be troubled with either
problem, depending on water intake characteristics. The principal irrigation
system management practices for controlling these losses are given in
Table 41.
Proper operation of most surface irrigation systems (except for graded
borders and level basins) requires water runoff to insure a uniform applica-
tion of water. Tailwater reuse systems can be installed to capture both
sediment and water runoff when maximum water stream sizes are used. Reuse
of tailwater return flows can result in the removal of 40 to 70 percent of
the sediment from the water runoff and can, in some cases, increase the
irrigation efficiency by 30 percent. Other sediment retention systems can
be used either separately or in conjunction with reuse systems to remove
much of the sediment and considerably reduce pollution problems. Where
97
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TABLE 41. IRRIGATION SYSTEM MANAGEMENT PRACTICES FOR SURFACE AND TRICKLE IRRIGATION SYSTEMS AND THEIR RELATED POLLUTION REDUCTION
1/2/
Specific control practice
Applicable
irrigation system
b
Contou
ditch
Graded
horde
QJ
(0 01
9 >
[b
« £
01
D.
O.
3 3
O Q>| TJ
S-d s
£9| 3
Install tallwater reuse system XXX
Land smoothing or leveling to X X X X
u
u
1-4
IM!
Coarse-textured soil — Medium-textured soil Fine-textured soil
(Sand to loamy sand) (Loam to silt loam) (Silty clay to clay)
Deep Surface water Deep Surface water Deep Surface water
percolation runoff percolation runoff percolation runoff
L M MS MS
X L L MM MM
control slope and shape
Hatch system to soil type and
slope (Table 39)
Proper stream size (maximum
noneroslve stream) with
reuse system
X X
X X
M-S
M-S
M-S
M-S
M-S
Reduce length of run with
reuse system
VO Use of cutback stream size
00 without reuse system
Use of alternate furrow
Irrigation
Increased use of automated
devices
Decreased set time with reuse
system
4/
Lining canals —
Replace system with sprinkler
system
X X X X
X X
X X
X X X X
X X
X X X X
X X X X
X
X
X X
X
X
X
M-S
L
L-M
L-M
M
M-S
M-S
L-M
L
L
L-M
L
L
L
M-S
L
L-M
L-M
M
L-H
L
S
L-M
L
L-M
L
L
L
M
L
L
L-M
M
L
L
S
L-M
L
L-M
L
L
L
— Carter, 1976; Interagency Task Force, 1978; Kruse and Heermann, 1977. Merrlam, 1977, Stegman, et al , in prep , Westcsen, 1977
— Ranges in percent reduction compared to an existing system without the specific control practice low (L) 0-10 percent,
moderate (M) 10-50 percent, substantial (S) 50-100 percent.
— Surface irrigation systems on coarse-textured soils should be carefully designed and operated because of the potential deep percolation
problems (Table 39).
4/
— The benefits of canal lining depend greatly on local soil conditions. The area affected by canals is usually much smaller than the
irrigated land, thus the benefit of canal lining Is usually small
-------
surface irrigation systems are used on land which has been leveled, smoothed,
or shaped, surface water runoff and soil loss are usually reduced. Where
surface systems are correctly designed for soil type and slope (Table 39),
deep percolation water losses will also be minimized if proper scheduling of
irrigation and proper stream sizes are used.
Surface system designs are best improved by adjusting slope, altering
the length of run (where field shape permits), and changing furrow stream
sizes to obtain proper advance and recession of the irrigation stream.
Stream sizes can be adjusted to advance water across the field in the fastest
time possible. They must not, however, exceed the maximum nonerosive stream
size if soil erosion is to be prevented.
Maximum nonerosive stream sizes and short set-times (a few hours) may
be best used with automated surface irrigation systems to obtain the maximum
irrigation efficiency. The maximum nonerosive stream size also can be used
with manually operated gated-pipe or other nonautomated systems. However,
the labor required for making frequent set changes using nonautomated systems
makes this system impractical in most cases.
The addition of devices to measure and control water volumes within
surface systems will considerably improve potential irrigation system
efficiencies. Automation of water control in any irrigation system will
increase the management capabilities of the system and, in turn, increase
the efficiency of the system [interagency Task Force, 1978, Kruse and
Heermann, 1977; Fischbach (ed.), 1977].
Reducing the stream size of water flows after water reaches the end of
the irrigated field and decreasing the length of water flow in the field
also are practices that can control runoff and deep percolation losses. How-
ever, these practices require considerable labor and system equipment inputs.
Irrigators may be reluctant to invest their management energies while other
control practices are more readily available (Carter, 1976).
Sprinkler Systems
The primary pollution hazards from sprinkler irrigation systems are
deep percolation losses caused by excessive water applications and surface
runoff which may be caused by excessive water application rates. Practices
for controlling these losses are given in Table 42.
Surface runoff of irrigation water usually will not occur with a
properly designed sprinkler system that applies water no faster than the
soil absorbs it. The automation of water application, at rates less than
the soil infiltration rates, eliminates surface water runoff with subsequent
reduction in sediment and nutrient losses. This automation is more easily
adaptable with sprinkler systems than surface systems.
Sprinkler systems are adaptable to a wide range of land classes and
soil types, and can be effectively used under varying conditions. However,
it must be remembered that after a sprinkler irrigation system is designed
99
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TABLE 42. IRRIGATION SYSTEM MANAGEMENT PRACTICES FOR SPRINKLER IRRIGATION SYSTEMS AND THEIR RATED POLLUTION REDUCTION -
I/
Specific control practice
Coarse-textured soil
(Sand to loamy sand)
Deep
percolation
Surface water
runoff
Medium-textured soil
(Loam to silt loam)
Deep
percolation
Surface water
runoff
Fine-textured soil
(Silty clay to clay)
Deep
percolation
Surface water
runoff
O
O
Design systems application
rates as to not exceed soil
intake rates —
Proper amounts of water
applied per irrigation
Increase use of automated
devices to control irrigation
depths
Increase uniformity by
improved design
Operate in periods of low
wind velocities
Change design of system —'
21
L-M
L-M
L-M
L-M
L-M
L-M
L-M
L-S
M-S
L-M
L-M
L-S
— Ranges in percent reduction compared to an existing system without the use of the control practice: low (L) 0-10 percent,
moderate (M) 10-50 percent, substantial (S) 50-100 percent.
— Depends greatly on soil type and type of irrigation system. Extra caution should be used when low pressure center-pivots
are placed on medium- and fine-textured soils, because of high application rates.
-------
and installed, the irrigator's management ability ultimately determines the
efficiency with which the system operates and the resulting pollution from
the system. Individual types of systems must be assessed to evaluate their
ability to reduce pollution problems (Stegman, et al., in prep.).
A change from surface to sprinkler irrigation is needed on soils having
high infiltration rates that cause excessive deep percolation or where land
slopes are uneven and steep (Table 39 and 40). Sprinkler irrigation systems
are more easily controlled through adjustments to application frequency,
depending on crop water-use requirements or soil moisture profile character-
istics (Stegman, et al., in prep.; Westesen, 1977). Thus, conversion from
surface to sprinkler irrigation may be suggested as a means of reducing
irrigation return flow on coarse-textured soils, especially on sloping lands.
Trickle Systems
Less water can generally be applied with trickier/drip irrigation
systems. Because only the plant root zone is supplied with water, little
water is lost to deep percolation and none to surface water runoff. Trickle
systems are amenable to customized crop-field designs, for sophisticated
automation, and installation and placement on any soil type or slope of land.
However, trickle/drip systems require excellent water filtration equipment
and demand skilled technical labor and management.
Trickle/drip systems have the highest capital costs of all irrigation
systems. Generally, only irrigation of specialty, high-value cash crops
such as vegetables and fruit can be considered an economical use of this
system. As with any irrigation system, economic considerations may limit
applicability in a specific situation.
ON-FARM WATER MANAGEMENT
Current on-farm water management practices alone or in combination with
other existing practices, such as nutrient and pesticide applications, may
contribute to inefficient irrigation practices and increase pollution from
irrigation return flows. Specific water management practices and their
rated pollution reduction are given in Table A3. While proper water manage-
ment practices under a given irrigation system are of utmost importance in
reducing pollution, these practices are highly related to the other control
practices discussed.
Irrigation Scheduling
Experience has shown that irrigators generally know when to apply water,
but may not know how much to apply at a given time. By employing irrigation
scheduling techniques (Jensen, et al., 1970), irrigators or competent con-
sultants (Gilley, 1978) can determine the amount of water required to refill
the crop root zone to meet crop water-use requirements. Several irrigation
101
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TABLE 43. ON-FARM IRRIGATION WATER MANAGEMENT OPTIONS AND THEIR RATED POLLUTION REDUCTION -1 -1
Coarse-textured soil
(Sand co loamy sand)
Management option
Irrigation
Scheduling
Specific control practice
a Adopt a water scheduling
procedure
Deep
percolation
M-S
Surface water
runoff
L-M
Medium- textured soil
(Loan to silt loam)
Deep
percolation
L-M
Surface water
runoff
L-M
(Sllty clay to clay)
Deep
percolation
L-M
Surface water
runoff
L-M
Employ floi. measuring
services
M-S
L-M
L-M
Increase use of automated L-M
devices to control irriga-
tion timing and amounts
Scheduling procedures L-M
allowing a soil moisture
deficit to.maxim!ze
rainfall -
L-M
L-M
L-M
L-M
O
rO
Adjust Crop
Water Demands
Alternate cropping
practices to modify
irrigation schedules
Reduction in prcplant
irrigation
4/
Follow leaching —
recommendations for
irrigation water quality
and crop
Modify selection of crop
type and variety
Use crops which use
less water
L-M
L-M
L-M
L-M
c Reduce irrigation,amounts
below crop needs -~
- Flschbach (ed ), 1977, Fitzsimmons, ct al , 1978, Interagency Task Force, 1978, Jensen, 1975, Mcrn.im, 1968, 1977,
2/
Walker, et al , 1978
— Ranges in percent reduction compared to existing practices low (L) 0-10 percent, moderate (M) 10-50 percent,
substantial (S) 50-100 percent
— This procedure should be used with caution on coarse-textured soils WLth low water-holding capacities.
— Some deep percolation is necessary to remove the salts from the root zone so that the cropland remains productive
— Severe yield reductions may result from this practice
-------
techniques can be adopted. These include: (1) soil measurements; (2) com-
puted water balance; or (3) evaporation devices (Jensen, 1975; Stegman,
et al., in prep.)- In some areas, irrigation scheduling may provide for a
soil-moisture deficit in the program to make the maximum use of rainfall.
Of course, this procedure should not be used in either low rainfall areas
or in areas of predominantly coarse-textured soils which have minimal soil-
moisture holding capacity.
Irrigation scheduling is necessary, but is only one method for improved
water management. Scheduling must be incorporated with other on-farm
irrigation management practices to achieve maximum irrigation water manage-
ment. Management has become so sophisticated that computers are used in
processing of scheduling data. In some cases, computer simulation tests are
used to determine the interactions of the various soil types, crop types,
rainfall characteristics, and irrigation system combinations for customized
scheduling problems.
The recognized value of carefully controlling the amounts and timing of
water applications usually justifies the increased equipment needs of a
control system. A water meter, for example, can be a valuable tool in the
proper application and scheduling of irrigation water. Most devices can be
automated, dependent on the degree of control required for a specific
irrigation system. The switch to automatic water measuring meters increases
capital outlays, but should reduce energy demand and management costs over
time.
Crops have different demands for water and times of peak water-use
needs. The modification of irrigation practices during the early part of
the irrigation season can limit pollution from irrigation exceeding the
crop use demands. For example, a light application of irrigation water
early in the crop season, when the water requirement and nutrient uptake
are low, will result in reduced nutrient leaching, as compared to a large
application of irrigation water.
Irrigation system operation can be changed to more closely match the
soil intake rates and crop water demands throughout the season. These
changes must be flexible enough to meet peak water demands, but the system
should be operated to this capacity only during peak water demand periods.
System changes should therefore consider the rate of crop water use during
the entire irrigation season. The ability of the irrigation system to reduce
pollution will depend on the ability of the system to supply limited amounts
of water when crop needs are low.
In some irrigated areas, on-farm water management decisions must
consider the quality of water used for irrigation. Irrigation waters
contain salts in varying concentrations. The water's suitability for
irrigation use is determined both by the kinds and amounts of salts in the
water. Poor water quality may cause reduced crop yields. Deep percolation
or drainage water reflects not only the application water quality but also
the salt content of the land. Different scheduling and water management
system practices can be selected to create favora'ble salt balances within
103
-------
the spectrum of water qualities used for irrigation [Ayers and Westcot, 1976;
Fischbach, (ed.) 1977; and Walker, 1977]. Decisions about leaching/drainage
control practices will then depend on the amount of water applied.
Adjust Crop Water Demands
A reduction in crop water demands during the growing season or a shift
of the demand from one time during the growing season to another can reduce
or modify the water application amounts. These changes also may reduce deep
percolation losses. Crop water demands during the irrigation season might be
modified by: (1) selection of different crop species (using a multiseason
crop mix, using double cropping or altering growing periods by changing
planting dates); (2) using crops which require less water; and (3) reducing
the irrigation amounts below crop water needs or stressing crops to reduce
water demands. These procedures most likely are limited to very specific
situations. Their imposition would be limited by agronomic considerations
and by economics. Clearly, if these procedures were simple to implement,
and, in general, economically rewarding, they would have already been widely
adapted. They have been presented here only because factors other than
irrigation return flows may force the use of some of these procedures at
some locations.
SOIL MANAGEMENT
The rated pollution reduction of specific soil management practices for
controlling surface water runoff and deep percolation losses from irrigation
return flows is given in Table 44. These practices are primarily beneficial
in the reduction of surface water runoff and the associated reduction in
sediments, nutrients, and pesticides in this runoff. Because soil moves
with water, controlling water runoff will also control soil erosion and the
resulting sedimentation. Soil management practices can be combined with
other irrigation management practices best suited to local conditions.
Many of the practices given in Table 44 are discussed in the
November 1975 U.S.D.A./E.P.A. publication "Control of Water Pollution from
Cropland - Vol. 1," by B. A. Stewart, et al., which judges the effectiveness
of specific practices used to control runoff and erosion from rainfall.
These practices have similar benefits in controlling runoff and erosion from
irrigation water. Some of the practices given in Table 44 will not be
practical under all types of irrigation systems. Their use in an overall
irrigation management program must be evaluated on an individual basis.
Carefully designed and managed sprinkler irrigation systems create
little or no surface runoff of irrigation water. The runoff that does occur,
especially during early season irrigations, can be safely disposed of with a
grassed waterway or outlet, which prevents excessive soil loss and the
formation of gullies. Vegetative residues left over from the winter protect
soils during critical periods of early erosive irrigations and rains.
Vegetative sediment filters, settling basins, and sediment traps represent
practices for sediment reduction. While they are not erosion control
104
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TABLE 44 SOIL MANAGEMENT OPTIONS FOR CONTROLLING IRRIGATION RETURN FLOWS AND THEIR RATED POLLUTION REDUCTION -1 -f
Medlun-texcured SOL!
Fine-textured soil
Management option
Land Modification
Cultural Practices
Supporting Practices
Specific control
practice
Land smoothing or
leveling
Terraces -
Sediment basin —
Grassed waterway or
outlet
Permanent vegetation -
Sod-based rotation —
Meadowless rotation
Conservation tillage —
0 (
Improved soil fertility -
Timing of operation
9/
Contouring —
Contour strip cropping —
Vegetative strips —
Diversions
Drainage
(Sand to
Deep
percolation
L
L
L
L
L
L
L
L
L-S
L
L
L
L
L
L
loanv sand)
Surface water
runoff
L
L
L
L
L
L
L
L
L-S
L
L
L
L
L
L
(Loan to
Deep
percolation
M
L-M
L
L
L
L
L
L
L-S
L
L
L
L
L
L
silt loan)
Surface water
runoff
M
L-M
M-S
L-M
L-M
L-M
L
L-M
L-S
L
L-M
L-M
L-M
L
L
(Siltv cl
Deep
percolation
M
L
L
L
L
L
L
L
L-S
L
L
L
L
L
L
Lay to clav)
Surface water
runoff
M
M-S
M-S
L-M
L-M
L-M
L
L-M
L-S
L
L-M
L-M
L-M
L
L
— Carter and Bondurant. 1976, Fitzsimmons, ct al , 1977 find 1978, Stewart, B A . CL al , 1975 and 1976 Walter, et al , 1977
— Ranges in reduction compared to continuous row cropping, conventionally tilled, up and down slopes, used as a basis for
comparison low (L) 0-10 percent, moderate (M) 10-50 percent, substantial (S) 50-100 percent The effectiveness of any
practices is relative to total amounts of nutrient rather than concentrations of these materials
— The use of terraces under some irrigation systems may not be practical By reducing surface water runoff, deep
percolation losses may be increased
— Applies primarily to surface systems
— Degree of control is dependent on type of vegetation and amount of ground cover Reduction of cash crop acreage will
reduce Income
— Reduction of cash crop acreage will reduce income
— Some types of conservation tillage practices under surface irrigation systems may result In poor water application uniformity
and may Increase deep percolation losses Their relative effectiveness is related to the arount of residue left on the
surface and the amount of surface storage created by the tillage operation
— Depending upon the fertility level of the standard for comparison, low to substantial effects have been reported
9/
— The use of this practice may not be practical under some types of irrigation systems
— The use of this practice may not be practical under sonc types of irrigation systems Reduction of cash crop acreage
will reduce income.
— Primarily at lower end of surface irrigation systems
-------
practices, they can be effectively used to trap sediments near their source.
An improvement in soil structure usually increases water intake rates.
