EPA-600/2-76-219
October 1976
Environmental Protection Technology Series
ASSESSMENT OF IRRIGATION RETURN FLOW MODELS
Robert S. Kerr Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Ada, Oklahoma 74820
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution. This
work provides the new or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-76-219
October 1976
ASSESSMENT OF IRRIGATION RETURN FLOW MODELS
by
Wynn R. Walker
Agricultural Engineering Department
Colorado State University
Fort Collins, Colorado 80523
Grant No. R-803477
Project Officer
Arthur G. Hornsby
Source Management Branch
Robert S. Kerr Environmental Research Laboratory
Ada, Oklahoma 74820
ROBERT S. KERR ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
ADA, OKLAHOMA 74820
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DISCLAIMER
This report has been reviewed by the Robert S. Kerr
Environmental Research Laboratory, U.S. Environmental Protec-
tion Agency, and approved for publication. Approval does not
signify that the contents necessarily reflect the views and
policies of the U.S. Environmental Protection Agency, nor does
mention of trade names or commercial products constitute
endorsement or recommendation for use.
11
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ABSTRACT
Throughout the Western United States irrigation return
flows contribute to the problem of water quality degradation.
Evaluating the effectiveness of alternative management strate-
gies involves models which simulate the processes encompassed
by irrigated agriculture. The development and application of
these models require multidisciplinary expertise. A work-
shop involving fifteen specialists in the varied aspects of
irrigation return flow modeling was held to review the status
of these models. Irrigation return flow and conjunctive use
models recently developed by the Bureau of Reclamation served
as focal points for the workshop. As the field verification
and potential applications of these models were discussed,
several general problems were identified where further inves-
tigation is needed. Particular emphasis was given to the
description of the spatially varied aspects of soil, crop,
and aquifer systems, and the proper alignment of model
objectives with available data. The large number and diver-
sity of existing models illustrate the individualistic nature
of irrigation return flow modeling. In order to affect more
widespread utilization of existing models, a systematic
procedure should be developed to update and disseminate this
modeling technology.
111
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CONTENTS
Page
Abstract iii
List of Figures vi
Acknowledgements vii
Sections
I Conclusions 1
II Recommendations 4
III Introduction 6
IV Workshop Assessments of Irrigation 11
Return Flow Modeling
V Workshop Summary 35
VI References 39
VII Appendices 41
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FIGURES
No. Page
1 Conceptual diagram of the irrigation
return flow system. 7
2 Conceptual diagram of block building
in irrigation return flow modeling. 13
3 Illustrative flow chart of the USER
conjunctive use model. 26
4 Flow chart of USER detailed irrigation
return flow modeling system. 29
VI
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ACKNOWLEDGEMENTS
The time and efforts contributed to the workshop by
the participants whose names and addresses are listed in
Appendix A is gratefully acknowledged. Their willingness in
traveling to Fort Collins and participating enthusiastically
in the workshop insured its success, and their reviews of
this report were especially helpful.
The author is indebted to the individuals who helped
with the local arrangements during the workshop. Ms. Lee
Kettering and Mr. Stephen W. Smith assisted in arranging
for participant travel and local activities.
Finally, the writer wishes to recognize the efforts
and advice given by the Project Officer,- Dr. Arthur G.
Hornsby, whose willingness to act as co-host at the work-
shop facilitated fulfilling the objectives of this
project.
Vll
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SECTION I
CONCLUSIONS
Irrigation return flows may contain significant concen-
trations of salinity, nutrients, insecticide and herbicide
residues, and sediments which affect the beneficial uses of
receiving waters. Management techniques are being developed
and implemented to control and reduce water quality degrada-
tion to a much smaller level while maintaining favorable
conditions for crop production. This requires a detailed
understanding and evaluation of the numerous processes that
affect return flow. The irrigation return flow model is a
vehicle for combining, in one simulation program, what is
known about these processes for the purpose of evaluating
alternative plans for their control.
Most modeling efforts have tended to express the
individual modeler's perception of the problem and its
solution in limiting the scope of the model application.
The resultant models, while individualistic in nature, are
generally composed of a multi-level structure in which
various processes are combined and integrated to simulate
the system. They vary from simplistic to highly refined
treatments of hydrologic and water quality related processes
and conditions. The structural level employed depends on
the scope and objectives of the study being conducted and
the experience of the modeler. In addition to the individ-
ualistic model structures are the often unique irrigation
return flow situations. A majority of workshop responses
favored using submodels of limited scope which consider
individual processes within the irrigated environment.
The flexibility for assembling submodels into the desired
operational format indicated by a specific problem is thus
possible. The need for careful documentation and field
verification was considered a prerequisite to testing the
capability of a model whatever its structure and scope.
Of the various segments of an irrigation return flow
system, modeling improvements are needed in the simulation
of root zone moisture and nutrient extraction, and ground-
water flow and chemistry. The effects of the design and
management of an irrigation system on subsurface moisture,
salt, and nutrient movement were also regarded as a signif-
icant research area requiring additional investigation.
Some segments of the return flow system such as evapotrans-
piration (except under trickle irrigation), soil chemistry,
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drainage, and surface hydrology of irrigated areas were
thought to be comparatively well defined and adequate at the
present time for most modeling purposes.
The USER models were introduced at the workshop for
assessment by the participants. The conjunctive use model
for river basins designed to simulate the basic irrigation
return flow system and to predict its regional impact was
considered a useful tool for such applications as: (1)
indicating the impact of an irrigated area on the region's
water resources; (2) producing regional water quality
planning and management information; and (3) assessing the
environmental impact of a proposed development or change.
The USER models for detailed simulation of irrigation
return flow systems were generally well received. Recog-
nizing that these models were among the most comprehensive
and user oriented available, the group concluded that fur-
ther improvements could be made in the areas of dynamic root
water-nutrient extraction, two-dimensional soil moisture
and salinity flow, nitrogen chemistry, and evaporation at
the soil surface. Most of the participants indicated
a desire to know more about the detailed models and the
opportunity to use them in their own research programs.
In most model applications to large systems, one of the
major problems is the spatially varied nature of field and
soil conditions. Soils for example may exhibit vastly
different characteristics from one locale to another in
the same irrigated area. Consequently, individuals who
expect to apply existing modeling technology should use
extreme care in determining the representative nature of
the data used as their input. Many parameters and data
are not normally distributed, and thus, the arithmatic
mean may not be a good estimation of central tendency.
Increased knowledge concerning parameter distributions in
the field should allow selection of more representative
model input data.