However, only a few control practices such as reduced tillage result in soil
structure improvements. These improvements may reduce runoff by allowing
more surface water to enter the soil profile. Conservation tillage systems
can be suited to a broad range of soil and climatic conditions, with good
options for fertilizer placement. Under many conditions, crop yields are as
good as, and sometimes higher than those obtained with the plow-based system.
Some of the practices require equipment modification, but human and machine
hours and soil compaction by implements are usually reduced.
The effectiveness of a particular system or machine depends on how the
soil surface is affected. The effectiveness is directly related to the
amount of residues left on the surface, the amount of residues mixed into
the upper few inches of topsoil, surface roughness, and ridges or residue
strips on the contour. Only fragmentary data are available to quantify the
runoff reduction that might be achieved by each practice. When used without
support practices, runoff will be slightly to moderately reduced. Runoff
may be reduced substantially if reduced tillage is used in conjunction with
contouring.
Rotating two row crops, or a row crop and a small grain, does not
provide the erosion control of a sod-based system. However, such rotations
help control some diseases and pests and can reduce the amount of pesticides
required. Direct runoff may be slightly reduced in the years the field is
in small grain, but it apparently has no residual effect on infiltration
capacity in the row-crop year. Rotations involving only row crops (corn and
soybeans, for example) would have approximately the same direct runoff
potential as continuous corn. However, given the present price structure,
there may be serious economic drawbacks to the use of certain crop rotations
and cover crops.
Change in land use is sometimes the only solution. Properly managed
hay or pasture furnishes adequate erosion control over a wide range of
slopes. Sometimes, a change to annual cropping of small grains and other
closely seeded crops, with appropriate tillage practices and residue manage-
ment, will suffice. Obviously, substantial local changes in land use will
often have serious adverse economic effects on the farmer and the region.
NUTRIENT MANAGEMENT
The loss of soil nutrients was occurring long before lands were
plowed, farmed or irrigated. Modern farming and irrigation practices
have only accelerated these nutrient losses. Nutrient losses cannot be
eliminated, but they can be reduced by the implementation of management
practices and application of technical information available today. A
number of practices will reduce surface runoff, soil erosion, or both and
thus control overland surface nutrient transport. In some cases, such as
leaching of nutrients by deep percolation, alternative practices must be
106
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used to reduce losses of soluble nutrients.
Nitrogen and phosphorus are the nutrients most commonly required by
crops, applied to the soil, and lost through present agricultural practices.
Where high rates of chemical fertilizer are applied to supplement the
nutrients present in the soil, the potential for pollution will usually be
highest.
Sufficient soil fertility research data are available to provide guide-
lines for the plant nutrients needed for optimum crop production. Thus, the
problem of applying excessive amounts of nutrients such as nitrogen and
phosphorus is not one of being able to predict the amount needed but rather
one of recognizing the environmental pollution caused by over-application.
It is fundamental to pollution control that fertilizers should not be added
unless they are needed. It is assumed that this first step of accurately
matching the rate of application of N and P nutrients to the needs of a given
crop has already been taken; that a realistic yield goal consistent with the
crop production capability of the soil and climate conditions has been
determined; and that proper management adjustments of recommended fertilizer
application have been provided.
Concentration of nutrients in the soil alone is not a good indicator of
potential losses. Nutrient concentration reflects both the amount and
timing of applications based upon the plant demands for the type of nutrient
applied. Both amount and timing strongly affect the efficient use of
nutrients by plants. The least possible nutrient loss will occur when
fertilizer is applied as close as possible to the time of use by the crop.
The management of nutrient application methods in relation to plant root
distribution and soil moisture is important in increasing the effective use
of fertilizers. Excessive application, along with improper timing and/or
improper method of fertilizer application contributes to nutrient losses
through direct runoff and deep percolation. (Stewart, B. A., et al., 1975,
1976; Harmon and Duncan, 1978).
A list of management alternatives for optimum nutrient application is
given in Table 45. The effectiveness of these management practices was
estimated for limiting nutrient losses from either surface or sprinkler
irrigation methods.
Mobile Nutrients Under Surface Irrigation
The application of nutrients in irrigation water, a practice referred
to earlier as fertigation, is becoming more extensive. Through careful
timing, the supply of mobile nutrients can more closely match the peak crop
demand. However, the proper management of irrigation and nitrogen applica-
tion is necessary even if the nitrogen is applied with the irrigation water.
107
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TA"-E « MANAGEMENT ALTERNATIVES FOR OPTIMUM (NOT EXCESSIVE) NUTRIENT APPLICATION
Nutrient application
method -'
SURFACE
Increment as needed
(Fcrtlgatlon)
Split (Preplant &
side-dress)
Side-dress only
Preplant (Spring)
Fall applied
SPRINKLER
Increment as needed
(Fcrtigatlon)
Split (Preplant S
side-dress)
Side-dress only
Preplant (Spring)
Fall applied
Coarse-textured soil _.
(Sands to loamy sands) —
Deep percolation Surface runoff
Hose
mobile
S
H
S
L-M
L
S
M
S
L-M
L
Least Host Least
mobile mobile mobile
UA */ 5/6/ SA 4/
NA
NA
& NA
NA
NA
NA
01
NA -' NA
NA -' NA
NA
NA
— ' NA i
NA
NA
NA
8/
NA - NA
Deep
Hose
Medium-textured soil -.
(Loams to sllty loams) -
percolation Surface runoff
Lease Host Least
mobile mobile mobile mobile
S
M
H-S
NA i/ 5/6/ NA 4/
NA
-' NA
L-H -' NA <
L
S
9/
NA
NA
NA
HI
NA - NA
-' NA ^ NA
NA
S — ' NA
M
L
NA ,
NA
NA
, NA
NA - NA
(Sllty clays to
soil ,.
clays) il
Deep percolation Surface runoff
Most Least Most Least
mobile irobllc mobile mobile
s NA *' '/I/ NA i/
M-S -' NA
S NA
M NA
NA
NA
NA
L NA - NA
S -' NA -' NA
"
S -' NA
H-S 1A
NA
NA
NA
M NA - NA
- Frere, 1976, Fried, et al , 1976, Homer, et al , 1978. McNeal and Pratt. 1978, Stewart. B A , et al . 1975 & 1976,
Harmon and Duncan, 1978
— The potential for leaching losses decreases for all the nutrient application practices as the soil
becomes finer-textured.
— Ranges In percent reduction of nutrient losses compared to existing methods without the specific
management method for nutrient application low (L) 0-10 percent, moderate (H) 10-50 percent,
substantial (S) 50-100 percent Comparison of the relative efficiency of various practices arc
made only within soil tcxtural classes and not across soil classes For example, an (S) rating
for a practice on sandy soil may connote a greater overall reduction of potential than an (S)
rating on the finer-textured soils
— NA Indicates these particular fertilizer management practices have no influence on irrigation-related
pollution problems
- Incorporation of fertilizer will result in a moderate reduction in nutrient losses by runoff and erosion
but would have little or no effect upon leaching
— The only surface runoff losses of major consequence will occur with the most mobile nutrients with
fertigatlon management practices. In surface irrigation when no tailwoter reuse system is employed or
with sprinkler application rates which exceed water intakes, and surface water runoff occurs
Rating depends a great deal on soil Intake rate. A well-structured soil with a higher water Intake rate
will have a higher Impact by this practice
8/
— Winter application of fertilizer on frozen ground nay increase loss of nutrients because of
surface water runoff
- There Is no need for a split application on these soils with proper water management practices
— This rating depends an the N form and amount of spring rain. If the early season rainfall is less
than 15 cm, both ammonium nitrate and ammonia fertilizers would have a rating of M. However, if the
spring rainfall is less than 15 cm, ammonium nitrate fertilizer only would be rated L (Watts, ct al , 1978)
108
-------
Nitrates and other mobile ions are easily leached with excessive water
applications. Without exception, however, the potential losses of mobile
nutrients are the highest within the coarse-textured soils. Both the amount
of fertilizer applied and the amount and timing of irrigation water must be
controlled to limit leaching and thus, deep percolation pollution on coarse-
textured soils. When nutrients are applied with irrigation water on medium-
or fine-textured soils, a reuse system should be used to reduce the amount
of runoff contributed to surface return flows. Soil incorporation of nitro-
gen fertilizers can moderately reduce nutrient losses caused by erosion or
runoff but has little or no effect on deep percolation.
Split application, the application of part of the nitrogen in spring
and the rest as summer side dressing, combines some of the timing features
of each of the other methods of application. Fertilizer applications in
the fall may be acceptable if leaching or surface runoff is not a problem.
Coarse-textured soils are the exception. Residual nitrate or preplant
nitrogen fertilizers may result in excessive losses of N if heavy spring
rains occur, making them unavailable for the next season's crop. The
problem of leaching residual nitrates is not as serious for irrigators on
fine-textured soils because much more of the residual N generally remains
in the root zone. However, some leaching losses do occur on well-structured
soils.
Relatively Immobile Nutrients Under Surface Irrigation
The less mobile nutrients (phosphorus and calcium) generally are not
applied during the irrigation season. Although management of these
nutrients also reflects quantity, timing and placement of nutrient applica-
tion, these practices have almost no effect on either deep percolation or
runoff of these relatively immobile nutrients. Application practices like-
wise have little influence on losses because these nutrients are incorporated
into the upper root zone where they remain. Losses of these nutrients are
primarily related to sediment losses in surface runoff water and are deter-
mined almost entirely by the factors that influence sediment losses.
Specific actions which reduce water runoff and erosion during the irrigation
cycle will also reduce pollution from preplant and fall-applied less
mobile nutrients. Where surface irrigation is used on erodible slopes and
medium-textured soils, phosphate losses can be expected.
Mobile Nutrients Under Sprinkler Irrigation
As shown in Table 45, the estimated reduction in mobile nutrient losses
with sprinkler irrigation is generally the same as the estimated reduction
under surface irrigation. The amount and timing of both water and fer-
tilizer applications can be adjusted with sprinkler systems to meet the
needs of the growing crop. Leaching losses of sprinkler-applied nitrogen,
however, can still be substantial if too much water is applied during the
growing season. If water applications are based on scientific irrigation
scheduling procedures, losses of mobile nutrients applied through sprinkler
systems will be reduced.
109
-------
There is no need for a split application of mobile nutrients with
sprinkler irrigation on either medium- or fine-textured soils. Use of proper
water management practices alone will reduce deep percolation and nutrient
losses on these soils during the growing season. It should be noted, how-
ever, that the application of excessive quantities of mobile nutrients may
leave residual amounts that are subject to leaching during the winter and
spring.
Relatively Immobile Nutrients Under Sprinkler Irrigation
The management practices with the relatively immobile nutrients under
sprinkler irrigation is the same as that for surface irrigation methods.
These nutrient fertilizers can be applied whenever they can be incorporated
into the soil. Placement practices that promote the efficient use of these
nutrients by plants increases their effectiveness and limits nutrient build-
up. The control of runoff and erosion will limit pollution problems from
the less mobile nutrient sources.
Specific Nutrient/Water Management Situations
When feasible, nitrogen fertilizers should be applied to irrigated
lands when potential excessive runoff from rainfall is minimal. A heavy
rainfall immediately following an irrigation will result in nitrate leaching,
especially in sandy soils, even if the irrigation application is perfect in
amount and timing. Losses of mobile nutrients under these conditions may
require changes in methods, forms, or time of application.
Leaching hazards with surface irrigation systems are somewhat greater
than with sprinkler systems because there generally is more water applied
per irrigation under surface irrigation. Slow release forms of the more
mobile fertilizers could be used as an option under these situations.
The selection of nutrient management options can be based on soil and
plant tissue analyses. Soil and plant tests determine how much of which
nutrients are needed, as well as the timing of application in specific
situations. Soil testing and the use of recommended nutrient rates can be
an important step in reducing pollution from fertilizers. However, nutrient
losses probably cannot be reduced to zero levels if crop production is to be
maintained at present levels.
PESTICIDE MANAGEMENT
Pesticides are used to control a pest or pests in a crop on a particu-
lar site at a particular time. When properly used and managed, pesticides
are crop production aids that relieve farmers of toilsome work and increase
crop yields. When improperly used, pesticides can cause environmental
problems which overshadow their potential benefits. Much effort goes into
the registration of a pesticide. The U.S. Environmental Protection Agency
closely regulates registration and labeling by the manufacturer. Yet, it is
110
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the user who is ultimately responsible for proper and discreet use of these
materials. The best information on proper use is printed directly on the
label. Rates, timing, pests controlled, methods of application, precautions,
prohibitions, etc., are there for each user to read and apply to any specific
situation. If these instructions are heeded, the risk of pesticide pollution
will be minimized from the start. That's why emphasis should be placed on
reading and following label instructions.
Principal factors affecting pesticides in irrigation return flow are
soil texture (with associated organic matter content) and the adsorptivity
characteristics of the individual pesticides. These were discussed in
Section 2. Most pesticides are found both in the water and adsorbed to the
sediments. The stronger the adsorption of the pesticide, the more likely it
is to move with the sediment. Reduced movement of both water and sediment
will have an impact on reducing pesticide losses, but the water phase usually
accounts for most pesticide movement. In coarse-textured soils, percolation
is of greatest concern. In fine-textured soils, runoff is more likely.
In Iowa's base report (Harmon and Duncan, 1978), estimated percentages
of applied pesticides dissolved in runoff water which reached surface streams
were 0 to 1 percent for the weakly adsorbed pesticides, 0 to 5 percent for
the moderately adsorbed, and 0 to 0.5 percent for the strongly adsorbed.
(A pesticide that is strongly adsorbed onto soil colloids is less likely to
be found dissolved in runoff water and more likely transported by sediments.)
The percentages transported by sediments were estimated at 0 to 0.1 percent
for the weakly adsorbed, 0 to 1 percent for the moderately adsorbed, and
0 to 2 percent for the strongly adsorbed pesticides. Under very unusual
soil-rainfall-pesticide conditions, these values could quadruple.
Wauchope (1978) states that for most commercial pesticides, total losses
are 0.5 percent or less of the amounts applied unless heavy rainfall occurs
within one to two weeks after application. Pesticides formulated as wettable
powders are lost more readily than other formulations. On slopes up to
10 percent, losses of these pesticides are estimated at 2 percent; on slopes
over 10 percent, pesticide losses are estimated at 5 percent.
Practices which reduce the need for a pesticide application or which
increase the effectiveness of the application and allow a lower application
rate reduce the risk of movement outside the area of application. The con-
trol of water loss is the key to reducing pesticide losses. Even the least
persistent pesticide will move if water runoff occurs shortly after applica-
tion. Because rainfall amount and intensity are unpredictable, some losses
may occur under even the best management systems. The goal should be loss
reduction to maintain water quality while pest management practices are kept
flexible enough to fit specific situations.
Management options which may affect pesticide losses in water and
sediments are given in Table 46. Those management practices which can reduce
runoff or percolation losses are given a moderate or substantial impact rat-
ing. A low rating implies that the overall impact is estimated to be minor
or non-existent under normal conditions. Where extreme conditions such as
high soil slope, an intense storm soon after application which produces large
111
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TABLE 46 MANAGEMENT PRACTICES TO REDUCE PESTICIDE CONTRIBUTIONS IN IRRIGATION RETURN FLOWS AND THEIR RATED POLLUTION REDUCTION -
II
ro
Coarse-textured soils
(Sands to loamy sands)
Management practice
Irrigation and soil management-
Timing of irrigation
Reduce excessive treatment
Use of alternative pesticide
Apply optimum dosage for pest
Optimum timing of application
Optimum placement of pesticide
Application techniques
Pesticide formulation
Alternative controls
Deep percolation
S
M
M
M
M
L
L
L
L
S
S
M
L
M
M
L
L
L
L
S
L
L
L
L
L
L
L
L
L
L
Surface water
runoff
U
L
L
L
L
L
L
L
L
L
L
M
L
L
L
L
L
L
L
L
L
L
S
L
L
L
L
L
L
L
L
L
L
Medium-textured soils
(Loams to silt loams)
Deep percolation
W M S
M
L
L
L
L
L
L
L
L
M
L
L
L
L
L
L
L
L
L
L
L
L
L
L
L
L
L
L
L
L
Surface water
runoff
U
S
M
M
L
M
L
L
L
si'
s
M
S
M
M
L
L
L
L
L
si'
s
s
M
L
L
L
L
L
L
L
L
M
Fine- textured soils
(Silty clays to clays)
Deep percolation
W
L
L
L
L
L
L
L
L
L
L
M
L
L
L
L
L
L
L
L
L
L
S
L
L
L
L
L
L
L
L
L
L
Surface water
runoff
U
S
M
M
L
M
L
M
L
si'
s
M
S
M
L
L
L
L
M
L
si'
s
s
s
L
L
L
L
L
L
L
L
S
— Rating to reduce pesticide pollution represents estimated potential Impact, not necessarily feasibility of practices
L - Low impact (0-10 percent), M - Moderate impact (10-50 percent.), S - Substantial imnact (over 50 percent)
-/ W - Weakly Adsorbed Pesticides
- M - Moderately Adsorbed Pesticides
- S - Strongly Adsorbed Pesticides
-' Refer to Tables 43 and 44
— For wettable powders
-------
runoff, careless application technique, poor judgment, etc., are involved,
Che impact may be higher. The influences of these management practices were
rated according to their ability to reduce pollution from the 1 to 5 percent
level that may be occurring over a broad region. In certain situations,
the impact may be higher or lower depending on the circumstances. If present
practices are well-managed, current losses may be negligible. On the other
hand, poor management can allow high losses which could be substantially
reduced by the adoption of better practices. Specific conditions will
dictate the ultimate impact of these practices. Table 46 should be viewed
as an aide to identifying management practices which will influence pesticide
movement in specific soil/pesticide groupings. Impact ratings are broadbased
and may not adequately describe local situations.