Another problem is that most detailed models are formu-
lated for small scale analyses where relatively large
quantities of data are available and where much is known
about the system being modeled. In scaling such models to
large land areas, the comparative lack of detailed data
may require the modeler to make assumptions which reduce
the modeling reliability. In addition, if in expanding the
model the sensitivity to certain assumptions is not suffi-
ciently understood, a great deal of misapplication may result.
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The workshop format effectively addressed each of the
objectives set forth by this project. The short time
allotted to each segment limited participant evaluation of
some important modeling problems. Most of the participants,
however, indicated it was time well-spent and believed their
own modeling capabilities were improved by gaining a per-
spective as to how the work of other disciplines interfaced
with their own. One conclusion indicated by most individuals
present was a need for increased interaction between the
different science disciplines (i.e., soil scientists, hydrol-
ogists, etc.).
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SECTION II
RECOMMENDATIONS
This study indicates that an important need in the
field of irrigation return flow modeling is the standardiza-
tion of modeling technology. This will provide more wide-
spread use of existing models, better interdisciplinary
coordination, and less duplication. Work should be under-
taken to identify and describe existing models relating to
the varied aspects of irrigation return flow systems. Then
a central program library comprised of these models could
be developed where the models as well as their documentation
and verification could be maintained. The library would be
a central point for the dissemination of these models to the
research and consulting communities when needed. A group of
interdisciplinary specialists should be formed to evaluate
each model submitted, test them on various computer systems,
and then develop a structure for classifying and updating.
In this way, the library may serve as a useful coordinating
tool for research workers as well as those interested more
specifically in planning.
A problem of improperly applying models no matter how
well documented will always exist. However, a report or
text written to describe the complex irrigation return flow
system and how it may be simulated would help to minimize
improper applications. It is probably necessary to develop
this documentation at two levels. One level should be struc-
tured with the planner or administrator in mind where as
the second would be written for use by the technical analyst.
An information source such as these suggested documents
should also be developed in coordination with a well exper-
ienced and interdisciplinary group of advisory personnel.
In the interim, the workshop concept is an effective
mechanism for increasing the utilization and understanding
of models already developed. Federal and state agencies
performing modeling development work should use the workshop
format to disperse their results to the various interested
individuals. The workshop is also an efficient vehicle for
acquainting administrators with the details of work being
accomplished under their direction. In addition, training
workshops would aid administrators in formulating specifica-
tions for research, implementation, and enforcement. The
USER models should be further exposed to a large group of
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potential users outside of the Bureau who will be involved
in future research and implementation activities as irriga-
tion return flow quality control begins on a large scale.
Publication of the results of the detailed USER model results
in the technical and scientific literature is strongly
encouraged.
Several areas of needed research were identified. A
procedure for describing the spatial variability of soil,
crop, and groundwater properties should be developed. The
sensitivity of models to their input data and assumptions
requires consideration in light of the variability of
systems under field conditions. Groundwater and aquifer
data are comparatively unavailable, and therefore, should
be emphasized in future data collection efforts. If irri-
gation practices involving low-tension, high-frequency
water applications find widespread utilization, such as drip
irrigation, additional work should be performed to determine
the evapotranspiration rates under these conditions. The
effects of an irrigation system design and operating prac-
tices should also be evaluated, especially for low-tension*
high-frequency irrigation regimes.
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SECTION III
INTRODUCTION
STATEMENT OF PROBLEM
The application of water to agricultural lands where
natural precipitation is inadequate for crop needs has
provided the United States and other countries in the world
with a more stable supply of food and fiber. Before the
widespread concern for environmental quality it was generally
accepted that the resulting increases in salinity, insecti-
cides, herbicides, sediment and other pollutant loads in
river systems were inevitable consequences. But under
present social pressures, water quality degradation and
control are receiving more attention. The capability for
evaluating management alternatives on irrigation return flows
was heretofore limited by the lack of suitable predictive
models based on physical aspects of the many processes
involved. These tools, which are commonly called models,
are being developed so that it is presently possible to
handle many of the complex processes related to irrigation
return flow.
A simulation model is generally a set of mathematical
relations applied to appropriate boundary conditions that
attempts to simulate the relationships between variables in
the natural system. It is used to understand the natural
system and evaluate its behavior under proposed management
alternatives. Researchers have been prolific in applying
modeling concepts to the various segments of the hydrology
in an irrigated area (Figure 1). To date, irrigation return
flow models of all types are somewhat individual in nature
because each model expresses the modeler's perception of
his own particular solution to a problem. Most models
are so unique that many active modelers find it is easier
and more reliable to program for individual purposes that
sift through the language barrier of another's program.
Consequently, application of modeling advances is extremely
slow and duplication or repetition is common.
One of the most exhaustive modeling efforts in the
field of irrigation hydrology was recently undertaken by the
Bureau of Reclamation, U.S. Dept. of Interior, with funding
support and program direction from the U.S. Environmental
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0 ..... Inflow to
Precipitation Cana|s
Evaporation
from Canals
Upstream
Surface Runoff from
Non-lrriaated Land
Ind. & Mun.
Wastes
Natural
Inflow
Evapotranspiration
from Crops
Other
Evapotranspiration
Irrigated Land
Applied to
Irrigated Land
Diverted
for
Irrigation
Irrigation
Return Flow
Groundwater
Contribution
River Flow
Downstream
Figure 1. Conceptual diagram of the irrigation return flow system (after
Skogerboe and Law, 1971).
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Protection Agency. A joint research effort entitled,
"Prediction of Mineral Quality of Return Flow Water From
Irrigated Land," was initiated in May, 1969. This project
was designed to first predict the mineral quality of irri-
gation return flows and then evaluate the impact such efflu-
ents could have on the remaining water resources in a river
basin. Specific emphasis was placed on salinity in terms
of its composition and concentration which would allow
evaluation of specific constituents on downstream salinity.
In order to evaluate the models developed, three case
studies were investigated which covered a variety of environ-
mental factors similar in range to those encountered in
the western U.S. river systems. Verification trials were
conducted in the Grand Valley in western Colorado, the
Cedar Bluff Irrigation District in Kansas, and an area in
the Vernal Unit of the Central Utah Project known as
Ashley Valley. For each of these areas, the research
involved characterizing field conditions, modeling to
predict quality of percolating irrigation water, determining
the effects of the percolation on local drainage effluents,
and evaluating management alternatives on the quality of
return flow. The product of this project was to be a set of
verified mathematical simulation tools which could be used
to evaluate the effects of existing and new irrigation activ-
ities on the quality of receiving streams.
Both the Bureau of Reclamation and the Environmental
Protection Agency are concerned about the veracity of model-
ing efforts as applied to regional planning. Consequently,
this workshop was organized and funded by this project to
consider the state-of-the-art with a view towards standard-
ization of modeling technology.
The extent to which this project may have accomplished
a significant step towards such a goal is not known, but it
did bring together modeling experts who deal with the
problems associated with irrigation return flows. In the
context of a workshop, these people were exposed to Bureau
models, exchanged ideas with each other, and offered an
initial assessment of the technology. This report summar-
izes that activity.
PURPOSE AND OBJECTIVE OF THE PROJECT
The quality of irrigation return flows has become a
matter of great concern in a number of areas where the
high degree of water resource development leads to
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substantial reuse of return flows. Specifically- in those
regions approaching complete development of water, several
locations may exist in which the flows are comprised primarily
of upstream return flows. Improving the quality of a water
resource where irrigation return flows are involved
requires the identification and control of individual hydro-
logic segments transporting the pollutants. Due to the
diversity in problem definitions and management strategies,
interdisciplinary efforts are necessary.
The major objective of this project was to organize
and conduct a workshop where individuals familiar with the
various aspects of irrigation return flow models could
critically review the work of the Bureau of Reclamation.
Their models were not presented as the total answer to the
long standing problem, but rather as one of the most recent
efforts to integrate models concerned with irrigation return
flows. Bureau models were, of course, intended to be
sufficiently general in nature and scope as to be applicable
to many different irrigated regions. Since model utiliza-
tion would be aided by a critical assessment from pro-
fessionals in and out of the government framework, this
workshop was designed to accomplish the following objectives:
1. Provide a vehicle for interchange of concepts,
methods, and ideas which will be useful in
modeling irrigation return flows systems;
2. Introduce the USER models to a wider audience of
users;
3. Provide direct contact with the USER modelers to
explain the philosophy, theory, and applications
of their models;
4. Provide a forum for assessing the present and
future needs for modeling irrigation return flow
systems; and
5. Stimulate an interdisciplinary awareness of the
commonality of problems which must be addressed
to manage irrigation return flow quality.
The participants invited to the workshop are listed in
Appendix A. Two other guests not listed were Mr. John T.
Maletic, Chief, Water Quality Office, Division of Planning
Coordination, U.S. Bureau of Reclamation, and Mr. Robert
Chandler, Graduate Research Assistant at Colorado State
University. This group included Irrigation Engineers,
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Drainage Engineers, Hydraulic Engineers, Soil Chemists,
Soil Physicists and Groundwater Hydrologists. In addition,
the participants came from a wide range of professional
areas including university researchers, government research-
ers, and government administrators. The one thing common
to each was extensive modeling experience and/or an interest
in improving such capability.
SCOPE OF REPORT
This report is intended to summarize the events, con-
clusions, and recommendations of the workshop. A compre-
hensive literature review of irrigation return flow
modeling was compiled earlier in a report by the Project
Officer entitled, "Prediction Modeling for Salinity Control
in Irrigation Return Flows," (Report EPA-R2-73-168), and
thus only selected abstractions will be made in order to
support certain of the workshops events. Detailed user
manuals of the USER models are forthcoming, and additional
details are described in existing publications so technical
information concerning the models will be omitted. However,
in order for the reader to also understand the Bureau
of Reclamation's objectives and results, the abstract,
conclusions, and recommendations from the summary report are
included as Appendix B. The remainder of this report
will include two sections. The first will address the
ideas presented in the workshop through a discussion of
irrigation return flow modeling. The second will be the
author's summary of the workshop and views concerning the
current state-of-the-art in irrigation return flow
modeling technology.
10
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SECTION IV
WORKSHOP ASSESSMENTS OF IRRIGATION RETURN FLOW MODELING
During the three day workshop five sessions covering
three discussion areas were conducted. The first discussion
area (Sessions 1 & 2) was provided to develop a common under-
standing of model philosophy, approaches, and problems asso-
ciated with modeling irrigation return flow systems. The
second discussion area (Sessions 3 and 4) was designed to
provide a forum for the USER modelers to describe and
discuss their various modeling capabilities. The third and
final discussion area (Session 5) was to provide an assess-
ment of the USER model efforts, exchange of modeling
philosophy, and assess the immediate research needs to
expedite development and utilization of models. The agenda
for the workshop is presented in Appendix C.
REVIEW OF IRRIGATION RETURN FLOW MODELING
Following the welcome of the participants and a state-
ment of the objectives and format of the workshop, a brief
review of the history of the existing USER models was pre-
sented. A discussion on the structure of irrigation return
flow models was then conducted to identify the multi-level
nature of models and suggest further consideration in
classifying existing models accordingly- After these intro-
ductory activities, seven of the participants presented
their review of some of the important aspects in modeling
irrigation return flows.
Modeling Philosophy
A mathematical model in the context being used in this
report may be loosely described as a set of mathematical
expressions describing the complex behavior of the irri-
gation return flow system. Generally these mathematical
expressions are coded in systematic computer language for
actual use. Modeling is the art of formulating models,
however, the only complete and true model of a system is the
system itself. It should be emphasized that there is
generally no unique model of a system since different
models will be produced by different analysts according to
their understanding and interpretation of the system.
The models describing irrigation return flow systems are
11
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generally simplified so that practical models can be
developed to simulate this complex system. For instance,
many hydrologic processes require simplification in order to
find solutions to the mathematical relationships. Models
of irrigation return flow systems are generally decomposed
into interrelated subsystems or blocks. Each subsystem may
be further subdivided into specific processes within the
system with the aim of reducing the number of variables in
each segment (Figure 2) to maximize flexibility and mini-
mize inaccuracy. In constructing the linkages between
various parts of the model, the relevance of each aspect
should be examined closely.
Several attributes can be identified which stimulate
the development of mathematical models. First is the
utility of simulating a process, studying its response
to a perturbation, and evaluating the results on the system
without being constrained by using the real system or real
time frame. And second, the ordering of natural processes
into functional relationships in developing the model forces
the modeler to detail, and thus understand the individual
processes.
The system response generated by a simulation model can
be used to evaluate the technical, legal, institutional,
and socio-economic implications of a system management scheme,
Thus, the answer to the question "Why model?" can range from
a need to understand the primary mechanisms or processes
occurring in a simple system to elaborate decision-making
tools needed to manage a complex system. A model that is
extremely useful for one user may be totally inadequate or
inappropriate for another's need.
Although it is obvious that a model's utility and scope
is limited by the modeler's knowledge of the system, there
are other important constraints which dictate the extent to
which a model may be refined and tested. For example:
(1) availability of data;
(2) accuracy of data;
(3) characteristic size of the system;
(4) characteristics of the mathematical statements
composing the model; and
(5) computer capability and/or core storage require-
ments .