Irrigation and Soil Management
Practices which help retain the soil and water substantially reduce
pesticide losses. These would be expected to help to the same degree that
they are rated for reducing deep percolation and runoff, as discussed in
earlier sections (Tables 43 and 44). Furthermore, the topsoil, because of
higher populations of micro-organisms and greater chemical activity, has the
greatest potential to decompose pesticides. Once removed from the topsoil,
the decomposition rate decreases and persistence increases.
In general, fine-textured soils with a relatively greater amount of
organic matter have greater potential for pesticide decomposition than
coarse-textured soils. However, the former also have greater adsorptive
capacity and slower water infiltration rates with the result that pesticide
pollution potential from runoff and erosion is greater from fine-textured
soils. On the other hand, deep percolation is a greater risk in coarse-
textured soils.
Excessive Treatment
When pesticides are applied more frequently than necessary or used as
preventative treatments, pollution risk is increased. The cost of a
pesticide treatment considerably reduces this type of activity in the crops
under consideration. In any case, excessive applications should be avoided.
Helping the grower identify potential pest problems and determining when
pesticides should be applied is a major activity of many consultants and
specialists. This concept is embodied in integrated pest management (IPM),
although the term has broader applications. To date, insect pests have
received major attention in IPM studies because of the ability of these
pests to proliferate rapidly. During the crop's growth, the populations
of destructive insects are monitored and control techniques applied if and
when needed to control the insect and produce the crop. In some cases,
natural enemies may be sufficient to hold the insect in check while insecti-
cides may be essential at other times. The same concept works for weeds,
diseases, and other pests, though weeds are more predictable and emergency
control measures for diseases are often not available.
113
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Pesticides are an important part of most IPM programs because they are
immediately effective in slowing or controlling the pest. However, they
should be used only as needed and not used indiscriminately. This type of
program has been successful in reducing the numbers of insecticide applica-
tions required in some crops, especially cotton. The key is that the pests
are monitored and a pesticide is only used if and when needed for maximum
benefit. Though not useable in all situations, the concept is sound and
should be used where applicable. Avoiding the wholesale use of pesticides
as preventive treatments will reduce the pollution potential of pesticides,
particularly in those situations where percolation or runoff are most likely
to present problems.
Alternative Pesticides
Once the Environmental Protection Agency has approved a pesticide label,
then on-farm selection must be made based on the pesticide's ability to
control a specific pest in a specific situation. Where suitable alternatives
exist, persistence and adsorption characteristics should be considered. It
is possible that the use of less persistent pesticides may result in a com-
pensating increase in the number of applications required to provide effect-
ive control. In cases where this is not true, then the use of less persis-
tent pesticides will reduce pollution potential. Where deep percolation is
more likely (coarse-textured soils), a pesticide that is strongly adsorbed
onto soil colloids will leach less than a weakly adsorbed pesticide.
While this option may be effective in reducing pesticide movement,
suitable alternative pesticides to control the pest in a particular situation
are not always available. Cancellation of registration for many of the
chlorinated hydrocarbon insecticides has lessened the impact of this
alternative.
Optimum Dosage
Applying the optimum dosage for the pest will result in the best
benefit-to-risk ratio. Overdosage will certainly increase the amount that
is available for runoff or percolation, and underdosage may force retreatment
which would increase the total amount applied. Dosage is most likely to
affect weakly adsorbed pesticides since their loss with water can happen so
readily. Persistence is also a function of dosage. Label instructions for
optimum dosage should be followed.
Optimum Timing
A pest is usually most vulnerable to a pesticide at some particular
point in its life cycle. By applying the pesticide at that time, both
effectiveness and efficiency of the pesticide is increased. A reduction of
both the amount needed and the frequency of application is thereby possible.
The time interval between pesticide application and irrigation should
be considered to avoid a pollution event. Normally, the two-week interval
114
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immediately after application is a critical period. Application of foliar
pesticides should be avoided for 8 to 24 hours before periods of rain and
sprinkler irrigation. This will maximize effectiveness and minimize plant
leaf washoff that can accumulate in runoff and soil particles. The impact of
optimum pesticide application timing shown in Table 46 is estimated to be low
because large differences in timing of pesticide applications normally do not
occur.
Optimum Placement
If a pesticide is optimally placed, its maximum effectiveness for pest
control is realized. Unless the pest comes into contact with the pesticide,
the pesticide is wasted and repeated applications will be necessary, particu-
larly for short residual pesticides.
Soil incorporation of wettable powder pesticide formulations has been
shown to decrease washoff from the soil. In this case, moderate impact may
be realized on fine-textured soils (Table 46).
Application Techniques
Techniques which place the highest proportion of the applied pesticide
dosage on the target are the most efficient for controlling a pest. A less
efficient method may require higher dosages to compensate for losses, there-
by increasing pollution potential. Method of application, spray volume,
nozzle selection, equipment calibration, sprayer operation, marking systems,
and climatic factors such as wind conditions can significantly affect
application efficiency. Aerial application efficiency is undoubtedly subject
to greater variation because of lower spray volumes and higher droplet re-
lease heights than most ground application equipment. Application directly
to the pest or host crop is probably the most efficient technique, but not
always feasible. Soil incorporation is essential for the performance of
some pesticides and any delay can mean significant loss of the pesticide
and poor pest control.
Application techniques require careful attention, because any practice
that reduces application efficiency may result in poor pest control and a
possible need to retreat. Although careful attention to sound application
techniques is vital for effective pesticide usage and pollution reduction,
it is unlikely that the overall pollution reduction will exceed 10 percent,
except where proper techniques are being ignored or slighted.
Pesticide Formulation
Wettable powder formulations of pesticides are readily lost if the first
rainfall or irrigation after treatment produces runoff (Wauchope, 1978).
Where alternative formulations are available for a pesticide, switching to
an emulsion, suspension, or granule could substantially reduce runoff losses
of the weakly or moderately adsorbed pesticides. Of the major use pesticides
in Tables 29 and 30, atrazine, cyanazine, and propazine are sold as wettable
powders but are available as suspensions too.
115
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Alternative Controls
Alternative control methods include natural control with predators,
crop resistance, crop rotation, crop competition, cultivation, tillage,
planting date, etc. All control alternatives should be considered and
pesticides used only when needed. This does not necessarily mean that
pesticide use will decline, but could insure that indiscriminate use will
be reduced. In situations where pesticide percolation or runoff present
high risks the decision to use an alternative pest control method may have
a substantial impact on pollution reduction; however, it is unlikely that
large-scale replacement of pesticides will occur unless wholesale changes
are made in society. The trends to larger farms, fewer farm laborers, high
labor costs, high production costs, monocultures, and reduced tillage has
brought about increased use of pesticides for pest control. These trends
are likely to continue though at a decreasing rate.
116
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SECTION 4
ECONOMIC FEASIBILITY OF FARM MANAGEMENT ALTERNATIVES
TO REDUCE POLLUTION
by
R. J. Supalla and R. R. Lansford
INTRODUCTION
Policies designed to reduce pollution from irrigated agriculture should
consider the impact each alternative approach will have on both water quality
and the income of agricultural producers. Policies which would have a
positive effect on water quality and a neutral or positive effect on the
agricultural economy obviously are desirable. On the other hand, policies
which would have a positive effect on water quality but a negative effect on
the economy are appropriate only if two conditions are met. First, it should
be the least costly method of achieving the desired result and, second, the
improvement in water quality should be worth more to society than the cost of
achieving it.
This section provides information which can be used to determine whether
the above conditions hold for a selected array of major management alterna-
tives. The basic intent is to provide an indication of how the implementa-
tion of these management options would affect the economic well-being of
agricultural producers. No attempt is made to answer the broad economic
question of whether any given action is justified in terms of a total array
of costs and benefits. An answer to this broad economic question would re-
quire quantification of the social value of water quality improvements, a
task which was outside the scope of this study. Thus, the approach used
herein assumes that implementing selected farm management improvements would
enhance water quality and proceeds to assess the economic feasibility of
making such management adjustments.
MANAGEMENT OPTIONS TO BE EVALUATED
Available research resources are not sufficient for presenting an
economic assessment of all available options for reducing pollution from
irrigated agriculture. Therefore, this analysis treats only those options
which were determined to have potentially large economic effects and/or
which were believed to be the ones most likely to be seriously considered by
policy makers. With these criteria in mind, the following management options
were selected for analysis: (1) alternative types of irrigation systems
(surface, with and without reuse, automated gated pipe and center pivot
sprinklers); (2) irrigation scheduling (quantity and timing of water appli-
cations); (3) surface system water application procedures (length of run,
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and cutback); (4) reductions in water applications below full irrigation
requirements; (5) fertilization practices; (6) pesticide usage; and (7)
reduced tillage. The economic effects associated with each of these options
are discussed below.
ANALYTICAL PROCEDURES
The effect of management changes on producer incomes could be analyzed
in several ways, but the least difficult and most easily understood approach
is enterprise budgeting (Appendix C). This technique permits the estimation
of the differences in costs and returns of various management practices for
one or more farm situations.
The first step in the budget analysis was the selection of the appro-
priate farm situations to be analyzed. This was the critical step in the
analysis because cost and return estimates are often significantly affected
by situational parameters such as location and size of farm, water source
(well or canal), depth to water, crops produced, level of managerial skill,
etc. For this analysis, two hypothetical farm situations were selected; one
"typical" of the Southern Plains, and another "typical" of the Central Plains.
For the Southern Plains, the hypothetical farm situation was assumed to
consist of: a 323.8 hectare completely-irrigated farm producing about 80.9
hectares of cotton, corn, grain sorghum and wheat; irrigation was from 2.47
cubic meters per minute wells with 68.8 meters of lift and natural gas-
powered pumps; soils were medium-textured with 0 to 1 percent slope, and
above average management was assumed. _L' It was further assumed that the
model farm contained some nonirrigated lands (either summer fallow wheat or
rangeland) which could be irrigated if sufficient water were available, and
that the acreage tracts for the farm were laid out in a manner which per-
mitted either surface or center pivot irrigation. This permitted the assump-
tion that total irrigated acreage and the average annual cropping patterns
were the same (center pivot corners would be used for the nonirrigated
options) when assessing the economics of alternative types of irrigation
systems.
The hypothetical situation analyzed for the Central Plains consisted of:
a 315.6 hectare farm producing 157.8 hectares of irrigated corn, 52.6 hect-
ares of irrigated alfalfa, 52.6 hectares of irrigated grain sorghum and 52.6
hectares of dryland grain sorghum; irrigation was from 3.04 cubic meters per
minute wells, with 30.5 meters of lift and diesel-powered pumps; soils were
— The "above average" management assumption is reflected in the cultural
practices, fertilization rates and expected yields employed in the budgets.
However, this does not appreciably influence the estimated cost of irrigat-
ing with alternative systems, and therefore has little influence on the
conclusions which follow from the analysis.
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medium-textured with 0 to 1 percent slope; and above average management was
assumed. It was further assumed that the model farm was laid out in acreage
tracts which would permit either surface or center pivot irrigation, with
total irrigated acreage and the average annual cropping pattern remaining the
same. When the economics of a center pivot sprinkler were considered, it was
assumed that dryland grain sorghum was planted in the corners.
Cost and return estimates for the above farm situations were developed
for use in analyzing alternative management practices using the Agricultural
Computer Network (AGNET) crop budget generator at the University of Nebraska-
Lincoln. See Appendix C for details regarding the machinery complements,
input prices and other cost factors used for both hypothetical farm situa-
tions.
RESULTS - WATER MANAGEMENT OPTIONS
Costs and Returns for Different Types of Irrigation Systems
One of the major policy alternatives available for reducing pollution
from irrigated agriculture is encouraging a change from a type of irrigation
system which may be causing excessive runoff or deep percolation, under a
given set of circumstances, to one which would improve water management and
subsequently reduce the pollutants (sediments, nutrients, salts, etc.) enter-
ing the water system. Within the Great Plains one can find nearly every
available type of irrigation system, but some are used more than others and,
in some instances, there are no appreciable differences in costs or in water
quality. Therefore, only four typical irrigation systems were considered:
surface with and without reuse (siphon tubes in the Southern Plains and gated
pipe in the Central Plains), automated gated pipe, and center pivot sprin-
klers.
The economic implications of shifting from one type of system to another
were analyzed by estimating the net returns for each irrigation system, by
crop, for both the Southern and Central model farm situations. This approach
provides an estimate of what the net return differences would be if farmers
initially installed one type of system instead of another, or if they changed
irrigation systems when their current one was fully depreciated. The analy-
sis does not address the cost differences associated with abandoning a par-
tially depreciated irrigation system with a low salvage value and installing
another type. This omission is of little significance, however, as long as
we accept the likely proposition that farmers will not be required to change
systems before their existing ones are fully depreciated.
Southern Plains—
Net return differences between irrigation systems can occur because of
differences in irrigation costs, tillage practices and/or gross returns
(yields). However, given that good management and sufficient water availa-
bility have been assumed, it seems reasonable to expect that the yields
(gross returns) for all types of irrigation systems would be the same. Also,
over much of the Southern Plains, tillage practices are very similar for the
119
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types of irrigation systems under consideration. This means that net returns
vary between irrigation systems only to the extent that irrigation costs vary,
as illustrated in Tables 47 to 50.—'
In the case of corn (Table 47), the least costly irrigation system is
furrow irrigation (siphon tubes) with reuse. However, there is little
difference in net returns between furrow irrigation with and without reuse
pits. The with and without reuse differences are small, because with reuse
one is able to produce with less water (80 percent irrigation efficiency
instead of 60 percent). Consequently, variable costs are reduced by enough
($12.21 per hectare) to more than offset the increase in investment costs
due to the reuse pit and associated equipment ($7.20 per hectare).
The automatic gated pipe alternative, in the case of corn, generates a
per hectare net return of $28.76 below the furrow irrigation with reuse pit
alternative. This difference occurs primarily because shifting to automated
gated pipe increases irrigation fixed costs by $35.82, while irrigation labor
costs are reduced by only $7.90 per hectare.
The center pivot sprinkler is by far the least profitable alternative.
In the case of corn, center pivot irrigation costs $117.84 per hectare more
than the most attractive alternative, furrow irrigation with reuse. Center
pivot costs are higher primarily because of the increased fuel costs to
pressurize the system and the investment cost of the pivot and associated
equipment. Irrigation labor and land leveling charges are somewhat lower,
but they do not come close to compensating for the cost components which are
increased.
Cotton is similar to corn in that furrow irrigation with reuse is the
least costly alternative ($149.94 per hectare) and there is little difference
in net returns between the furrow irrigation with and without reuse pits
(Table 48). The irrigation variable costs are $11.86 per hectare lower with
furrow irrigation when reuse pits are used, because irrigation water pumpage
is reduced five hectare-centimeters. Fixed costs are increased by $7.19
because of the investment in the reuse pit and associated machinery. The
resulting net difference due to reuse is only $4.67.
Net returns from cotton production using automatic gated pipe or center
pivot irrigation are, respectively, $29.88 and $118.39 per hectare below that
expected from furrow irrigation with reuse pits. These differences occur in
the case of cotton for the same reasons discussed above in the case of corn.
Grain sorghum follows a pattern for the four alternative irrigation
systems that is similar to corn and cotton (Table 49). Again, furrow
— It could be argued that purchased inputs such as fertilizer will vary
between options because of such things as less leaching, but this effect
was ignored on the grounds that it would be very small and, in any case,
difficult to estimate.
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TABLE 47. PER HECTARE COSTS AND RETURNS FOR CORN (GRAIN)
BY TYPE OF IRRIGATION SYSTEM FOR THE SOUTHERN HIGH PLAINS. 1979
Type of irrigation system
Cost and
return items
Purchased inputs
Labor, except
irrigation
Variable machinery
costs
Fixed machinery
costs n ,
j_ /
Other costs —
21
Irrigation —
Labor
Variable machinery
Fixed machinery
Interest on
operating expenses
Total irrigation
Summary
Total cost _,
Gross returns —
Net returns to land
and management
without
reuse pits
_ _ _ _
139.19
25.45
167.21
41.49
38.62
21.74
64.87
69.80
5.76
162.17
574.13
722.62
148.49
Furrow irrigation
with
reuse pits
- - - dollars per
139.19
25.45
167.21
41.49
38.10
21.74
52.66
77.00
4.94
156.34
567.78
722.62
154.84
automatic
gated pipe
hectare - - -
139.19
25.45
167.21
41.49
38.67
13.84
53.40
112.82
4.47
184.53
596.54
722.62
126.08
Center
pivot
sprinkler
_____
139.19
25.45
167.21
41.49
41.83
5.93
102.25
158.78
7.19
274.15
689.32
722.62
33.30
— Includes interest on operating capital and an overhead charge.
2J The amount of water applied to the root zone was assumed to be 33.5
centimeters for each system. The gross applications were 55.9 centimeters
for furrow without reuse, 41.9 centimeters for furrow with reuse and
center pivot, and 39.4 centimeters for automatic gated pipe.
- Yield:
8156 kilograms; price: 8.86 cents per kilogram
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TABLE 48. PER HECTARE COSTS AND RETURNS FOR COTTON
BY TYPE OF IRRIGATION SYSTEM FOR THE SOUTHERN HIGH PLAINS, 1979
Type of irrigation system
Cost and
return items
Purchased inputs
Labor, except
irrigation
Variable machinery
costs
Fixed machinery
costs . .
Other costs —
without
reuse pits
59.30
20.88
202.00
57.23
33.19
Furrow irrigation
with
reuse pits
59.30
20.88
202.00
57.23
32.59
automatic
gated pipe
59.30
20.88
202.00
57.23
33.09
Center
pivot
sprinkler
59.30
20.88
202.00
57.23
35.93
costs . .
Other costs —
21
Irrigation —
Labor
Variable machinery
Fixed machinery
Interest on
operating expenses
Total irrigation
Summary
Total cost „ ,
Gross returns —
Net return to land
and management
57.23
33.19
19.77
59.75
69.80
5.29
154.61
527.21
892.42
365.21
57.23
32.59
19.77
48.63
77.00
4.55
149.94
521.94
892.42
370.48
57.23
33.09
12.85
49.49
112.82
4.15
179.32
551.82
892.42
340.60
57.23
35.93
5.93
93.65
158.78
6.42
264.78
640.12
892.42
252.30
— Includes interest on operating capital and an overhead charge.
2J The amount of water applied to the root zone was assumed to be 30.5
centimeters for each system. The gross applications were 50.8 centimeters
for furrow without reuse, 38.1 centimeters for furrow with reuse and
center pivot, and 46 centimeters for automatic gated pipe.
3/
- Yield: lint, 616.5 kilograms; cottonseed, 874.2 kilograms;
Price: $1.26 per kilogram lint, $132.27 per tonne cottonseed
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TABLE 49. PER HECTARE COSTS AND RETURNS FOR GRAIN SORGHUM
BY TYPE OF IRRIGATION SYSTEM FOR THE SOUTHERN HIGH PLAINS. 1979
Type of irrigation system
Cost and
return items
Purchased inputs
Labor, except
irrigation
Variable machinery
costs
Fixed machinery
costs j.
Other costs —
21
Irrigation —
Labor
Variable machinery
Fixed machinery
Interest on
operating expenses
Total irrigation
Summary
Total cost _ ,
Gross returns —
Net returns to land
and management
without
reuse pits
_ _ _ _
88.46
25.45
107.66
41.49
25.25
19.77
57.18
69.00
5.11
151.06
439.37
434.89
-4.48
Furrow irrigation
with
reuse pits
- - - dollars per
88.46
25.45
107.66
41.49
24.68
19.77
46.45
77.00
4.40
147.62
435.36
434.89
-.47
automatic
gated pipe
hectare - - -
88.46
25.45
107.66
41.49
25.18
12.85
47.54
112.82
4.03
177.24
465.48
434.89
-30.59
Center
pivot
sprinkler
------
88.46
25.45
107.66
41.49
27.95
5.93
89.35
158.78
6.33
260.39
551.40
434.89
-116.51
— Includes interest on operating capital and an overhead charge.
2/
— The amount of water applied to the root zone was assumed to be 29.0
centimeters for each system. The gross applications were 48.3 centimeters
for furrow without reuse, 36.1 centimeters for furrow with reuse and 34.0
centimeters for automatic gated pipe.
3/
— Yield: 5604.2 kilograms; price: 7.76 cents per kilogram
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TABLE 50. PER HECTARE COSTS AND RETURNS FOR WHEAT
BY TYPE OF IRRIGATION SYSTEM FOR THE SOUTHERN HIGH PLAINS. 1979
Type of irrigation system
Cost and
return items
Purchased inputs
Labor, except
irrigation
Variable machinery
costs
Fixed machinery
costs 1 ,
Other costs —
21
Irrigation —
Labor
Variable machinery
Fixed machinery
Interest on
operating expenses
Total irrigation
Summary
Total cost _ ,
Gross returns —
Net returns to land
and management
without
reuse pits
66.72
6.38
46.70
18.88
16.65
18.78
57.18
69.00
3.78
148.74
304.07
296.44
-7.63
Furrow irrigation
with
reuse pits
A 11
66.72
6.38
46.70
18.88
16.06
18.78
46.45
77.00
3.09
145.32
300.06
296.44
-3.62
automatic
gated pipe
•
66.72
6.38
46.70
18.88
16.14
11.86
47.54
112.82
3.14
175.36
330.18
296.44
-33-74
Center
pivot
sprinkler
66.72
6.38
46.70
18.88
17.96
5.93
89.35
158.78
6.33
260.39
417.03
296.44
-120.59
— Includes interest on operating capital and overhead charge.
21
— The amount of water applied to the root zone was assumed to be 29.0
centimeters for each system. The gross applications were 48.3 centimeters
for furrow without reuse, 36.2 centimeters for furrow with reuse and
34.0 centimeters for automatic gated pipe.
- Yield: 2690 kilograms; price: 11.02 cents per kilogram
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irrigation with reuse pits is the most profitable alternative, generating a
net return to land and management of -$0.47 per hectare with total irrigation
costs of $147.62. Furrow irrigation without reuse, automated gated pipe, and
center pivots were estimated to be $4.01, $30.12 and $116.04 per hectare less
profitable than furrow with reuse, respectively.
Wheat followed the same economic pattern as grain sorghum, with the
furrow irrigation without reuse pits alternative being the most profitable
and the center pivot being the least profitable alternative (Table 50).
Indeed, the differences were the same, because the amount of water applied
was assumed to be the same for both crops.
To summarize for the Southern Plains, furrow irrigation with reuse pits
appears to be the most viable alternative for reducing deep percolation and
runoff of irrigation water. Gross irrigation water applications can be re-
duced by about 20 percent below the alternative of furrow irrigation without
reuse pits with a slight improvement in net returns for all crops. Gross
irrigation water applications can be reduced another 5 to 10 percent by
going to the automatic gated pipe system; however, net returns on the average
are reduced 20 to 25 percent primarily because of the larger capital invest-
ment requirement for the automatic gated pipe system. The center pivot
system required about the same amount of irrigation water be pumped as that
required for the furrow irrigation with reuse pit, but the per hectare net
returns are greatly reduced.
Central Plains—
The results of the analysis of alternative irrigation systems in the
Central Plains (Tables 51 to 53) are quite similar to those for the Southern
Plains, except the magnitude of the differences between irrigation systems
tends to be smaller. The principal regional differences occur because lifts
and water application levels are generally lower and land leveling charges
higher in the Central Plains. As was the case for the Southern Plains, all
variations in net returns for each crop considered are due to variations in
irrigation costs, because tillage practices and purchased inputs do not vary
by type of irrigation system.
Gated pipe with reuse was found to be the least costly system for all
crops in the Central Plains example, but the with and without reuse cost
differences were very slight for all crops (Tables 51 to 53). Estimated
total irrigation costs with reuse were $160.71, $149.05 and $140.55 per
hectare for alfalfa, corn and grain sorghum, respectively, whereas without
reuse they were $168.20, $154.86 and $144.55 per hectare. The differences
are small, because with reuse one is able to produce with less water (80
percent irrigation efficiency instead of 60 percent) and consequently vari-
able costs are reduced by enough to essentially offset the investment costs
of reuse pit and associated equipment. The variations across crops are
primarily due to variations in the amount of water required.
For all crops, automated gated pipe irrigation is considerably less
expensive than center pivot sprinklers and only slightly more expensive than
gated pipe with reuse. The automated system was also more expensive than
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TABLE 51. PER HECTARE COSTS AND RETURNS FOR ALFALFA
BY TYPE OF IRRIGATION SYSTEM CENTRAL GREAT PLAINS. 1979
Type of irrigation system
Cost and
return items
Purchased inputs
Labor, except
irrigation
Variable machinery
costs
Fixed machinery
costs . ,
Other costs —
21
Irrigation —
Labor
Variable machinery
Fixed machinery
Interest on
operating expense
Total irrigation
Summary
Total cost , ,
Gross return —
Net returns to land
and management
Gated pipe
without
reuse pits
497.90
18.68
165.55
27.28
81.34
14.83
53.67
95.16
4.55
168.20
958.95
965.79
6.84
Gated pipe
with
reuse pits
i 11
497.90
18.68
165.55
27.28
80.92
14.83
45.54
96.34
4.00
160.71
951.04
965.79
14.75
Automatic
gated pipe
1» 4-
497.90
18.68
165.55
27.28
81.07
11.12
43.86
109.32
3.66
167.95
958.43
965.79
7.36
Center
pivot
sprinkler
497.90
18.68
165.55
27.28
84.75
7.41
108.28
131.97
7.68
255.35
1049.51
965.79
-83.72
— Includes interest on operating capital and an overhead charge.
21
— The amount of water applied to the root zone was assumed to be 40.6
centimeters for each system. The gross applications were 67.8 centimeters
for gated pipe without reuse, 50.8 centimeters for gated pipe with reuse
and 47.8 centimeters for automatic gated pipe and center pivot systems.
3/
— Yield of 14.6 tonnes per hectare at $66.15 per tonne.
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TABLE 52. PER HECTARE COSTS AND RETURNS FOR CORN
BY TYPE OF IRRIGATION SYSTEM CENTRAL GREAT PLAINS. 1979
Type of irrigation system
Cost and
return items
Purchased inputs
Labor, except
irrigation
Variable machinery
costs
Fixed machinery
costs . ,
j_ /
Other costs —
21
Irrigation —
Labor
Variable machinery
Fixed machinery
Interest on
operating expenses
Total irrigation
Summary
Total cost , ,
3/
Gross returns —
Net returns to land
and management
Gated pipe
without
reuse pits
______
139.19
20.66
58.64
88.14
25.28
11.86
44.11
95.16
3.73
154.86
486.77
722.62
235.85
Gated pipe
with
reuse pits
- - dollars per
139.19
20.66
58.64
88.14
24.93
11.86
37.56
96.34
3.29
149.05
480.61
722.62
242.01
Automatic
gated pipe
i .
139.19
20.66
58.64
88.14
25.06
8.90
36.35
109.32
3.01
157.57
489.26
722.62
233.36
Center
pivot
sprinkler
139.19
20.66
58.64
88.14
28.05
5.93
88.19
131.97
6.25
232.34
567.02
722.62
155.60
— Includes interest on operating capital and an overhead charge.
21
— The amount of water applied to the root zone was assumed to be 32.5
centimeters for each system. The gross applications were 54.1 centimeters
for gated pipe without reuse, 40.6 centimeters for gated pipe with reuse
and 38.4 centimeters for automatic gated pipe and center pivot systems.
3/
— Yields of 8156 kilograms per hectare at 8.86 cents per kilogram.
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TABLE 53. PER HECTARE COSTS AND RETURNS FOR GRAIN SORGHUM
BY TYPE OF IRRIGATION SYSTEM CENTRAL GREAT PLAINS. 1979
Type of irrigation system
Cost and
return items
Purchased inputs
Labor, except
irrigation
Variable machinery
costs
Fixed machinery
costs . ,
Other costs —
Gated pipe
without
reuse pits
75.86
17.77
37.88
73.78
14.65
Gated pipe
with
reuse pits
i 1 -i
75.85
17.77
37.88
73.78
14.38
Automatic
gated pipe
i ,
75.86
17.77
37.88
73.78
14.58
Center
pivot
sprinkler
75.86
17.77
37.88
73.78
16.85
costs . ,
Other costs —
21
Irrigation —
Labor
Variable machinery
Fixed machinery
Interest on
operating expense
Total irrigation
Summary
Total cost 0/
3/
Gross returns —
Net returns to land
and management
73.78
14.65
11.86
34.45
95.16
3.09
144.55
364.49
487.06
122.57
73.78
14.38
11.86
29.58
96.34
2.77
140.55
360.21
487.06
126.85
73.78
14.58
8.90
28.66
109.32
2.50
149.38
369.25
487.06
117.81
73.78
16.85
5.93
68.08
131.97
4.92
210.90
433.04
487.06
54.02
— Includes interest on operating capital and an overhead charge.
2/
— The amount of water applied to the root zone was assumed to be 24.4
centimeters for each system. The gross applications were 40.6 centimeters
for gated pipe without reuse, 30.5 centimeters for gated pipe with reuse
and 28.7 centimeters for automatic gated pipe and center pivot.
— Yield: 6,276.6 kilograms; price: 7.76 cents per kilogram
128
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gated pipe without reuse except in the case of alfalfa. It is more costly
than gated pipe with reuse, because the relatively high investment costs
associated with the system are not entirely offset by lower labor and vari-
able machine costs. When compared to gated pipe without reuse, however, the
improved efficiency of the automated system lowers variable costs enough to
more than offset the higher investment costs in the case of high water-using
crops such as alfalfa. It also should be noted that automated gated pipe is
much more attractive for the Central Plains situation than it is in the
Southern Plains. This difference occurs because for the Southern Plains
example, automated gated pipe was compared to siphon tubes which are consid-
erably cheaper than gated pipe.
The most costly type of irrigation system for all crops is center pivot
sprinklers, costing $255.35 per hectare for alfalfa, $232.34 for corn and
$210.90 for grain sorghum. These costs are 50 to 59 percent higher than the
least costly options for each crop, due primarily to the additional fuel
costs for pressurizing a center pivot and to the cost of the machine itself.
From the above cost analysis it appears that policies to require or
encourage different types of irrigation systems as a method of improving
water quality could have relatively severe economic consequences. For
example, a shift from gated pipe without reuse to center pivot sprinklers
would increase production costs (reduce net returns) by $21,033 for the
entire farm ($90.56 x 52.6 hectares alfalfa + $80.25 x 157.8 hectares corn
+ $68.55 x 52.6 hectares grain sorghum).
The only pollution-reducing irrigation system changes which apparently
could be implemented without substantial economic losses are the installa-
tion of reuse pits or a shift from gated pipe without reuse to automated
gated pipe. It was estimated that a shift from gated pipe without reuse to
gated pipe with reuse would increase annual net returns by $1,613 ($4.28
x 52.6 hectares + $6.16 x 157.8 hectares + $7.91 x 52.6 hectares = $1,613).
This action is therefore clearly desirable from both a water quality and an
economic point of view. On the other hand, to go a step further and shift
from gated pipe with reuse to automated gated pipe would reduce annual net
returns by $2,229. This decrease appears small enough to make automated
gated pipe a viable alternative, depending on the magnitude of the potential
water quality improvements.
As a final note, it is important to point out that the above analysis
assumes equal yields for all irrigation systems. This is probably a reason-
able assumption if well capacities are great enough to provide all plant
water needs even in dry years. But, if this is not the case, then the more
efficient systems would be more attractive economically than depicted above.
Irrigation Scheduling
Another important irrigation management option for improving water
quality is irrigation scheduling. Many irrigators apply excess water to
their crops because of limited information regarding the amount of water
needed at particular times during the season. Excess water applications,
129
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which often increase runoff and leaching, can be reduced by following one of
several available scheduling techniques.
The financial impact from requiring irrigators to follow a particular
scheduling practice depends on the amount of water saved, the cost of sched-
uling, impact on fertilizer needs, and the yield effect, if any. Individual
irrigators will therefore be affected differently, but a good idea of the
nature of the financial impact can be obtained by examining two illustrative
cases, one for the Central Plains and another for the Southern Plains.
Improved irrigation scheduling appears to have substantial potential
for improving water quality in the Central Plains where water is plentiful
and where over-irrigating is quite prevalent. For example, a study of
irrigation practices in the Benedict area of east central Nebraska revealed
that irrigators who followed technical scheduling procedures applied an
average of 34.8 centimeters, which was estimated to be 35 percent less than
the average for all irrigators in the area (Noffke, et al., 1975). Assuming
a diesel-powered gravity system and 15.24 meters of head, the cost savings
from pumping 18.8 (53.59 - 34.8) fewer centimeters per hectare amounts to
about $25.29 per hectare. In addition, it has been estimated that a savings
of as much as 56.0 kilograms per hectare of nitrogen can be obtained with
scheduling and at the current price of 22 cents per kilogram, this amounts
to a savings of $12.32 per hectare. Therefore, given no change in yields
and assuming a scheduling service can be purchased for $6.18 per hectare,
the net gain from scheduling in our Central Plains example totals $31.43 per
hectare ($25.29 + $12.32 - $6.18 = $31.43).
For the Southern Plains, the issues associated with scheduling are the
same, but the potential impacts are less. Impacts are likely to be less
because water is less plentiful and thus there is generally less over-
irrigating. In contrast to the Central Plains illustration where a 35 per-
cent reduction in water use was assumed, the potential savings in the South-
ern Plains is likely to be 10 percent or less. Assuming a natural gas-
powered gravity system without reuse and 68.6 meters of head, the cost sav-
ings from pumping 5.08 centimeters less (about 10 percent) would be only
$5.88 per hectare. Likewise, the reductions in nitrogen leaching would
likely be proportionately less, perhaps 6.8 kilograms, for a per hectare
fertilizer savings of $1.50. Thus, a "typical" net gain from scheduling in
the Southern Plains totals only $1.20 per hectare ($5.88 + $1.50 - $6.18 =
$1.20).
It seems reasonable to conclude from the foregoing analyses that
irrigation scheduling will usually have a positive economic impact on
irrigators and that, especially for the Central Plains, this impact could
be very substantial. Thus, scheduling is an attractive management practice
from both a water quality and an economic point of view.
Reduce Water Applications Below Full Irrigation Requirements
The foregoing analysis of scheduling treats the economic consequences
of policies to avoid water applications in excess of crop needs. It is
130
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possible, although not necessarily desirable, to go a step further and
reduce water applications below crop needs. This potentially could improve
water quality beyond what is possible with scheduling to meet crop needs, but
yield reductions are likely to make the economic consequences quite severe.
Yield decreases from reducing water applications below crop needs will
vary widely from region to region and by crop. However, one can get an idea
of the potential magnitude of these effects by examining one situation for
which the necessary data are available. John Shipley of Texas A & M Univer-
sity estimated yield/water-applied relationships for wheat, corn and grain . ,
sorghum in the Northern High Plains of Texas (Shipley, 1977; Larsen, 1978).-
By using his equations to approximate yield reductions from less water use
and given the available data on variable irrigation costs for ditch irriga-
tion with reuse, one can assess the net economic effect from reducing water
applications below crop needs.
Shipley found that the production function for corn was of the form
Y = 980.8 + 234.65X - 1.56X2 (3)
where Y = yield in kilograms per hectare and X = total water applied in
centimeters. This means, for example, that if gross water applied to corn
was reduced from 61.0 to 40.6 centimeters, yield would decrease by 1,553.54
kilograms per hectare. At 8.86 cents per kilogram, this is a reduction in
gross returns of $137.64. Total costs would be reduced by approximately
$46.40 (irrigation variable costs, $23.52; seed, $5.19; and fertilizer,
$17.69), for a net economic loss of $91.24 per hectare, or $4.47 per centi-
meter reduction in water applied.