12
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/
Soil Moisture Movement input
Process Model
Dissolution
a
Precipitation
Output
X
Soil Subsystem Model
Irrigation System Model
Input^
\
\
\
Surface
Hydrology
System
\
Soil
Hydrology
System
Conveyance
8
Operation
System
Figure 2. Conceptual diagram of block building in irrigation
return flow modeling (after Hornsby, 1973).
13
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Some of these limitations may be recognizable by only the
most astute modeler. Thus the fact that a completely
general irrigation return flow model does not exist merits
mention.
In recognizing these factors, it is not difficult to
understand why there are nearly as many models as modelers.
In recent years, this situation has meant a great deal of
duplication and a rather slow dissemination of the modeling
advances. Although interdisciplinary research which has
been touted for years may have significantly reduced dupli-
cation and increased information exchange, little has been
accomplished, especially in irrigation return flow modeling.
Today/as problems of water quality degradation associated with
irrigation return flows become increasingly acute, the con-
cerned research community cannot afford to "reinvent the
wheel" for each problem area. The necessity for developing
standardized model packages seems apparent although efforts
to standardize may be impaired due to the complexity of
nature.
During the workshop, participants were asked to respond
to three questions regarding the above comments. Five
choices were delineated in numerical fashion, ranging
from 5 for strong agreement to 1 for strong disagreement.
Values of 2 and 4 were for intermediate responses with 3
being neutral. A summary of the results of the question-
naires completed by the participants excluding both the
writer and Project Officer is shown in Appendix D.
The first question was intended to assess the work-
shop participants' opinions concerning the numerous models
that have been developed to describe irrigation return flow
systems. When asked about the general availability of
information concerning these models, 51% felt such informa-
tion was unavailable for most potential" users and only 16%
felt otherwise. The general acceptance and use of other
models was also considered poor by 50% of the group with
only 8% having a positive reaction. An interesting view
of the participants themselves and their interest in improv-
ing their own capabilities was .indicated in three other
responses under the first question. A desire to have de-
tailed operational information about the existing models
consisting of documented users manuals describing input
requirements, methods of solution, operational character-
istics, format of results, sensitivity to model parameters,
and verification results was expressed by approximately 80%
of the individuals. Since most of the participants are
14
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actively involved in research supported by state and federal
funding, it would seem important that attempts be made to
provide these informational needs in order to improve the
effectiveness of on-going and future research. In addition,
the definition of certain terminology in models changes as
research develops a new and better understanding of the
physical processes and thus a system to centrally update
the terminology would help as new problems come into focus.
The block building concept illustrated in Figure 2 is
probably the best way of overcoming problems associated
with using the many existing irrigation return flow models.
Several other scientific professions have already taken
steps to standardize some of the more commonly employed
computational tools. For instance, most computer centers
have both statistical and optimizational packages as part
of their user libraries from which one can select a par-
ticular program to fit his specific needs. Of course,
such libraries must not only be very well documented and
verified but also written with a view towards use on
various kinds of computers. Each program in the library
must have the capability of being updated as new methods
become available. Most of the workshop participants
thought that such a program library would narrow the gap
between research and application.
Question number three in Appendix D evaluated the par-
ticipants' attitudes towards the idea of a central library
of irrigation return flow models. The goal of one gener-
alized, all inclusive model was rejected by 84% of the
responses with 59% strongly disapproving. Most agreed that
such a model may be useful but would be so large that many
computers could not accommodate it and updating would be
difficult. An alternative to the generalized model would
be a set of generalized subsystem models, such as surface
hydrology, root zone, groundwater models, etc. This idea
would be similar to a library of second level models such
as the soil subsystem model in Figure 2. The response to
this idea was 33% strongly in favor, 33% slightly in favor,
and 17% disapproving. It is apparent that even at the sub-
system level, generality would be difficult. In addition,
within such a subsystem there is research and application
by more than one discipline, and a need exists to further
subdivide these systems and allow modelers to formulate
specific subsystem models for individual circumstances.
The subdivision of subsystem models would consist of a
library of subroutines and functions simulating individual
components of the irrigation return flow system. This
15
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suggestion is represented within the third level of Figure 2
(individual segments of the soil moisture movement process
model). This question drew the most positive response from
the group (75%) with 50% in strong agreement. Although one
session of the workshop was devoted to discussing this con-
cept, most of the participants needed more time to consider
what the process level models might include. A partial list
included potential evapotranspiration, infiltration, root
extraction patterns for moisture and nutrients, horizontal
and vertical unsaturated flow, saturated flow, ion exchange,
mineral dissolution and precipitation, nutrient transforma-
tions, and various irrigation system characteristics. Some
of these processes would require alternative methods of
solution so several process models describing the same
phenomenon would need to be developed. For example, calcu-
lation of potential evapotranspiration can be facilitated
by numerous methods depending on climatic data, geographic
location and desired level of fine tuning. There was some
concern about matching time intervals and data requirements
since some, process models vary in each respect.
Based on the assumption that a library of process
models could be developed and documented, a congruent set of
programs representing the various formulations of subsystem
models was discussed. Such subsystem models might include
one of the irrigation system, crop system, root zone sys-
tem, and groundwater system. Combining two or more sub-
system models and possibly a mixture of process and sub-
system models would lead to programs utilized in the field
to evaluate the various irrigation return flow systems.
And finally, combinations of the previously noted models
could be utilized for the basin-wide models needed to eval-
uate the effects of different management alternatives.
Thus, a four level program library was felt to be an impor-
tant step in furthering interdisciplinary research and
specifically attacking irrigation return flow quality
problems in the future. Although various members of the
workshop expressed concern over a number of potential
problems, it was agreed that such an effort would require
substantial time and funding as well as close coordination
among modelers.
Formulating Irrigation Return Flow Models
The third session of the workshop involved seven short
presentations discussing the current status of various
aspects of irrigation return flow modeling and where unsolved
16
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problems may be encountered. After this session, all of the
participants were asked to rate components of irrigation re-
turn flow models for adequacy and degree of development for
general applicability. A summary of responses is found un-
der question 2, Appendix D. Some of this information is
interesting in view of the multidisciplinary nature of the
group. Concerning models which describe the efficiency of
individual irrigation systems as affected by field, soil,
and crop characteristics along with climate and irrigator
practice, 41% of the participants felt such analyses were
currently inadequate while 16% disagreed. Models evaluating
evapotranspiration, unsaturated flow, saturated flow and
drainage, and surface hydrology and routing, however, were
considered adequate by 67%, 42%, 50%, and 57%, respectively.
Models of root moisture extraction and root zone chemistry
were rated evenly with respect to adequacy as 25% responded
both ways to each model. Other models felt to be inade-
quate were plant chemistry (64%) , saturated zone chemistry
(33%), and management and optimizational models (50%).