The corresponding production function for grain sorghum was
Y = 2,434.1 + 231.5X - 2.59X2 (4)
where Y = yield in kilograms per hectare and X = total water supplied in
centimeters. Thus, a reduction in water applied to grain sorghum of from
40.6 to 30.5 centimeters is estimated to reduce yields by 478 kilograms per
hectare. At a price of 7.67 cents per kilogram, this reduces gross returns
by $36.66 per hectare. Total costs would be reduced by $18.31 (irrigation
variable costs, $11.76: fertilizer, $5.34; and seed, $1.21), for a net
economic loss of $18.35 per hectare or $1.82 per centimeter reduction in
water applied.
In the case of wheat, Shipley's estimated production function is of
the form
— The Shipley analysis considered changes in irrigation timing as well as
the quantity of water applied, and this is reflected in the estimated
yield/water-applied relationships.
131
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Y = 1,091.3 + 100.8X - .94X2 (5)
where Y = yield in kilograms per hectare and X = total water applied in
centimeters. This means that if the water applied to wheat is reduced from
40.6 to 30.5 centimeters, yields decrease by 343 kilograms per hectare, for
a reduction in gross returns at 11.02 cents per kilogram of $37.80 per hect-
are. Production costs would be reduced by $15.69, (irrigation variable costs,
$11.76; and fertilizer, $3.93) resulting in a reduction in net returns of
$22.11 per hectare, or $2.19 per centimeter reduction in water.
Shipley's analysis indicates quite clearly that the economic losses
from reducing water applications below crop needs are quite severe, espe-
cially for corn. However, it is important to note that a program which
precluded full irrigation of corn would probably result in a substitution
of sorghum for corn, making the economic losses less severe than they appear
to be from Shipley's analysis. The precise impact would depend on the
relative profitability of corn and sorghum, crop rotation constraints and
the specific dimensions of any program which prevented full irrigation.
Another factor which policy makers should keep in mind is that the
economic impact of reducing water applications below crop needs will probably
vary widely by geographic area, soil type, etc. Thus, there may be situa-
tions where programs to encourage or require less than full irrigation are an
economically-feasible means of improving water quality, but given the magni-
tude of the illustrated consequences, this appears unlikely.
Other Water Management Practices
There are numerous water management practices, in addition to schedul-
ing and reducing water applications below crop needs, which could be used to
potentially improve water quality. The most important of these appear to be
reducing the length of run and using a cutback system.
Length of Run—
Irrigation efficiency can be increased by reducing the length of field.
However, there are some trade-offs that should be considered before recom-
mending that length of run be reduced. The primary trade-off considerations
are: (1) increased labor requirements, (2) reduced machinery efficiency,
and (3) loss of productive land by increasing the turn row areas. If length
of runs are reduced, more irrigation time will be required if for no other
reason than because of the increase in the number of irrigation sets. In
addition, machine and labor efficiency may be reduced and productive land
lost if the tractor operator has to turn around twice as often because of
shorter fields. There are methods to reduce the impacts on machinery and
loss of productive land but these alternatives tend to be capital intensive.
One of the more popular of the alternatives is the use of underground pipe
with laterals and hydrants across fields. However, it does not appear that
this will save additional irrigation pumpage above furrow irrigation with
reuse pits, and it would clearly reduce net returns per hectare because of
the capital investment.
132
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Cutback--
The cutback system involves reducing the volume of water being placed
on a field when the field is two-thirds to three-quarters irrigated. Typi-
cally, siphon tubes or gated pipe are used with this system. The major
economic factor associated with this system is the increased amount of labor
required. To fully use this system, full-time irrigators may be required.
For example, on corn, the hours of irrigation labor per year per acre may
almost triple to perhaps three hours per acre. In many areas of the Great
Plains it would be almost impossible to find this much temporary labor during
the irrigation season. Irrigation water savings would be expected to be in
the same magnitude as that for automatic gated pipe. Economically, it appears
that it is almost a direct trade-off between fixed capital investment for the
automatic gated pipe system and the increased annual irrigation labor costs
for a manual system.
ECONOMICS OF REDUCED TILLAGE
One of the practices available for reducing runoff from irrigated lands
is reduced tillage. Reduced tillage techniques disturb the soil less than
conventional practices and leave more crop residue on the land surface, thus
reducing runoff.
The central economic question associated with the reduced tillage option
is: how is the profitability of agriculture affected by a shift from con-
ventional to a reduced or conservation tillage system? If net returns are
substantially reduced by a shift to conservation tillage, it is probably a
feasible means of improving water quality only if large public subsidies are
available. On the other hand, if conservation tillage yields are near or
equivalent to net returns, it is a near "costless" means of improving water
quality. Unfortunately, the answer to this central question will depend on
the region, the crop, the type of conservation tillage, and other factors,
and it is not possible to consider all situations. Accordingly, this
analysis will consider only two illustrative situations as a means of assess-
ing the general magnitude of the impact.
The situations to be considered involve gated pipe and center pivot
irrigated corn in the Central Plains. The first situation consists of
comparing conventional gated pipe with reuse irrigation (Table 52) with a
reduced tillage system. The assumed reduced tillage system differs from
conventional tillage in the following ways: two tandem discings are elimi-
nated and replaced by a once-over rotary till-plant operation; one cultiva-
tion is replaced by a rotary till operation; and net water application is
reduced by 2.54 centimeters. It is further assumed that the tillage shifts
occur for all row crops on the model farm and that all other inputs remain
the same.
The second illustrative situation consists of comparing conventional
center pivot irrigated corn in the Central Plains with a reduced tillage
system which differs from the conventional in the following ways: two tandem
discings are eliminated and replaced by a rotary till-plant operation;
133
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additional herbicide (broadcast instead of band) is substituted for two
cultivations; and net water application is reduced by 5.1 centimeters.
Again, it is assumed that this shift occurs for all row crops and that no
other changes in the farm operation occur.
The results of the analysis indicate that the gated pipe reduced tillage
system yields slightly higher net returns ($5.69 per hectare) than conven-
tional tillage (Table 54). The reduced tillage net returns are higher,
because of slight irrigation costs and labor savings. Total machinery costs
do not change much because the cost savings from the reduced number of field
operations are essentially offset by the need to purchase a new $12,000 rotary
tiller. It also should be noted that yields have been assumed constant,
whereas in actual practice they could vary by perhaps plus or minus 20 per-
cent.
The results for the center pivot reduced tillage alternative are quite
different. They indicate that reduced tillage yields somewhat lower net
returns, $140.73 per hectare compared to $155.60. The principle reason for
this difference is the $37.06 increase in purchased inputs to meet greater
herbicide requirements. Essentially, the slightly lower labor, irrigation
and machine costs fail to compensate for the higher herbicide requirement
that is necessary for effective weed control. Again, projected yields have
been assumed constant.
In general it appears that there are economically-attractive oppor-
tunities for adopting reduced tillage in irrigated agriculture. However,
the machinery, irrigation, and labor savings are not very great and, there-
fore, reduced tillage is not attractive in instances where purchased input
requirements are considerably higher or where there is a likelihood of
negative yield effects. From an economic perspective only, reduced tillage
options appear to merit serious consideration, providing considerable caution
is exercised for each specific situation.
FERTILIZATION PRACTICES
Another area of management practices related to water quality is that
associated with fertilization practices. Problems associated with fertil-
ization can occur in two ways: excessive applications, and application at
the wrong time or in the wrong form.
Excessive fertilization occurs when amounts in excess of crop needs
are applied because of poor information. Programs to discourage excessive
use of fertilizer, especially nitrogen, could potentially improve water
quality and would definitely have a positive economic effect. For example,
if only 56 kilograms per hectare excess nitrogen is applied, and this is
apparently quite common, net returns are reduced by $12.32 per hectare, with
nitrogen costing 22.0 cents per kilogram.
The economic consequences of applying fertilizer in different forms or
at different times are more difficult to assess. 'One must consider the
prices of fertilizer for different application procedures and the
134
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TABLE 54. PER HECTARE COSTS AND RETURNS FOR CORN UNDER CONVENTIONAL
AND REDUCED TILLAGE SYSTEMS. CENTRAL GREAT PLAINS, 1979
Cost and
return items
Purchased inputs
Labor, except
irrigation
Variable machinery
costs
Fixed machinery
costs
Other costs
Irrigation
Labor
Variable machinery
Fixed machinery
Interest on
operating expenses
Total irrigation
Summary
Total cost
Gross returns
Net returns to land
and management
Corn: gated
Conventional
tillage
_______
139.19
20.66
58.64
88.14
24.93
11.86
37.56
96.34
3.29
149.05
480.61
722.62
242.01
pipe
Reduced -
tillage
- dollars per
139.19
18.43
58.29
87.87
24.76
11.86
35.06
96.34
3.11
146.38
474.92
722.62
247.70
Corn: center pivot
Conventional
tillage
hectare - - -
139.19
20.66
58.64
88.14
28.05
5.93
88.19
131.97
6.25
232.34
567.02
722.62
155.60
Reduced-^
tillage
_ _ _ _
176.25
14.01
54.29
87.18
31.26
5.93
75.59
131.97
5.41
218.90
581.89
722.62
140.73
— Differs from conventional tillage in the following ways: two tandem
discings are eliminated and replaced by a once-over rotary till-plant
operation; one cultivation is replaced by a rotary tiller; and net water
application is reduced by 2.54 centimeters.
21
- Differs from conventional tillage in the following ways: two tandem
discings are eliminated and replaced by a rotary till-plant operation;
additional herbicide (broadcast instead of band) is substituted for two
cultivations; and net water application is reduced by 5.1 centimeters.
135
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availability of labor and equipment. Thus, the implications of policies to
modify fertilization practices are very farm-specific. For some producers,
the method of fertilization which is most favorable from a water quality per-
spective may also be the most economical. However, other producers facing
different equipment and labor availability situations might find it very
difficult to comply with a given policy. Therefore, it will probably be
necessary for policy makers to take a very flexible and cautious approach
to this matter.
PESTICIDE USAGE
The last management practice to be discussed concerns pesticide use.
There are three important managerial dimensions of the pesticide use situa-
tion: when to apply, amount to apply, and type of pesticide to use.
Programs to discourage excess pesticide applications, including appli-
cations when not needed, have positive economic effects. They also have
potentially positive water quality effects, because if pesticide use is
truly excessive, one can reduce it and lower costs without lowering yields
(gross returns). Therefore, educational and/or regulatory programs to dis-
courage excess pesticide use would appear to be a desirable option to improve
water quality in cases where there is indeed a relationship between pesticide
use and water quality.
In the case of policies to preclude or discourage the use of a par-
ticular pesticide, the economic effects could be quite adverse depending on
the specific circumstances. If a close substitute exists for a pesticide
which is causing water quality problems, i.e., one which can control the pest
at a similar cost, the economic impact would be minimal. On the other hand,
a policy which precludes the use of one or more pesticides for which good
alternatives are not available could be economically disastrous. There is a
potential for very adverse economic effects because of the likelihood of
yield reductions. Even small yield reductions, such as 10 percent, from poor
pest control would substantially effect net returns. Therefore, policies to
preclude pesticide use should not be implemented without careful, situation-
specific assessments of potential yield reductions, especially in cases
where no close substitutes for the pesticide in question are available.
136
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SECTION 5
CONTROL PROGRAM STRATEGIES FOR IRRIGATION RETURN FLOWS
by
J. R. Gilley and M. Twersky
Agencies and individuals have widely differing judgments regarding what
constitutes best management for a specific situation. Some believe that the
public water quality objective is paramount while others consider economic
stability of the agricultural enterprise to be most important. A third
group argues that it is possible to strike a reasonable balance between
these two elements. For this reason, and because of the variation in
climate, soil and current irrigation management practices used throughout
the Great Plains, no single control measure or group of control measures
can be thought of as best. This chapter accordingly presents an approach
for choosing alternative irrigation management systems for a particular
situation under varying circumstances, given the information found in
Sections 1 through 4 of this manual. Presented are two illustrations of
how the suggested approach might be used.
DEVELOPMENT OF A CONTROL PROGRAM
Determining whether a nonpoint pollution problem may exist and, if so,
what measures may be taken to alleviate it most effectively involves a
logical sequence of decisions. These decisions should result from a step-
by-step procedure such as that illustrated in Figure 23 and the flow charts
given in Figures 24 and 25.
The general procedure is summarized in Figure 23, the Master Flow Chart,
which shows the steps required for the development of control program
strategies to reduce nonpoint pollution from irrigated lands. The procedure
incorporates the information gathered in response to the series of questions
embodied in the flow chart for assessing deep percolation (Figure 24) and
the flow chart for assessing surface runoff problems (Figure 25).
It must be remembered that these flow charts will be most effective
when used as a guide by a group of specialists familiar with the local area.
From the practices given in Tables 39 through 46, these specialists can
develop a list of specific controls to reduce pollution resulting from
irrigation return flows in the area. This type of local input is essential
in arriving at the best possible choice of control practices. The final
step of the Master Flow Chart, the formulation of a control program strategy
based upon the most favorable environment and economic tradeoffs, is best
completed by persons having knowledge of the specific land area and
conditions.
137
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CONSIDER A SPECIFIC LAND AREA
• LOCATION
• SOIL
• TOPOGRAPHY (SLOPE)
• CLIMATE
• CROP
• EXISTING IRRIGATION SYSTEM
• EXISTING MANAGEMENT PRACTICES
(Sections 1
DETERMINE SPECIFIC WATER QUALITY PROBLEM
RESULTING FROM IRRIGATION RETURN FLOW
(Section 2)
I
DEVELOP ACCEPTED PRACTICES FOR
CONTROL OF DEEP PERCOLATION
(Figure 24
-------
DEEP PERCOLATION
RETURN TO
MA5TER
FLOW CHART
IS NITROGEN
LEACHING
A PROBLEM?
SELECT APPROPRIATE
CONTROL PRACTICES
FOR:
a. IRRIGATION SYSTEM
MANAGEMENT
(TABLE 41 OR 42)
b. ON-FARM WATER
MANAGEMENT
( TABLE 43}
c. SOIL MANAGEMENT
(TABLE 44)
d NUTRIENT
MANAGEMENT
(TABLE: 45)
WILLTH&5E CONTROL
PRACTICES INTRODUCE
A SURFACE WATER
KVNOff PROBLEM?
SELECT APPROPRIATE
WATER RUNOFF
CONTROL PRACTICES
f ROM TOBLES
41 thru 46
TRADEOFFS BETWE5N
PgE? PeRCOLATlOW
^ SURFACE WATER
(?UNOFF A1AV HAVE
TO BE MAPE.
IS PESTICIDE
LEACHING
A PROBLEM?
SELECT APPROPRIATE
CONTROL PRACTICES
FOR/-
a. IRRIGATION SYSTEM
MANAGEMENT
(TABLE 41 OR 42.)
b. ON-FARM WATER
MANAGEMENT
(TABLE 43)
C 50IL MANAGEMENT
(TABLE 44)
d PESTICIDE
MANAGEMENT
STABLE 46)
WII/U THESE CONTROL
PRACTICES INTRODUCE
A5URFACE WATE-I?
RUNOFF PROBLEM?
SELECT APPKOPRIAT5
WATER RUNOFF
CONTROL PRACTICES
FROM TABLE5
44 -thru 46
TRADEOFFS BETWEEN
PEEP PERCOLATION
g SURFACE WATER
RUNOFF MA/ HAVe
T08EMAI7E.
15 SALINITY OF
IRRIGATION RETURN
FLOW A PROBLEM?
SELECT APPROPRIATE
CONTROL PRACTICES
FOR-
a IRRIGATION SYSTEM
MANAGEMENT
(TABLE 41 OK 42)
b ON-FARM WATER
MANAGEMENT
(TABLE 431
SELECT APPROPRIATE
CONTROL PRACTICES.
TKAPEOffS BETWEEN
POLLUTION R£Pi>crioM
£ SALINITY CONTROL
FOR CROP
PRODUCTION WILL
TO BE- MAP£.
Figure 24. Flow chart for assessing deep percolation problems.
139
-------
f
16 SURFACE WATER
RUNOFF A /7
A PROBLEM *
-@ — +-
SURFACE WATER RUNOFF
j
NO PROBLEM
AT THIS TIME
RETURN TO
MASTER
cflflfr
)
SE-tgCT APPROPRIATE
CONTROL PKACTICK
FOR
a IRRIGATION SYSTEM
(TABLE 41 OR 42)
b. ON- FARM WATER
MANASEMEKJr
GEMENT
C SOIL
d. NUTRICUT
MANAGEMENT
(TABLE 45)
WlULTHefECOMTROL
PRACTICE? INTRODUCE
A PEEP PERCOLATION
SELECT < EVALUATE
APPROPRIATE
UeACHlM6 CONTROL
FROM
41
MAV
TO
A PROBLEM?
seueor APPROPRIATE
COWTPOU PKACTlCK
IPRI9ATION 5VSTEM
MANAS^MENT
f. ONI -FARM WAltR
MANAGEMENT
(TABL£
C
d.
MANAGS/VIENT
CONTROL
PRACTICES INTROPUCf
A Pg^P Pg^COlATION)
PROBLEM ^
5eUCT ^ eVALUATE
APPROPRIATE
LEACHING CONTROL
PRACTICES FROM
MflY HAVE TO BE!
I
ARE 5EPIMENTS
A PROBLEM'7
CONTROL PRACTICES
FOR-
0. IRRIGATION SYSTEM
MANAQEMENT x
(TABLE 41 012 41)
b. ON-FAI?M \MATCR
MAMAfiE/ViewT
(TABLE
(TMIE44)
.
d.