In each of these ten categories of models, a substantial
fraction of the participants was non-committal. This would
tend to exemplify the various backgrounds present where
certain kinds of models would not be of great familiarity.
The specific interests of the group as a whole were indica-
ted in question four when asked which of the aspects of the
model of conjunctive use and regional water quality simila-
tion prepared by the USER was of the most interest person-
ally. Most were interested only in the mechanisms immedi-
ately surrounding the irrigation return flow system itself
rather than how these flows reacted in the total river
basin context. The results of these responses indicate
some unfamiliarity with aspects of irrigation return flow
modeling lying outside individual experience and interest.
As part of the workshop experience, it was felt that
the seven individual presentations would aid the group as
a whole in visualizing this problem in its entirety. The
first presentation by Dr. Robert W. Hill concerned modeling
irrigation efficiency, an area the questionnaire identified
as inadequate at present for modeling irrigation return
flows. Many models which attempt to describe irrigation
have been developed for large valley or river systems.
Such models are generally inadequate in assessing the effects
that the irrigation system itself has on subsequent return
flows. At the farm level, the measure of irrigation impact
is termed irrigation efficiency which represents the fraction
of water diverted for irrigation that is utilized by the
crops. Specific subsets of irrigation efficiency such as
17
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farm efficiency and application efficiency have been defined
to segregate return flows into field tailwater and deep
percolation. One of the most important parameters affecting
efficiencies is application uniformity, or the uniformity
with which water is added to the root zone. From a distri-
bution standpoint this variable is difficult to define par-
ticularly for furrow irrigation cases. However, application
uniformity is highly dependent on soil and field variability
which to date has not been successfully simulated.
Water infiltrating the soil surface enters a two to
five foot zone of soil where the bulk of crop roots are
located. The total water extracted from this zone is depen-
dent on plant characteristics and the soil moisture status,
but is reasonably well estimated by examining the energy
balance at the surface. The patterns of root water extrac-
tion were the subject of the second discussion by Dr. R.
John Hanks. To date most models of the soil profile have
either assumed moisture is taken up uniformly or taken up
in decreasing fractions with depth. However,- the true root
uptake patterns are known at least to be non-linearly dis-
tributed and change with the irrigation season. An equation
was presented describing the best current estimate of
uptake and root growth (Nimah and Hanks, 1973a, 1973b).
Recent research indicates that relative crop yields in terms
of dry matter are related to relative transpiration (Hanks,
1974) . Thus, accurate description of root uptake is not
only important in simulating root zone moisture movement, but
also in evaluating the effects of various irrigation prac-
tices on yield. In some cases, therefore, it may be impor-
tant to consider the daily distribution of evapotranspira-
tion. Also, with the increasing use of trickle irrigation,
it is becoming more and more important to be able to deline-
ate the crop transpiration from the soil surface evapora-
tion. Most empirical estimating procedures include a crop
coefficient to correct for physiological plant characteris-
tics, and the effects of canopy cover on soil evaporation,
but because of changes in the volume of soil wetted under
low-tension, high-frequency irrigation systems (especially
trickle irrigation systems), additional work may be
required for predicting evapotranspiration under such
irrigated systems. A useful reference on evapotranspira-
tion was published by the American Society of Civil
Engineers (Jensen, 1974) . The modeling of root extraction
patterns is also constrained by lack of an adequate pre-
dictive model for root growth-root activity as a function of
time and space. A list of references presented at the
workshop on the subject of plant root extraction of water
18
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and solutes is given in Appendix E. Moreover, there are
some questions on the relationships between relative yields
and salinity concentrations in the root zone. Research is
currently underway to help find answers to this question
(Childs and Hanks, 1975).
The chemical characteristics of solute transport in the
root zone and soil regions below the root zone was the
topic discussed by Dr. Marvin Shaffer in the third presen-
tation. A typical model of this system might consist of
the following principal parts:
(1) Nitrogen. The nitrogen added to or found in
soils undergoes such transformations as hydrolosis
of urea; mineralization of organic nitrogen;
immobilization of ammonia and nitrate; and nitri-
fication. Rates for these reactions generally
tend to be slower than soil chemical reactions and
may be handled by a kinetic approach. The possi-
bility of modeling nitrogen kinetics with what
was called a transition state approach was
introduced.
(2) Inorganic chemistry. The bulk of salinity in soil
and irrigation supply water and return flows is
almost exclusively derived from chemical weather-
ing of rocks and minerals. Thus, solubility of
minerals is a major chemical reaction. Other
chemical reactions including cation exchange, for-
mation of ion pairs, and bicarbonate buffering
are also important considerations in soil and
water chemistry- Since these types of reactions
take place relatively fast, a chemical equilibrium
approach is generally used to model these parame-
ters .
(3) Constituent redistribution. Since moisture is
being extracted by roots and percolating into or
out of the root zone, an important aspect of
modeling soil chemistry is predicting the move-
ment and redistribution of salts and nitrogen.
During this discussion, a concept of layered segments in
the soil system was presented as a method of computing the
changes in root extraction and soil properties with depth.
In outlining the primary chemical reactions which must be
considered in western soils, the kinetic and chemical
19
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equilibrium approaches to simulating these reactions were
also discussed in terms of irrigation. The need for addi-
tional data to further refine modeling capability for
mineralization, immobilization and denitrification seems to
be the most critical area for further research.
The capability to simulate the unsaturated movement of
water, salts, and nutrients in the soil profile gives some
interesting alternatives for controlling the salinity stem-
ming from irrigation return flows.
In the fourth presentation by Dr. James D. Oster, field
and lysimeter data were presented to show the various tech-
niques for managing the soil chemical system. Interestingly
enough, the suggestion was made that salinity distributions
in the soil profile are primarily a function of the irriga-
tion system itself. Recent studies have shown that very
few natural waters will cause yield reductions if leaching
fractions are greater than 10%, and in some crops like
alfalfa, the leaching fraction may be as low as 3%. The
mass emission of salts from irrigated lands can be con-
trolled to some extent by irrigation management, more
specifically, controlling the leaching fraction. Two pri-
mary factors are involved, namely, salt pickup and salt
deposition. Salts are picked up by chemical weathering
of minerals and solubilization of soluble salts primarily
as a function of depth of water passing through soils and
chemical characteristics of the irrigation water. A reduc-
tion in leaching fraction generally results in less salt
pickup, but as dissolved salts in the soil solution are
concentrated by evapotranspiration, there is a tendency
for calcium and magnesium salts to precipitate out as their
solubility product constants are exceeded. Thus, a reduc-
tion in the leaching fraction may result in more deposi-
tion of these salts,and sodium will tend to predominate
even while the salt content of return flows may be reduced.