45)
6 PE5TICIPE
MANAGEMENT
ITML£ 46)
I
WlU
CONTROL
PSRCDLAT10M
$aecr^ EVALUATE
APPROPRIATE
L6ACHIM6 CONTROL
FROM
MAY HAVB TD
Figure 25. Flow chart for assessing surface water runoff problems,
140
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In some cases, an alternative control practice for deep percolation
problems may introduce an increased surface water runoff problem. This
situation or its opposite, when a control practice for surface water runoff
causes an increased deep percolation problem, requires tradeoffs between
deep percolation control and surface water runoff control. Here, compromises
will be necessary. The following examples demonstrate the development of a
control program using two different sets of existing circumstances.
RECOMMENDATIONS FOR TWO SITE-SPECIFIC CASES
Example of a Deep Percolation Problem
In Finney County, Kansas, a large area of sandy soils is being irri-
gated. There is a potential for excessive deep percolation water in this
area, resulting from irrigation with center pivot sprinklers. The initial
step is to obtain information for the first part of the Master Flow Chart
in Figure 23. For this example, these data are:
LOCATION: Finney County, Kansas (Subarea 1103)
SOIL: Coarse-textured
SLOPE: In excess of 3 percent
CLIMATE: Precipitation is about 44 cm per year (Figure 5).
Most of the precipitation occurs as rain in the
spring and summer (Figure 6).
CROPS: Corn is the major row crop (85 percent) and forages
constitute the remaining 15 percent.
EXISTING
IRRIGATION There is extensive use of center pivots in this
SYSTEMS: area.
EXISTING Water from groundwater sources is applied to row
MANAGEMENT crops mainly through the center pivots. Other
PRACTICE: management practices also are considered to be at
a medium level.
Because of the above factors, the potential exists for contamination of
groundwater supplies by nitrate-nitrogen leaching below the crop root zone.
The extent of sandy soil in this area enhances the possibility of nitrate
movement to the groundwater supply. Because farm management practices in
this area are considered at a medium level, it is assumed that more water
than needed to meet crop needs has been applied. It is also assumed that
higher-than-needed amounts of nitrogen have been applied. Excess precipi-
tation or irrigation when crop nitrogen demands are low also increases the
possibility of nitrate leaching to groundwater.
141
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A flow chart for assessing potential deep percolation problems is
given in Figure 24. Nitrate leaching has been identified as a potential
problem. Several control practices can now be selected from among several
irrigation management options: sprinkler irrigation system management
(Table 42), on-farm water management (Table 43), soil management (Table 44)
and nutrient management (Table 45).
The specific sprinkler irrigation control practices given in Table 42
for use on coarse soils with deep percolation problems offer only low to
moderate pollution reduction. Because the management level has been identi-
fied as medium, it is likely that most of these practices already are used
in this area. Therefore, the existing deep percolation problem will probably
not be solved by any of the practices chosen from Table 42.
The on-farm water management alternatives are given in Table 43. This
table shows that on coarse soils like those in this example, irrigation
scheduling options can reduce the possibility of deep percolation of water.
Some specific control practices suggested in Table 43, such as the installa-
tion of flow measuring devices and other automated devices, are an integral
part of irrigation scheduling. Reductions of preplant irrigations and
reducing irrigation amounts below crop needs are other practices which could
be used. However, the economics of the latter alternative, particularly on
sandy soils, must be weighed carefully before it can be recommended.
Soil management options (Table 44) offer few choices for the implementa-
tion of specific control practices to reduce pollution on coarse-textured
soils. The single exception is the improvement of soil fertility. However,
the soil fertility management level is actually medium, above the standard
level shown on the table. Therefore, no beneficial soil management option
exists for this situation.
The nutrient application methods, given in Table 45 indicate that
nutrient applications through fertigation, split-application, or side-dress
application offer moderate to substantial pollution reduction. One or a
combination of these practices should be incorporated into the management
system of this example. The fertigation option is well suited to incorporate
with irrigation scheduling, especially on coarse-textured soils.
In summary, irrigation scheduling in combination with nutrient applica-
tion methods are the control practices most likely to reduce deep percolation
problems in this particular instance.
As shown in Figure 24, the next question to be addressed is: Will
these control practices introduce a water runoff problem? On coarse-textured
soils, this is highly unlikely. Proper water application also reduces the
possibility of surface water runoff on the steeper slopes (6 percent and
above) even though the possibility of surface runoff is low on coarse-
textured soils such as those in this example. If the soil texture on a
particular site is sandy loam rather than sand or loamy sand, improper water
applications on steeper slopes may cause a runoff problem (Table 40). If
this is the case, the information in Tables 41 through 45 must be recon-
sidered to include specific control practices which will reduce both deep
142
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percolation and surface water runoff. In some cases, tradeoffs between deep
percolation and surface water runoff will have to be made.
From an economic viewpoint (Section 4), especially in areas with declin-
ing groundwater tables, proper application of water in amounts not exceeding
crop needs can save the excessive energy costs associated with pumping and
distributing the irrigation water. Also, decreased nitrate leaching leaves
more nitrogen in the soil for use by the crop, thus reducing nitrogen appli-
cation requirements and lowering nitrogen fertilizer costs.
In this example, irrigation scheduling and associated nutrient manage-
ment practices represent a possible solution for the control of potential
pollution of groundwater from IRF on sandy soils in Finney County, Kansas.
The procedure used to select the appropriate nitrate-nitrogen leaching
control practices can also be applied to pesticide and salinity problems.
The flow chart given in Figure 24 can be used to assess these two problems.
It must be reemphasized that the important interrelationships that exist
between the options presented in Tables 41 through 46 affect the choice of a
particular control practice in a given situation. In most cases tradeoffs
and compromises will probably be necessary.
Example of a Surface Watery Runoff Problem
Surface water runoff from a furrow irrigation system without reuse can
have a medium to high potential pollution rating (Table 39). Chosen for
this example is a surface irrigation system in Wyoming. The management of
the system indicates a higher potential for surface water runoff than deep
percolation. The specifics of the situation are:
LOCATION: Goshen County, Wyoming (Subarea 1018)
SOIL: Fine silt to loam (Fine-textured)
SLOPE: 1-2 percent
CLIMATE: Precipitation is about 38 cm per year (Figure 5).
Most of the precipitation occurs in the form of
rain in the spring and summer (Figure 6).
CROP: Sugar beets.
EXISTING
IRRIGATION
SYSTEM: Furrows with siphons without reuse.
EXISTING Water from a surface stream is conveyed to fields in
IRRIGATION open ditches and is applied to the crop through siphon
PRACTICE: tubes. Water reuse is not part of the irrigation
system. The level of management is low to medium.
143
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On these soils with slopes in the range of 1-2 percent, surface runoff
is usually larger than deep percolation losses. In this management system
it is often necessary to apply more water than needed by the crop to insure
water flow across the length of the field. With surface systems, the higher
application of water also insures a better distribution of water in the
field. Thus, surface water runoff will exceed deep percolation. Sediments
are carried in this runoff as well as potential pollutants of nitrogen and
phosphorus compounds adsorbed on the sediments. However, the sediments
themselves constitute a major pollution problem. The flow chart for assess-
ing surface runoff problems is given in Figure 25, and the portion devoted
to sediments is used to determine the appropriate control practices for the
Wyoming example.
Specific control practices for surface irrigation management are given
in Table 41. For medium-textured soils some of the management alternatives
that have higher pollution reduction values are: (1) installation of tail-
water reuse systems; (2) matching the irrigation systems to the soil type
and the slope (Table 39); and (3) reducing the length of run with reuse
systems. Reduction of the length of run will require redesigning of the
irrigation system and create higher labor costs. The use of a reuse system
will also require capital expenditures and increase energy costs. Thus,
the reduction of the length of run or the installation of reuse systems will
involve some economic and environmental tradeoffs. Because it is more
desirable in a surface irrigation system to have a high initial flow of
water at the furrow head, it may be more desirable to install a reuse system.
In cases of severe problems, it may be necessary to redesign the system or
replace the system with a sprinkler system. An economic analysis of the
situation must be included if these alternatives are considered.
On-farm irrigation water management options which can help alleviate
sediments contained in surface water runoff are given in Table 43. For
soils with surface water runoff problems the only highly rated pollution
reduction option is the adoption of irrigation scheduling procedures. For a
situation of this kind, the incorporation of irrigation scheduling will
probably not reduce pollution as much as would the installation of a reuse
system.
The soil management options are given in Table 44. Sediment basins
have a high potential for reducing surface runoff problems and can be in-
stalled as part of the reuse system. Several other practices having low to
moderate ratings, such as terraces, contouring, sod-based rotation and others
may not be practiced under the present irrigation system. Others such as
conservation tillage, grassed waterways and vegetative strips could be
included to reduce surface water runoff.
An evaluation of nutrient application methods is given in Table 45.
Surface runoff losses of most mobile nutrients from medium-textured soils
may occur when fertigation is used in conjunction with surface irrigation
and a tailwater reuse system is not employed. Soil incorporation of the
fertilizer can moderately reduce the loss of nutrients with sediments and
water runoff.
144
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These various control practices may or may not introduce a deep perco-
lation problem. If conservation tillage practices are used, it is likely
that the rate at which water advances down the furrow will be slowed, requir-
ing longer set times. This will increase the amount of water infiltrated at
the upper end of the irrigation set and may increase the deep percolation
loss. If a deep percolation problem is created, other appropriate practices
must be chosen and tradeoffs will have to be made.
From an economic standpoint, the installation of reuse systems and a
sediment basin or the switch to gated-pipe with reuse, are perhaps the best
means of reducing pollution problems caused by sediments in surface water
runoff. In such systems, the maximum nonerosive stream and reuse systems
should be used to recapture and pump back the runoff to the irrigation
system to attain the most economical use of water and energy.
It must be remembered that even the most careful water management
efforts to reduce surface water runoff may be accompanied by increases in
deep percolation. In such cases, tradeoffs between solutions to the two
different problems will have to be made.
In this example, it should be pointed out that any change will (1) cost
the grower money for capital improvements and (2) will probably increase
energy requirements, especially if pumping of water (either initial applica-
tion or reuse) is selected. Such investments may not pay him any additional
financial return.
SUMMARY
This section has illustrated the development of control program
strategies to reduce deep percolation and surface runoff in irrigation
return flows. It is apparent that no single control measure or group of
measures can be considered the best management system. However, the
procedures illustrated can be used to identify accepted practices having
higher probabilities of improving irrigation management and reducing pollu-
tion from return flows. The selection of a control program from among the
accepted practices requires local input to insure local acceptability.
145
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Vlachos, E. C., P. C. Huszar, G. E. Radosevich, G. V. Skogerboe and W. Trock.
1978. Socio-economic and Institutional Factors in Irrigation Return
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Environmental Protection Agency, Ada, Oklahoma. Pg. 55-57.
Walker, W. R. 1977. Irrigation Return Flow Modeling, A State of the Art.
ASAE Paper No. 77-2503. Paper presented at the 1977 Winter Meeting
of the Amer. Soc. Agric. Engr., Chicago, Illinois. 18 pg.
Walker, W. R., G. V. Skogerboe, and R. G. Evans. 1978. Best Management
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Warnick, C. C. 1977. Federal Government's Role in Water Resources.
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154
-------
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155
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APPENDIX A
COUNTY AND SUBAREA DATA
TABLE A-l. ESTIMATED PERCENTAGE OF COUNTY AREA IRRIGATED AND PERCENTAGE OF
COUNTY CROPLAND IRRIGATED FOR ALL COUNTIES IN THE IRRIGATED GREAT PLAINS
Subarea State County
1002 Montana Jefferson
Madison
Beaverhead
Gallatin
1003 Montana Toole
Liberty
Chouteau
Glacier
Lewis & Clark
Cascade
Meagher
Pondera
Broadwater
Teton
1004 Montana Garfield
Fergus
Musselshell
Judith Basin
Golden Valley
Petroleum
Wheatland
Percent
of county
irrigated
2.0%
4.0
4.0
5.0
0.4
0.5
0.7
1.3
1.8
2.0
2.4
4.4
6.0
7.0
0.2
0.6
0.8
1.0
1.2
1.3
3.7
Cropland
Total -.
hectares —
(1,000)
22
59
85
111
50
227
482
156
43
177
26
237
47
226
1,871
58
222
30
122
33
Not Available
35
500
in county
Percent „,
irrigated -
35.0%
56.0
73.0
30.0
0.9
0.7
1.5
6.4
37.0
7.7
57.0
7.9
39.0
19.0
4.0
3.0
13.0
4.0
12.0
—
38.0
(Continued)
156
-------
TABLE A-l (Continued)
Subarea State
1005 Montana
1006 Montana
1007 Montana
1008 Montana
Wyoming
1009 Wyoming
Montana
Wyoming
Wyoming
County
Hill
Valley
Phillips
Elaine
Daniels
Me Cone
Sheridan
Roosevelt
Stillwater
Park
Sweet Grass
Carbon
Big Horn
Hot Springs
Fremont
Park
Washakie
Big Horn
Campbell
Powder River
Johnson
Sheridan
Percent
of county
irrigated
0.5%
1.0
1.3
1.8
0.3
0.3
0.4
0.9
2.0
2.6
3.2
4.2
1.6
2.5
2.8
3.0
3.8
5.4
0.1
0.5
2.4
4.3
Cropland
Total .j.
hectares —
(1,000)
422
290
164
158
1,034
225
192
277
314
1,008
83
51
37
74
245
107
12
64
58
21
48
310
48
56
24
47
175
in county
Percent -,
irrigated -
0.9%
4.4
11.0
12.4
0.5
1.0
0.6
1.7
11.2
34.5
41.0
30.5
19.4
100.0
100.0
95.0
100.0
92.0
2.7
8.0
100.0
61.0
(Continued)
157
-------
TABLE A-l (Continued)
Cropland in county
Percent Total ,
of county hectares —
Subarea State County irrigated (1,000)
1010 Montana Fallon
Wibaux
Custer
Prairie
Daws on
Rosebud
Treasure
Richland
Yellowstone
1011 N. Dakota Billings
Slope
Golden Valley
Dunn
Montana Carter
N. Dakota McLean
Mercer
Williams
McKenzie
1012 S. Dakota Meade
Wyoming Weston
S. Dakota Custer
Wyoming Crook
S. Dakota Pennington
Lawrence
Fall River
Wyoming Niobrara
S. Dakota Butte
0.4%
0.4
0.9
1.2
1.2
1.3
2.7
2.7
3.7
<0.1
<0.1
0.1
0.3
0.5
0.5
0.5
0.9
1.3
0.2
0.3
0.4
0.4
0.5
0.7
1.0
1.3
2.9
90
62
56
Not Available
174
64
19
171
128
764
48
126
92
185
55
378
127
342
213
1,566
129
20
20
54
91
19
36
35
52
456
Percent _,
irrigated —
1.8%
1.6
14.7
—
4.0
26.0
36.0
8.7
19.5
<0.1
0.2
0.4
0.7
8.6
0.7
1.2
1.5
4.3
1.5
10.0
7.4
5.5
3.6
7.4
13.3
25.0
33.0
(Continued)
158
-------
TABLE A-l (Continued)
Subarea State County
1013 N. Dakota Mclntosh
Hettinger
Sioux
Sheridan
Logan
Adams
S. Dakota Corson
Ziebach
Dewey
McPherson
Harding
Haakon
Stanley
Campbell
N. Dakota Stark
Bowman
S. Dakota Perkins
Potter
N. Dakota Grant
Emmons
Morton
Oliver
S. Dakota Walworth
Sully
Hughes
N. Dakota Kidder
Burleigh
1014 S. Dakota Hyde
Brule
Jackson
Shannon
Mellette
Tripp
Lyman
Washabaugh
Percent
of county
irrigated
<0.1%
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
0.1
0.1
0.1
0.1
0.2
0.2
0.2
0.3
0.3
0.3
0.4
0.4
0.6
0.6
0.6
1.2
<0.1
<0.1
0.1
0.1
0.1
0.1
0.2
0.2
Cropland
Total ,
hectares —
(1,000)
176
243
65
158
157
159
134
60
89
151
75
101
88
104
221
145
175
134
202
219
232
74
105
136
76
190
227
3,896
79
106
37
29
51
186
160
37
in county
Percent „/
irrigated —
<0.1%
<0.1
<0.1
0.1
0.1
0.1
0.2
0.2
<0.1
<0.1
0.7
0.7
0.5
0.2
0.2
0.4
0.7
0.3
0.5
0.5
0.6
1.0
0.7
1.2
1.6
1.1
2.3
0.1
<0.1
0.7
1.8
0.8
0.2
0.6
1.7
(Continued)
159
-------
TABLE A-l (Continued)
Subarea State County
1014 S. Dakota Gregory
(cont.) Jones
Bennett
Todd
Buffalo
Charles Mix
1015 Nebraska Cherry
Boyd
Dawes
Keya Paha
Sheridan
Sioux
Rock
Brown
Box Butte
Holt
1016 S. Dakota Marshall
Aurora
Edmunds
Faulk
Jerauld
Hanson
Douglas
N. Dakota Wells
S. Dakota Clark
Sanburn
Hutchinson
Brown
Hand
N. Dakota Stutsman
Eddy
Percent
of county
irrigated
0.2%
0.3
0.3
0.3
0.5
0.7
0.9
1.5
2.0
3.0
2.8
3.5
6.3
7.5
11.7
12.2
0
0
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
0.1
0.1
0.1
0.2
0.2
0.2
0.3
Cropland
Total j,
hectares —
(1,000)
134
70
74
64
31
186
1,244
183
62
83
49
145
32
66
50
147
253
1,070
136
105
192
146
75
74
83
280
164
88
174
335
188
396
115
in county
Percent „/
irrigated -
0.3%
1.0
1.2
1.4
2.0
1.0
7.3
3.2
8.3
11.5
12.3
58.0
25.0
47.0
22.0
30.0
0
0
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
0.1
0.2
0.2
0.2
0.3
0.3
0.4
(Continued)
160
-------
TABLE A-l (Continued)
Cropland in county
Percent
of county
Subarea State County irrigated
1016 S. Dakota Davison
(cont.) Yankton
Spink
Beadle
N. Dakota LaMoure
Foster
Dickey
1017 S. Dakota McCook
Day
Codington
Hamlin
BonHomme
Miner
Lincoln
Deuel
Kingsbury
Minnehaha
Lake
Clay
Brookings
Moody
Turner
Union
Nebraska Dixon
Knox
Cedar
1018 Wyoming Natrona
Converse
Nebraska Garden
Banner
Wyoming Platte
Carbon
Goshen
Albany
0.4%
0.4
0.6
0.7
0.7
0.8
1.4
0
<0.1
<0. 1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.2
0.5
0.5
0.8
1.1
2.0
2.0
3.1
6.0
1.8
2.4
3.0
3.6
5.2
5.2
8.0
9.3
Total ,
hectares —
(1,000)
82
94
282
203
247
135
210
3,804
118
200
122
110
111
98
123
110
152
160
110
85
158
106
136
97
92
169
141
2,398
15
23
75
81
78
61
119
39
Percent .,
irrigated —
0.5%
0.6
0.9
1.0
0.9
1.0
2.0
0
<0.1
<0.1
0.1
0.1
0.2
0.1
0.2
0.1
0.2
0.2
0.7
0.6
1.0
1.3
2.3
2.7
5.3
8.3
3/
3/
~* 1
17.0
8.5
36.0 ;
_'
39.0 /
-*/
(Continued)
161
-------
TABLE A-1 (Continued)
Cropland in
Percent Total ,
of county hectares —
Subarea State County irrigated (1,000)
1018 Nebraska Morrill
(cont.) Scotts Bluff
1019 Colorado Denver
Arapahoe
Elbert
Washington
Nebraska Kimball
Wyoming Laramie
Nebraska Cheyenne
Colorado Adams
Logan
Nebraska Deuel
Colorado Sedgwick
Weld
Morgan
1020 Nebraska Grant
Hooker
Thomas
Arthur
McPherson
Blaine
Logan
Loup
Garfield
Wheeler
Custer
Sherman
Greeley
Nance
Valley
Boone
Howard
Platte
12.4%
43.7
0.1
0.4
0.6
2.0
4.3
4.5
4.7
7.5
8.4
10.0
10.3
15.9
17.7
0.4
0.4
0.7
0.9
1.1
2.4
3.0
3.8
3.9
6.2
9.7
11.0
12.9
14.2
18.0
18.3
20.8
25.0
94
90
675
<1
54
91
305
161
129
226
194
221
95
91
369
122
2,058
31
6
6
14
19
18
29
22
38
34
188
71
58
66
71
119
74
135
999
countv
Percent .,
irrigated —
48.0%
91.0
39.0 -
1.5
3.0
4.0
6.5
24.0
6.5
12.3
18.0
12.0
16.0
44.5
48.0
2.6
14.0
20.3
11.5
12.8
25.0
15.5
25.5
15.0
27.6
34.3
23.0
32.5
24.4
37.5
27.0
41.0
31.7
(Continued)
162
-------
TABLE A-l (Continued)
Cropland in
Percent Total .