The overall resultant effect from minimum leaching is often
an increase in salt concentration but a decrease in the mass
emission of salts. It should be noted that these factors
must also be viewed in conjunction with salt displaced
from saline soil and aquifers underlying many irrigated areas,
As less irrigation return flows are contributed to ground-
water basins, less of the saline water from the aquifer
will be displaced into receiving waters, and initially at
least, little change in salt concentration would be noted.
In addition, a reduction in irrigation return flows may also
result in reduced rates of salt pickup from these saline
20
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aquifers. The end results are difficult to predict because
of the complex interaction between irrigation return flows
and salt pickup.
The modeling of groundwater and drainage flows asso-
ciated with irrigation systems, as reviewed by Dr. A.W.
Warrick, comprised the fifth presentation. Most surface
and sprinkler systems can be treated as one-dimensional
flow patterns in both the root zone and soil or aquifer
beneath. The analysis of one-dimensional moisture flow can
be adequately determined using various mathematical pro-
cedures such as finite difference techniques, but the simu-
lation of two-and three-dimensional flow patterns encoun-
tered under trickle or subsurface irrigation systems are
generally more difficult. The studies reported have
employed one-,two-, and three-dimensional programs using
transient and steady-state solutions. Groundwater models
were also reviewed including those incorporating groundwater
chemistry options, a bibliography of which is listed in
Appendix F. In any of these cases, the spatial variability
has not yet been accounted for in either hydraulic param-
eters or input data. The discussion reviewed some of the
most important studies evaluating the interfacing of un-
saturated and saturated flow models. A problem that arose
several times during the workshop was that there is diffi-
culty in matching models with data in many aspects of hydro-
logic and water resource systems. Often the analyses gener-
ated by a computer are deemed infallible when, in fact,
basic limitations and sources of error have been ignored.
The results are conclusions which can be incorrect, and
since many of the results of models are involved in signif-
icant legal decisions, the criterion for judging model
accuracy is an important issue. Most of the group felt that
one-dimensional description of flows below the root zone
was adequate for modeling purposes, although dispersion
and some two-dimensional flows must be considered under
certain circumstances. An observation was made during
this session that the chemistry models were sometimes more
accurate than the associated moisture flow models except
where nitrogen may be concerned. Since the chemistry model
depends on accurate simulation of the moisture movements,
some emphasis should be given to using reliable moisture
flow models as a means of improving the overall capa-
bility to predict the quality of irrigation return flows.
The previous five discussions reported detailed infor-
mation concerning the formulation of irrigation return
flow models. In the sixth presentation by Professor Kenneth
K. Tanji, some thoughtful and timely remarks were presented
21
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on the application of the largely research oriented models
to local and regional problems. During the past twenty
years or so, most of the concerted effort by engineers and
scientists has dealt with modeling small-scale irrigation
problems. The models that evolved were primarily research
submodels, and the utility of employing them for river
basins, irrigation projects, and other large-scale systems
has been questioned. As alluded to earlier, very few small-
scale models have been verified in large-scale applications.
Three primary reasons were identified for this observation.
First, lack of adequate field data appears to be universal
for broad-scope models. For example, the models of surface
hydrologic processes are well supplied by climatic, land
use, and stream gauging data whereas the subsurface hydrol-
ogy is generally inadequately defined spatially and with
time. Groundwater hydrology models encounter deficiencies
with data used to estimate storage capacity, water and
salt inputs from the unsaturated zones above, and the
transient mixing at the unsaturated-saturated zone inter-
face. The second consideration is the high level of refine-
ment advanced in chemical simulation models. There is an
inconsistency involved in applying such finely-tuned sub-
models to field conditions where spatial variabilities and
lack of detailed characterization of soils and substrata
materials constrain such models to somewhat simplistic
analyses. A question of how much field data would be
required to alleviate this inconsistency is not known.
It is apparent that the crop root zone data are much more
abundant than that for deeper underlying regions. An
example of a problem resulting from this disparity of data
is the paucity of information available on salt pickup and
deposition from the natural geologic sources. And finally,
most modeling efforts attempt to simulate the transient
condition of the natural system. These short-term predic-
tions at times tend to reflect the initial conditions rather
than improved management conditions. Thus, water quality
objectives may be more meaningfully met through the steady-
state approaches in order for the results to better reflect
long-term conditions. The detailed analyses of selected
segments of an irrigation return flow system are well-
founded but are limited in terms of practicality to large-
scale uses, and perhaps simpler models should be used.
Certainly simpler models are more compatible in many cases
with available field data. Most of the participants also
agreed that field applications and studies require time
periods of five years or more which is generally not the
practice today.
22
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In the final presentation by Dr. M. Leon Hyatt, thoughts
and discussions as to how the use of a systems analysis
approach in modeling might be implemented were presented.
The thorough and thoughtful planning required for effective
salinity management must be coordinated with the goals and
needs of the public at large. The various consequences of
manipulating systems to affect management objectives will
incur difficult political, institutional, and social con-
straints. At the beginning of efforts to implement improve-
ments, or to implement existing or future public laws, the
best technical input available will be required regardless
of the level of refinement. Management level models must
be basin-wide in scope because of the tradeoffs between
various potential areas of development. In addition, the
various state and federal agencies involved in water quality
management need a range of modeling capabilities to specifi-
cally suit their needs. An outline summarizing the concepts
requiring consideration when using a systems approach for
basin-wide salinity management was presented. This outline
as given at the workshop is as follows:
A. Modeling Salinity Systems
1. Designating Appropriate Variables
2. Delineating Relationships Among Variables
3. Setting the Model Complexity
a. Accuracy
b. Management Questions
c. Data Availability
d. Variable Identification
e. Solution Methods
4. Understanding the Dynamic Nature of Salinity
Systems and Associated Problems
a. Equilibrium or Steady-State Conditions
b. Time Increment
c. Non-steady State Condition
d. Data Limitations
e. Type of Definition Required--Processes
to be Looked At
f. Extreme Events
5. Conservation of Mass in Salinity System
a. Data Limitations
b. Events or Processes to be Described
i. Hydrologic
ii. Salinity Including Diffuse and Point
Sources
iii. Municipal, Industrial, Agricultural,
and Other Natural Events
23
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c. Consideration of Both Surface and Subsurface
Waters
d. Size of Spatial Units in Basin
e. Incorporation of Subroutine Elements (other
Models)
6. Consideration of Instream Flow Uses
a. Fish and Wildlife
b. Recreational
B. Methods of Modeling
1. Optimization
2. Statistical
3. Mathematical Simulation
a. Focus of Workshop Participants
b. Difficulty of Verification
The basic physical and chemical characteristics of a river
basin must be defined to at least some degree before
planners and policy makers consider the political, insti-
tutional,, and social problems associated with water qual-
ity management. Models provide a tool to evaluate manage-
ment options. Depending on their complexity, relative
effects of different options can be evaluated even though
insufficient data may be available to verify the accuracy
of a model in its ability to predict a current condition.