of county hectares —
Subarea State County irrigated (1,000)
1021 Nebraska Lincoln
Keith
Buffalo
Dawson
Kearney
Hall
Merrick
Phelps
1022 Nebraska Wayne
Lancaster
Cuming
Stanton
Saunders
Burt
Madison
Col fax
Pierce
Antelope
Dodge
1025 Kansas Norton
Jewell
Decatur
Rawlins
Cloud
Clay
Nebraska Hayes
Kansas Cheyenne
Colorado Kit Carson
Nebraska Webster
Furnas
Frontier
Kansas Republic
9.2%
12.9
35.0
42.0
47.2
53.0
53.4
55.0
2.8
3.3
4.4
6.9
8.2
9.4
13.7
15.4
16.3
22.0
22.5
1.3
1.8
2.0
2.5
3.0
3.0
6.6
7.2
7.7
9.2
10.4
10.5
10.9
167
104
154
143
107
94
84
109
962
98
155
125
79
154
103
113
95
109
144
115
1,290
121
136
138
162
125
108
73
153
295
73
109
88
132
county
Percent „,
irrigated -
36.0%
33.0
56.0
74.0
58.5
78.0
79.0
71.0
3.3
4.7
5.2
9.7
10.5
11.3
18.0
17.0
22.3
33.7
26.7
2.5
3.0
3.5
4.2
4.5
4.7
16.7
12.6
14.7
18.9
17.8
30.0
15.3
(Continued)
163
-------
TABLE A-l (Continued)
Percent
of county
Subarea State County irrigated
1025 Nebraska Dundy
(cont.) Colorado Yuma
Phillips
Nebraska Hitchcock
Perkins
Kansas Thomas
Nebraska Red Willow
Harlan
Franklin
Kansas Sherman
Nebraska Gosper
Chase
1026 Kansas Russell
Ellsworth
Rooks
Dickinson
Lincoln
Ottawa
Trego
Phillips
Saline
Smith
Ellis
Logan
Colorado Cheyenne
Kansas Osborne
Mitchell
Graham
Gove
Wallace
Sheridan
12.9%
13.0
13.0
13.0
13.2
14.6
16.9
17.4
17.6
19.2
20.5
24.2
0.2
0.2
0.5
0.7
0.7
0.9
1.0
1.1
1.3
1.7
1.7
1.9
1.9
2.1
2.2
2.2
3.1
12.3
12.7
Cropland in
Total ,
hectares —
(1,000)
73
263
152
95
165
226
95
84
72
179
56
102
3,275
121
100
126
160
119
101
119
125
106
142
127
136
153
139
136
117
140
111
124
2,402
county
Percent _,
irrigated —
42.3%
30.5
15.0
25.5
18.4
17.9
31.4
30.0
36.5
29.4
44.0
54.8
0.3
0.4
1.0
1.0
1.2
1.6
2.0
2.1
2.3
2.9
3.2
3.9
5.5
3.5
3.0
4.4
6.2
26.0
24.0
(Continued)
164
-------
TABLE A-l (Continued)
Percent
of county
Subarea State County irrigated
1027 Kansas Washington
Nebraska Gage
Jefferson
Nuckolls
Saline
Butler
S award
Thayer
Adams
Fillmore
Polk
Clay
York
Hamilton
1102 Colorado Kiowa
El Paso
Lincoln
Las Animas
Pueblo
Bent
Crowley
Otero
Prowers
1103 Kansas Kingman
Ness
Reno
Sedgwick
Rice
Harvey
Rush
McPherson
Hodgeman
1.3%
7.6
11.4
12.9
19.0
20.4
24.0
24.4
37.2
39.6
44.6
44.6
52.5
64.5
0.4
0.5
0.5
0.6
2.0
6.4
8.8
10.5
13.2
1.0
1.2
1.3
1.9
2.4
2.9
3.0
3.3
4.0
Cropland in
Total ,
hectares —
(1,000)
147
172
101
99
112
117
122
110
116
132
95
112
124
121
1,680
192
42
147
26
53
47
30
39
186
762
133
191
225
186
139
105
139
173
115
county
Percent „,
irrigated —
2.0%
10.0
16.8
19.6
25.3
26.4
29.0
33.0
46.6
44.7
52.5
58.7
63.3
74.0
0.8
6.8
2.2
27.3
23.7
53.7
60.7
88.0
29.8
1.8
1.8
1.8
2.6
3.2
3.9
4.0
4.4
7.9
(Continued)
165
-------
TABLE A-l (Continued)
Percent
of county
Subarea State County irrigated
1103 Kansas Barton
(cont.) Lane
Hamilton
Kiowa
Greeley
Stafford
Pratt
Pawnee
Ford
Edwards
Kearny
Scott
Finney
Gray
Wichita
1104 Kansas Clark
Oklahoma Harper
Colorado Baca
Kansas Meade
Morton
Seward
Stevens
Stanton
Grant
Haskell
1105 Oklahoma Woods
Kingfisher
Major
4.4%
4.4
4.7
5.0
6.6
7.3
7.5
10.5
11.5
15.2
20.0
25.9
28.0
30.0
31.0
0.8
2.7
5.3
13.5
15.0
21.3
25.2
37.0
45.2
56.6
0.4
2.4
2.9
Cropland in
Total ..
hectares —
(1,000)
191
133
158
105
141
142
138
164
205
114
129
155
190
172
136
3,679
80
88
286
138
113
94
145
169
110
127
1,350
128
155
102
385
county
Percent _,
irrigated -
5.3%
6.0
7.7
8.9
9.4
10.5
10.3
12.6
16.0
21.4
34.5
31.3
49.6
39.5
42.9
2.4
8.3
12.3
25.0
25.0
38.0
32.8
38.5
60.5
67.0
1.0
3.6
7.0
(Continued)
166
-------
TABLE A-l (Continued)
Subarea State
1106 Kansas
Oklahoma
1108 New Mexico
1109 New Mexico
Texas
Oklahoma
Texas
Oklahoma
Texas
1110 Oklahoma
Texas
Oklahoma
Texas
Oklahoma
Texas
Cropland in
Percent Total ,
of county hectares —
County irrigated (1,000)
Barber
Harper
Comanche
Alfalfa
Harding
Mora
Co If ax
Quay
Union
Oldham
Beaver
Cimarron
Hutchinson
Texas
Dal lam
Hartley
Ochiltree
Moore
Sherman
Hans ford
Dewey
Blaine
Hemphill
Woodward
Roberts
Canadian
Ellis
Lipscomb
0.1%
0.2
0.6
0.7
0.5
1.2
1.4
2.9
1.9
3.4
3.8
10.3
14.3
16.7
21.3
23.7
25.0
39.5
43.7
60.9
0.5
0.7
0.7
1.2
2.8
2.9
3.0
4.8
101
148
74
146
469
14
13
25
102
154
36
40
195
174
42
301
131
88
134
92
139
136
1,508
91
124
35
86
19
123
92
75
645
county
Percent _,
irrigated -
0.4%
0.3
1.8
1.1
20.3
45.8
52.3
21.5
52.0
32.5
9.0
28.0
77.0
29.7
63.0
100.0
43.7
100.0
74.8
100.0
1.5
1.3
4.7
4.5
33.4
5.5
10.4
15.6
(Continued)
167
-------
TABLE A-l (Continued)
Subarea State
1112 Texas
1113 Texas
Oklahoma
Texas
Oklahoma
Texas
Oklahoma
Texas
Oklahoma
Texas
Oklahoma
Texas
Oklahoma
Texas
Percent
of county
County irrigated
Potter
Randall
Deaf Smith
Swisher
King
Foard
Motley
Wilbarger
Roger Mills
Beckham
Baylor
Cottle
Kiowa
Custer
Grady
Childress
Hardeman
Hall
Washita
Wheeler
Donley
Collingsworth
Greer
Armstrong
Knox
Tillman
Gray
Harmon
Caddo
Jackson
Briscoe
3.6%
14.2
39.3
50.6
<0.1
0.3
0.5
0.8
0.8
1.0
1.1
1.0
1.4
1.5
1.6
1.7
1.7
2.3
2.6
2.9
2.9
3.3
3.7
4.1
4.8
5.5
5.8
7.9
10.6
11.6
13.2
Cropland in
Total ,
hectares —
(1,000)
19
117
206
162
504
15
50
40
106
80
95
54
60
164
137
108
64
81
84
176
58
37
79
78
59
84
156
76
75
165
140
63
2,384
county
Percent „/
irrigated —
44.3%
28.8
74.5
72.5
1.0
1.2
3.2
1.8
2.8
2.5
4.5
3.8
2.3
2.8
4.0
5.0
3.7
6.4
3.8
12.0
18.4
9.7
7.8
16.4
12.6
8.3
18.6
14.8
21.3
17.3
47.7
(Continued)
168
-------
TABLE A-l (Continued)
Cropland in county
Subarea State County
1205 New Mexico Roosevelt
Texas Lynn
New Mexico Curry
Texas Crosby
Bailey
Hockley
Floyd
Lubbock
Lamb
Castro
Farmer
Hale
1206 Texas Stonewall
Kent
Fisher
Jones
Taylor
Dickens
Garza
Haskell
1208 Texas Ector
Howard
Martin
Andrews
Midland
New Mexico Lea
Texas Dawson
Yoakum
Cochran
Terry
Gaines
Percent
of county
irrigated
8.7%
11.0
22.3
28.6
31.5
38.7
43.0
52.3
66.0
66.3
66.5
77.6
<0.1
0.3
0.3
0.4
0.5
1.3
2.2
4.0
0.2
0.9
2.1
2.2
2.3
3.6
8.3
20.7
23.8
24.3
42.0
Total ,
hectares —
(1,000)
140
154
172
133
121
160
178
190
183
178
180
232
2,021
41
22
97
127
91
65
46
130
619
2
67
92
52
22
35
173
108
103
178
220
1,052
Percent »•
irrigated -
39.3%
17.0
47.0
51.0
56.0
57.0
62.0
63.8
95.0
84.6
82.0
84.8
0.5
3.0
0.8
0.7
1.3
5.0
11.5
6.8
20.0
3.0
5.5
16.6
24.8
100.0
11.2
41.3
46.8
32.0
73.7
(Continued)
169
-------
TABLE A-l (Continued)
— Based on figures from the 1974 Census of Agriculture (for each of the
ten Great Plains states), U.S. Dept. of Commerce, Bureau of the Census,
Washington, D.C.
2/
— Based on 1976 and 1977 figures for irrigated hectares (sources
referenced in Section 1), divided by figures for total cropland obtained
from 1974 Census of Agriculture (for each of the ten Great Plains states),
Dept. of Commerce, Bureau of the Census, Washington, D.C.
3/
— In the case of these four Wyoming counties, the 1976 figures for
irrigated hectares were larger than the 1974 figures for total crop-
land. Based on this information, an accurate percent of total
cropland which is irrigated could not be determined.
— Of all the counties listed, Denver County had the lowest number of
irrigated hectares—only 70. Because the county is the site of the
city of Denver, however, there is also little total cropland.
170
-------
APPENDIX A
TABLE A-2. SUBAREAS, ALL OR PART OF WHICH ARE INCLUDED IN THE
I/
IRRIGATED GREAT PLAINS. AND THE DRAINAGE BASIN EACH REPRESENTS -
Subarea number
Drainage basin
it represents
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
Missouri River Headwaters
Missouri-Marias
Missouri-Musselshell
Milk
Missouri-Poplar
Upper Yellowstone
Bighorn
Tongue-Powder
Lower Yellowstone
Missouri-Little Missouri
Cheyenne
Missouri-Oahe
Missouri-White
Niobrara
James
Missouri-Big Sioux
North Platte
South Platte
Loup
Platte
Elkhorn
(Continued)
171
-------
TABLE A-2 (Continued)
Drainage basin
Subarea number it represents
1025 Republican
1026 Smoky Hill
1027 Upper Kansas (Big Blue)
1102 Upper Arkansas
1103 Arkansas in Kansas
1104 Upper Cimarron
1105 Lower Cimarron
1106 Arkansas-Keystone
1108 Upper Canadian
1109 Canadian in Texas
1110 Lower Canadian
1112 Red River Headwaters
1113 Red-Washita
1205 Brazos Headwaters
1206 Middle Brazos
1208 Colorado (Texas) Headwaters
— From U.S. Water Resources Council, 1970.
172
-------
-------
TABLE B-l (continued)
Herbicide Crop
Common Name
Methazole
Metolachlor
Metrlbuzin
MSMA
Oryzalln
Paraquat
Pcndlmethalln
PerCluldone
Profluralin
Prometryn
Propachlor
Propazlne
Simazine
Terbacll
Terbutryn
Trifluralin
2,4-D
2,4-DB
Vernolate
Trade Name Alfalfa Corn Cotton Sorghum Soybeans
Probe X
Dual X -1
Lexbne, Sencor X X — '
various X -'
Surflan X
Paraquat XXX X
Prowl XX X
Destun X
Tolban XX X
Caparol X
Bexton, Ramrod X X -'
Mllogard X -'
Princep X X
Sinbar X
Igran X
Treflan X - X -
various X -' X -'
Butoxone, Butyrac X X
Vernam X
Chemical. .
Class -'
—
AM
TZ
AS
AM
CT
NA
—
NA
TZ
AM
TZ
TZ
DZ
TZ
NA
PO
PO
CB
Transport
Mode -
S
SW
W
S
S
s
s
w
s
s
w
s
s
w
sw
s
sw
s
sw
Acute
Oral
Toxic lev .
Class -'
3
3
3
3
4
2
3
3
3
3
3
3
3
3
3
3
2
2
3
Mobility, .
Class -
2
3
4
1
2
1
1
4
1
3
4
2
2
4
3
1
4
4
3
Average , .
Persistence -
2
2
2
-
3
5
3
2
3
2
1
4
4
5
2
3
1
1
1
(Continued)
-------
TABLE B-l (continued)
— Chemical type designations: AL, aliphatic acids; AM, amides and
anilides; AR, aromatic acids and esters; AS, arsenicals;
CB, carbamates and thiocarbamates; CT, cationics; DZ, diazines;
NA, nitroanilines; NT, nitriles; PH, phenols and dicarboxylic
acids; PO, phenoxy compounds; TZ, triazines and triazoles;
UR, ureas.
2/
— Where movement of herbicides in runoff from treated fields occurs,
JS_ denotes those chemicals that will most likely move primarily with
the sediment, W denotes those that will most likely move primarily
with the water, and SW denotes those that will most likely move with
both sediment and water.
— Acute oral LD classes:
1 - less than 50 mg/kg (most toxic)
2 - 50 to 500 mg/kg
3 - 500 to 5,000 mg/kg
A - less than 5,000 mg/kg (least toxic)
A/
— Mobility Class:
1 - Immobile
3 - Slightly mobile
5 - Mobile
— Persistence Class - Residual Life
1.
2.
3.
4.
5.
0-2
2-6
6-12
1-3
over 3
months
months
months
years
years
— Represents major use.
-------
APPENDIX B
TABLE B-2. INSECTICIDES AND MITICIDES COMMONLY USED IN FIVE CROPS IN THE GREAT PLAINS
Insecticide
Common Name
Aldicarb
Azinphos methyl
Carbaryl
Carbofuran
Carbophenothlon
!_, Chlordimeform
O\ Chlorobenzilate
Chlorpyrifos
Demeton
Diazinon
Dicrotophos
Dicofol
Dimethoate
Disulfoton
EPN
Endosulfan
End r in
Ethoprop
Fonophos
Trade Name
Temik
Guthion
Sevin
Furadan
Trithion
Galecron
Acaraben
Lorsban
Systox
Spectracide
Bidrin
Kel thane
Cygon
Di-Syston
EPN
Thiodan
Endrin
Mocap
Dyfonate
Crop
Alfalfa Corn Cotton Sorghum Soybeans
x I'
X XX
X X - X X - X -
X X - X
X X X X X
X
X
X X
X XX
X - X X X X
X
X
X XX-
X X X X -1 X
X X -'
X X
X
x -'
x~
Chemical.. .
Class -'
CB
OP
CB
CB
OP
N
OCL
OP
OP
OP
OP
OCL
OP
OP
OP
OCL
OCL
OP
OP
Transport
Mode -
W
S
SW
W
S
W
S
U
w
SW
w
s
w
s
s
s
s
u
s
Acute
Oral
Toxicitv,
Class -
1
1
2
1
1
2
3
2
1
2
1
3
2
1
1
1
1
2
1
(Continued)
-------
TABLE B-2 (continued)
In;
Common Name
Ma lath ion
Methyl parathion
Methidathion
Me thorny 1
I—1 Methoxychlor
'vj Mevinphos
Naled
Parathion
Fhorate
Phosmet
Phosphamidon
Propargite
Terbufos
Toxaphene
Trichlorfon
secticide Crop
Trade Name Alfalfa Corn Cotton Sorghum Soybeans
Malathion X - X X X X
LI
Methyl parathion X X X - X X
Supracide X — X
Lannate X X
Marlate X - X
Phosdrin XX X
Dibrom X X
Parathion X X X X -1 X
Thiraet X X - X X
Imidan X
Diraecron X
Omite X
Counter X —
Toxaphene X X X— X— X -
Dylox X X X -
Chemical. ,
Class -
OP
OP
OP
CB
OCL
OP
OP
OP
OP
OP
OP
S
OP
OCL
OP
Transport
Mode -
W
SW
U
U
S
W
S
S
SW
S
W
U
U
S
W
Acute
Oral
Toxicitv ,
Class -
2
1
1
1
4
1
2
1
1
2
1
3
1
2
2
(Continued)
-------
TABLE B-2 (continued)
— Chemical type designations: CB, carbamates; N, miscellaneous
nitrogenous compounds; 0_, cyclic oxygen compounds; OCL, organo-
chlorines; OP, organophosphorus compounds; PY, synthetic pyrethrin;
j>, aromatic and cyclic sulfur compounds.
2/
— Where movement of insecticides in runoff from treated fields occurs,
j>, denotes those chemicals that will most likely move primarily with
the sediments, W, denotes those that will most likely move primarily
with the water, SW, denotes those that will most likely move with
both sediment and water, and U_, denotes those whose predominant
mode of transport cannot be predicted because properties are unknown.
3/
— Acute oral LD n classes:
1 - less than 50 mg/kg (most toxic)
2 - 50 to 500 mg/kg
3-500 to 5,000 mg/kg
4 - less than 5,000 mg/kg (least toxic)
4/
— Represents major use.
-------
APPENDIX C
BASE DATA AND PROCEDURES FOR THE CROP BUDGETS
AND IRRIGATION COST ESTIMATES
CROP BUDGETING PROCEDURES
The model farm budgets were developed with the aid of a computerized
crop budgeting program, which is available on the Agricultural Computer
Network (AGNET) at the University of Nebraska-Lincoln. This program automates
many of the tedious calculations which are required in preparing crop budgets.
A benchmark, or "typical", farm unit was selected for the Southern and
Central Plains areas. Each benchmark farm represents the farming or ranching
operation of an "above average" operator in the area. The selection of a
benchmark farm directly affects the production costs shown in the crop enter-
prise budgets. The relative level of management is reflected in the cultural
practices employed, fertilization rates, and expected yields. The inventory
of machinery, the size of each machine, and the amount of time each machine
is used annually determine the fixed machinery cost per hectare. Thus, the
size of the farm, the cropping plan, and the machinery inventory are impor-
tant determinants of crop production costs. A description of the benchmark
farm is in the text and the machinery inventories are shown in Table C-l.
Current prices for the machinery used on each benchmark farm were
collected from major machinery manufacturing companies. Cost factors, based
largely on information in the Agricultural Engineer's Yearbook, were applied
to determine repair rates, annual fixed costs, and accomplishment rates.
Fuel consumption factors were based on Nebraska tractor test data.
Irrigation costs were calculated using current prices of irrigation
equipment, engineering performance standards, and typical water application
rates. A detailed description of the irrigation cost procedures is discussed
later in this appendix.
Tillage practices, plant population, yields, custom operations, and
other practices which are typical in the area of the benchmark farm were
based on farm record data, workshops with growers, consultation with ex-
tension specialists in other disciplines, and the judgment of farm manage-
ment specialists.
Yields shown in the budgets are estimates of those which an above-
average producer might average over several years,, including years of low
yields due to drouth, hail, insect damage, etc. Consequently, no allowance
179
-------
TABLE C-l. MACHINERY INVENTORIES FOR THE CENTRAL AND SOUTHERN
PLAINS FARM SITUATIONS
Central Plains
Southern Plains
130 hp diesel tractor
90 hp diesel tractor
Old usable tractor (no market
value)
Truck, 350 bushel grain box
Pickup, 3/4 ton
Tandem disc, 21'
Corn machinery, 30" spacing
Shredder, 6 row
Anhydrous applicator, 6 row
Planter, 6 row
Cultivator-tiller, 6 row
Combine, 6 row cornhead
20' grain platform
Wagon, 425 bushel
Auger, 62' 8"
Dryer, cont. flow, 350 bushel/hr.
remove 8 points moisture
Wet corn handling bin, 3300 bushel
Chisel sweeps, 20'
Field conditioner, 32'
Grain drill, 16' 10"
Irrigation equipment
Pivot sprinkler, diesel
Gravity wells, 2 diesel
reuse pits, pumps, and return
pipe
Gated, conveyor, and reuse pipe
Layout beginning of season
and pick up at end of season
Pipe
Bale loader
110 hp diesel tractor
90 hp diesel tractor
60 hp gasoline tractor
40 hp gasoline tractor
Pickup, 1/2 ton
Shredder, 4 row
Tandem disc, 15'
Moldboard plow, 6 bottom
Packer, 15'
Chisel, 15'
Sprayer, 15'
Corn-Grain Sorghum Equipment,
8 row lister
Rolling cultivator
Bed planter
Sand fighter
Cotton Equipment, 6 row
Box float
Lister
Rolling cultivator
Bed planter
Wheat Equipment
Offset disc, 15'
Grain drill, 20'
Irrigation Equipment
Ditcher
Siphon tubes
was made for the expense of crop insurance, or the income from crop insurance
proceeds.
Labor requirements were calculated from machinery accomplishment rates,
with an additional 20 percent added for "non-field" time required for crop
production, such as getting machinery ready, driving to and from fields,
hauling fertilizer, buying seed, chemicals, and other supplies.
Interest on operating expenses was charged at 9.5 percent for the
portion of the year that cash was tied up.
180
-------
Farm overhead expenses were estimated to be 5 percent of other cash
expenses based on past studies of farm records. Overhead expenses include
items which are normally not allocated to individual farm enterprises, yet
which are necessary to keep an ongoing business running. These include
pickup expense, farm share of car expense, farm publications, unallocated
farm utilities, cost of attending farm meetings, income tax preparation
expense, etc.
Production input prices were estimated based on prevailing market con-
ditions (Table C-2).
TABLE C-2. PRODUCTION INPUT PRICES USED IN
ESTIMATION OF CROP BUDGETS
Fertilizer
N-Anhydrous
Liquid or dry
P20
18-46-0 starter
10-34-0 liquid
Herbicides
Corn and sorghum
Soybean
Aerial spray 2,4-D
Corn or sorghum
Pastures
Insecticides
Corn (rootworm)
(13.96/ha. for 101.6 cm rows)
Aerial spray
Rootworm beetle
Alfalfa weevil or greenbug
Western bean cutworm/armyworm
Spider mites or grasshoppers
Seed
Corn—single cross
(80,000 kernels per bag)
Sorghum
Alfalfa
Wheat
Interest
Labor
Machinery operations
All other
0.22/kilogram (kg.)
0.44/kg.
0.44/kg.
0.22/kg.
243.56/tonne
209.82/tonne
12.35/hectare (ha.)
13.59/ha.
9.27/ha.
14.83/ha.
19.77/ha.
11.12/ha.
11.12/ha.
17.30/ha.
12.35/ha.
51.00/bag
0.01/kg.
2.25/kg.
0.15/kg.
9.5 percent
4.00/hour
3.00/hour
(Continued)
181
-------
TABLE C-2. (Continued)
Energy
Electricity
Natural gas (Central Plains)
Natural gas (Southern Plains)
Propane
Diesel
0.053/kwh
50.00/1000 cu. meters
62.50/1000 cu. meters
0.09/liter
.13/liter
Estimated Irrigation Costs
Irrigation costs were estimated with the use of AGNET's "pump" computer
program at the University of Nebraska-Lincoln. Energy prices used in the
cost computation were 1979 expected prices. The irrigation equipment, well
drilling, and land-shaping costs used in the budgets were collected by a
telephone survey of selected dealers in August, 1978 (Tables C-3 and C-4).
I/
TABLE C-3. INVESTMENT COSTS FOR SELECTED IRRIGATION SYSTEMS. CENTRAL PLAINS -
Type of irrigation system
Item
Well
Pump
Power unit
Gearhead
Fuel tank
Pipe, main or gated
Leveling or sloping
Reuse system
Sprinkler system
Electric generator
Gated pipe
without
reuse pits
$2,800
4,400
5,900
1,200
500
9,100
19,500
0
0
0
Gated pipe
with
reuse pits
$2,800
4,400
5,900
1,200
500
9,100
19,500
4,200
0
0
Automatic
gated pipe
$2,800
4,400
5,900
1,200
500
16,250
19,500
4,200
0
0
Center
pivot
sprinkler
$2,800
5,000
12,000
2,600
500
1,000
1,950
0
28,000
1,000
— The Central Plains investment costs assume that a 3.04 cubic meters per
minute well with 30.5 meters of lift is used for all irrigation systems.
182
-------
TABLE C-4. INVESTMENT COSTS FOR SELECTED
IRRIGATION SYSTEMS. SOUTHERN PLAINS -
Type of irrigation system
Item
Well
Pump
Power unit
Gearhead
Pipe, main or gated
Leveling or sloping
Reuse system
Sprinkler system
Electric generator
Other
Without
reuse pits
$ 5,300
7,925
2,725
1,700
500
2,250
0
0
0
0
With
reuse pits
$ 5,300
7,925
2,725
1,700
500
2,250
4,200
0
0
0
Automatic
gated pipe
$ 5,300
7,925
2,725
1,700
10,625
2,250
4,200
0
0
0
Center
pivot
sprinkler
$ 7,950
13,088
8,175
4,350
1,500
225
0
28,000
1,500
500
— The Southern Plains investment costs assume a single 2.47 cubic meters
per minute well for three gravity irrigated systems and one and one-half
wells for the center pivot system. The lift is 68.6 meters for all four
systems.
Fixed irrigation costs (depreciation, interest on the investment, and
insurance) were calculated from the investment costs using the following
factors:
Depreciation rates:
Wells
Power units
Nat. gas or propane
Diesel (w/o reuse)
Diesel (with reuse)
Gearhead
Fuel tanks and lines
Pipe
Sprinkler system
Reuse system
Percent
4.0
11.1
9.09
12.5
56
0
6.67
6.67
4.0
Years of life
25
9
11
8
18
20
15
15
25
Interest was figured at 4.5 percent of original investment on all items
except leveling. (This is equivalent to 9.0 percent on the average un-
depreciated balance). Interest and taxes on the investment in leveling
were figured at 7 percent.
Variable irrigation costs (energy, lubrication, repairs, and service
labor) were calculated using engineering formulas and anticipated 1979
energy prices. Power units were assumed to be operating at 85 percent of
the Nebraska performance standards.
183
-------
APPENDIX D
METRIC CONVERSIONS
CONVERSION TABLES
To compute the United States Customary (or English) system equivalent of
a quantity given in metric units requires the use of an appropriate conver-
sion factor. Conversion factors for units of length, area, volume, weight,
and concentration used in this manual are presented in the first five tables
which follow. Table D-6 presents the conversion factors for several special
unit combinations deemed helpful for the reader.
TABLE D-l. UNIT CONVERSIONS FOR LENGTH
To convert from:
Metric unit
centimeter
meter
ii
ii
kilometer
Symbol
cm
m
m
m
km
Multiply by:
Conversion factor
0.394
39.4
3.28
1.09
0.621
To obtain:
English unit
inch
inch
foot
yard
mile
TABLE
D-2. UNIT
CONVERSIONS FOR AREA
To convert from:
Metric unit
square centimeter
square meter
M II
II II
square kilometer
square kilometer
hectare —
Symbol
2
cm
2
m
2
m
2
m
2
km
km
ha
Multiply by:
Conversion factor
0.155
1,550
10.8
1.20
0.386
247
2.47
To obtain:
English unit
square inch
square inch
square feet
square yard
a/
square mile —
acre
acre
-
— 1 square mile = 640 acres.
h/ 2
— A hectare-is actually 1 square hectometer (hm ).
1 hm = 10,000 m = 0.01 km .
184
-------
TABLE D-3. UNIT CONVERSIONS FOR VOLUME
To convert from:
Metric unit Symbol
Multiply by:
Conversion factor
To obtain:
English unit
milliliter
ii
b/
liter —
ii
"
M
ii
cubic meter
ii ii
hectare-centimeter
hectare-meter
mL^7
mL
L
L
L
L
L
3
m
3
m
ha -cm
ha-m
0.0338
0.0610
1.06
0.264
0.0284
61.0
0.0353
35.3
28.4
0.973
8.11
fluid ounce
cubic inch
cl
quart (liquid) -
d/
gallon —
e/
bushel -
cubic inch
cubic foot
cubic foot
bushel
acre-inch
acre-foot
a/
— To prevent confusion between the lower-case letter "1" and the
number "1", the word 'liter1 can be either spelled out or represented
by an upper-case "L", as done here.
— Liter is a special name for the cubic decimeter (dm ).
c/
1 L = 1 dm33= .001 m3
1 mL = 1 cm
— 1 quart (liquid) =0.86 quart (dry).
— The conversion is for the U.S. gallon:
1 gallon = 4 quarts (liquid).
e/
— The conversion is for the U.S. bushel:
1 bushel = 4 pecks = 32 quarts (dry).
185
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TABLE D-4. UNIT CONVERSIONS FOR MASS TO WEIGHT -
a/
To convert from:
Metric unit
gram
kilogram
tonne —
it
Symbol
g
kg
t
t
Multiply by:
Conversion factor
0.0353
2.20
1.10
2,200
To obtain:
English unit
ounce
pound
ton^7
pound
a/
— These mass-to-weight conversion factors are for the normally-expected
elevations and locations.
— The tonne, or metric ton, is equal to 1,000 kilograms.
c/
— The ton is called the short or net ton and is equal to 2,000 pounds.
This is distinguished from the long ton of 2,240 pounds.
TABLE D-5. CONCENTRATION IN WATER
Multiply by:
To convert from: Abbreviation Conversion factor To obtain:
Parts per million
ppm
ppm
ppm
1.0
0.1
2.72
milligrams/liter
kilograms/hectare-
centimeter
pounds/acre-foot
Parts per billion
ppb
ppb
ppb
1.0
0.1
0.043
micrograms/liter
grams/hectare-
centimeter
ounces/acre-foot
186
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TABLE D-6. UNIT CONVERSIONS FOR SPECIAL COMBINATIONS
To convert from: Multiply by: To obtain:
Metric unit Symbol Conversion factor English unit
tonne/hectare
kilogram/hectare
kilogram/ tonne
t/ha
kg/ha
kg/t
0.446
0.892
2.00
tons/acre
pounds/acre
pounds/ ton
Many publications dealing with the metric system are available. The
following are only a few of those that may be helpful to the reader who
wishes to pursue the subject further:
Metric Manual. 1975. J. J. Keller and Associates, Inc., Neenah,
Wisconsin.
SI Metric Handbook. 1977. John L. Feirer, The Metric Company;
Charles Scribner's Sons, New York.
The International System of Units. 1977. National Bureau of
Standards Publication 330, U.S. Department of Commerce,
Washington, D.C.
The Metric Encyclopedia. 1975. A. L. LeMaraic and J. P.
Earamella, ed., Abbey Books, Metric Media Book Publishers;
Somers, New York.
System International d'Unites, Metric Measurement in Water
Resources Engineering. 1976. Peter C. Klingeman. The
Universities Council on Water Resources, Lincoln, Nebraska.
187
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