Often planners will have to balance the need for a better
simulation of a situation against the time available for
a decision to be made. Consequently, many different
models may be developed for a given situation, and the
modelers and planners will wish to assure themselves that
the concepts upon which the model is based make use of
basic principles as well as the physical and chemical
characteristics of a river system. In addition, models
may also indicate what data should be collected in order
to refine planning strategies.
COMMENTS ON THE USER MODELS
The USBR model development program with EPA funding
evolved into two distinct phases. The first was the formu-
lation of a river basin conjunctive use model, and the
second involved a series of interfaced detailed models of
the irrigation return flow system.
USBR Conjunctive Use Model
The USBR conjunctive use model was designed to assess
the water management alternatives on a river basin scale.
24
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The functional operation of the model divides a hydrologic
system into subunits called "nodes" which generally conform
to the subbasins in an area identified by natural watersheds
and U.S. Geological Survey gauging stations within each node,
The delineation of flows is as illustrated in Figure 3. As
one can readily see, the model considers the vast array of
water uses in a river system. The workshop participants
were given the opportunity to comment on model details
and the Bureau's attempts at model verification.
In a previous section, the interests of the various
workshop participants with regards to various segments of
the model were presented. However, in questions 5 and 6
of Appendix D, the participants were asked for more specific
responses to the model itself. The model obviously attempts
to integrate the interests and expertise of several disci-
plines, and the group was asked to cite their opinion of
the model's usefulness to them in accomplishing certain
objectives. The first use of the model presented was in
indicating the impact of an irrigated area on the total
flow system. Sixty-two percent (12% strongly) felt that
they could effectively use the model in this regard, and
38% gave a neutral reaction. No negative reactions were
expressed. Approximately the same result was obtained
concerning a question on using the model for regional water
quality planning since the USER has already utilized the
model for environmental reports on a number of new irriga-
tion developments. The Bureau will undoubtedly use the
model in future environmental impact statements for proposed
water resource developments, irrigation or otherwise, as
required (63% felt they could utilize the model in this
manner). The capability of scaling the results generated
by the model in a small study area to a large one was
disagreed upon. Twenty-five percent felt it would be of
little use to them while 12% responded positively and 63%
reacting neutrally. The conjunctive use model probably
was not intended for a small study area because data and
results are generally more explicitly defined. An
interesting point was made by the USER personnel present
concerning the coordination of the basin scale model with
the highly scientific models of the individual hydrologic
segments. Using available data which was noted earlier as
generally insufficient for the detailed models, the con-
junctive use model could perform a "first cut" analysis
and give valuable qualitative results. For example, this
model could identify critical irrigated areas where fur-
ther data could be collected for the detailed model in
25
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Surface Evaporation
) Bank Storage and/or Seepage
y Recreation and/or Flood Control
(_S«dim«nt Inflow
Surface
Reservoir
Consumptive Use
10 Segments 7~
With Piston
Figure 3. Illustrative flow chart of the USER conjunctive
use model.
26
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answering specific questions. Thus, the concept of using
higher level models as planning tools should be considered
for analysis of interactions of complex phenomenon, identi-
fying important hydrologic variables, and delineation of
data collection requirements. In fact, the third item in
question 5 asked just how the participants felt about such a
use and 50% felt favorably towards this concept. The use
of the conjunctive use model as an irrigation return flow
model and one capable of evaluating other models was not
endorsed. Only 25% felt mildly that such use could be made
and 25% felt otherwise.
The data necessary for the conjunctive use model comes
from different kinds of monitoring systems or studies,
all of which have varied objectives, accuracy, and time
interval characteristics. The group was invited to assess
the resolution of these factors keeping in mind the model's
purpose and scope. In simulating the irrigation return flow
system itself, 50% felt the model's use of soil moisture
movement data was inconsistent, and 25% agreed also in
the area of soil chemistry. In both cases, 25% and 37%,
respectively,reacted in reverse. Thus, in the soil environ-
ment itself, the model could be improved by updated mathe-
matical concepts and by better coordination between the
soil moisture movement simulation and available data. The
highest ratings for inconsistency were given to groundwater
movements and chemistry- In handling of groundwater move-
ment, where it was noted earlier that data are severely
limited, 50% of the group disagreed with the modeling
approach with 12% strongly disagreeing. At the same time
12% felt mildly in the opposite vein. The simulation of
chemistry in the groundwater received a negative rating of
76%. The conjunctive use model was rated favorably in the
segments describing surface distribution of irrigation diver-
sions, consumptive use, and reservoir operation and chem-
istry.
The conclusions which stem from this phase of the
workshop are as follows. The conjunctive use model is well
suited to assessing the potential impact of irrigation
developments and in giving overall planning direction in
the use of the detailed irrigation return flow model. The
use of this model as a tool to consider existing irrigation
return flow systems or to verify their models is not
recommended. This agrees with the intent of the personnel
who developed the model, namely, to design a model for the
evaluation of irrigation return flows rather than to design
27
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a model which details the flow itself. Other comments were
detailed directly under question 11 and 12, Appendix D.
Detailed Irrigation Return Flow Models^
The second aspect of the USER models deals with detailed
simulation and prediction of return flow quality. The
models simulate the plant-soil-water system encompassing
the region between the soil surface and the area of a ground-
water return point such as a tile or open drain. The flow
of this system is illustrated in Figure 4 and consists of
separate models for evapotranspiration, unsaturated and
saturated water flow, dissolution-precipitation of slightly
soluble salts, cation exchange, ion pairing, nitrogen
transformations and uptake by crops, and the movement and
redistribution of salts and nutrients. This interfaced
system allows a dynamic nonsteady-state approach to simu-
lating the following specific chemical constituents: cal-
cium, magnesium, sodium, bicarbonate, carbonate, chloride,
sulfate, nitrate-nitrogen, ammonium-nitrogen, and urea-ni-
trogen contained in soil, aquifer, and drainage flows.
Within the soil and substrata regions, organic nitrogen
and gypsum concentrations are predicted along with exchange-
able calcium, magnesium, sodium, and ammonium. These
models and their verification were discussed during sessions
3 and 4 of the workshop.
Most of the workshop participants were interested in
the recent improvements in modeling unsaturated zone chem-
ical behavior. The flow of water, salt, and nutrients in
this zone of the soil was modeled with a revised version
of the model presented by Dutt and Shaffer (1972), which
several of the participants had actually helped formulate
or were familiar with its functions. As the discussion
proceeded, several suggestions were made for future improve-
ments and were first solicited in question 7, Appendix D.
The distribution of water under different irrigation
regimes was considered an area for improvement by 62% of
the responses while 38% disagreed. The rapidly expanding
use of trickle irrigation and, in fact, the entire concept
of low-tension, high-frequency irrigation would seemingly
require such improvements to accurately depict the chemistry
in the first few feet of the soil. Nitrogen chemistry
seemed to be another area of major concern of the group as
illustrated by the 50% rating given to suggesting its
improvement with only 25% in disagreement. The same
28
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Irrigation
Scheduling
Drainout
Unsaturated
Flow
Saturated
Flow
Interface
for
Chemistry
Unsaturated
Chemistry
Saturated
Chemistry
Drain
Effluent
Prediction
Figure 4.
Flow chart of USER detailed irrigation return
flow modeling system .
29
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response also applied to predicting other nutrient and
fertilizer movements, root extraction patterns, and estima-
tion of evapotranspiration. Overall, 63% felt continued
efforts should be made to enhance the model's capability to
simulate salt movement, exchange, and reactions in the soil.
This overall question may seem critical in identifying
weaknesses in the unsaturated chemistry model although the
workshop atmosphere was otherwise. Most felt this model
was good, and the suggestions were intended as guidance for
whatever additional changes may be forthcoming. Question
8 asked of the participants whether they would consider
interfacing this model with their own. Fifty percent
reacted strongly in favor, while 75% indicated this model is
useful to their on-going or anticipated research. From a
purely technical standpoint, the exposition of the unsat-
urated chemistry model may have been the most beneficial
result of the workshop.
The saturated flow and chemistry models also received
considerable interest from the group although these models
are oriented primarily towards drainage design and evalua-
tion. Again, 50% of the participants rated the model's
utility favorably, while 12% had little current interest.
General
- The overall assessment of the USSR models was positive
by most of the workshop participants. Their evaluation of
the conjunctive use model indicated several problems to
which it should be readily applicable and certain instances
where its utility would be limited. The interest in the
detailed models was also significant. There were, of course,
suggestions for improvement of a qualitative nature that
were determined by the questionnaires and during the dis-
cussions.
The USER models are structured in a subroutine format
that most believe is the most feasible method of modeling
the irrigation return flow system. Since various partici-
pants were not experienced in the terminology of each
subreutine function, they were asked if sufficient detail
had been given for a user to determine when it is appropri-
ate to use a particular subroutine. The majority who did
comment indicated sufficient detail had been given, but
a substantial effort must be made in each case to under-
stand the actual workings of the models. In order to maxi-
mize the utility and reliability of USER models, the follow-
ing specific suggestions were made:
30
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1. Conduct more sensitivity studies to determine the
effects of field data variability, model assump-
tions, and model resolution in time and space;
2. Reevaluate the methods of simulating groundwater
movement and chemistry to include salt pickup,
saturated flow conditions where a drainage system
is not present, anisotropy, and increased emphasis
on two-dimensional flows; and
3. Update the unsaturated flow program to improve
simulation of bare soil evaporation, root extrac-
tion patterns, layered soils, and hysteresis
effects.
There were also various comments suggesting changes in
USSR techniques which are found under question 6, Appendix
G. The reader may wish to read these himself for any impli-
cations that are present.
SUMMARY AND FUTURE DIRECTION
The final session of the workshop was a summary and
general discussion period in which problems in irrigation
return flow modeling were discussed and suggestions made
for future research.
Discussion about the characteristics of data and their
use in models reoccurred frequently at the workshop. As
pointed out earlier, a substantial disparity exists between
available data obtained from surface and subsurface elements
of the hydrologic system and that required by adequate
models. Thus when models are applied to systems involving
irrigation return flows, errors and unknown variables tend
to get lumped together in the groundwater because of the
usual lack of groundwater data for verification and/or
inadequate definition of subsurface hydrology. When this
occurs, the model's ability to reflect the effects any water
management policy may have on return flow quality is restric-
ted by the assumptions regarding groundwater behavior.
The participants indicated in a take-home questionnaire,
Appendix G, what they felt were the important research needs,
In question 4, responses a and e noted the groundwater
modeling problems. The two points raised here regarded the
acquisition of adequate data on groundwater hydrology and
the investigation of the linkage between unsaturated and
saturated flow regimes.
31
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An important point raised during the proceedings was
the observation that current theoretical developments are
inadequate to treat the spatial variability of soil proper-
ties with a reasonable degree of confidence. In small-scale
applications of return flow models, data may be much less
variable than in large areas. An important research need
is the definition of data requirements in modeling large
tracts of irrigated land area. Soil mapping should be made
in accordance with anticipated use of the data. Also,
consideration should be given to additional analyses of
specific parameters as an aid to both the verification and
interpretation of computed results. For example, chloride
ion in most western soils is not significantly adsorbed by
the soil, and thus, the ratio of chloride concentrations
in the soil solution and irrigation water may provide a
reasonable check on deep percolation leaching of soluble
salts or the concentrating effects of evapotranspiration.
Problems associated with "reinventing the wheel" in
modeling were noted as substantiation of the need for
formulating a standardized library of well documented
subprograms in the field of irrigation return flow modeling.
Responses to question 4 in Appendix G contain several
references to improving calibration, increasing sensitivity
and documentation, etc. A systematic and uniform program
for coordinating modeling efforts would aid each of these
needs. An additional need was noted for intermediate level
models which would perform somewhere between the USER
conjunctive use model and their detailed irrigation return
flow model. Again, the library concept would answer this
need by having such models available so the building blocks
for an intermediate level model could be readily assembled.
Maybe one of the most neglected parameters in models
discussed was the influence of the design and operation of
the irrigation system itself. Uniformity differences, for
example, can result in corresponding differences in an irri-
gated field. A stronger emphasis is being placed on irri-
gation systems which maintain a uniform, low-tension, soil-
water regime because some reported results have shown
improved yields, higher quality produce, water and labor
savings, and more effective fertilization (Second Inter-
national Drip Irrigation Congress Proceedings, 1974).
With these advances in this area, it may be necessary to
characterize certain soil properties such as infiltration
rates throughout the irrigation season to adequately model
the moisture flow. The practices of the irrigator himself
play an important role in determining how much moisture may
32
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be applied to the soil profile. Irrigation efficiency has
been used to describe the effectiveness of irrigation prac-
tices. Yet, recent studies have demonstrated that the con-
cept of irrigation efficiency when applied to return flow
studies may be misleading since these values vary widely
during the irrigation season. Thus, the problem noted
earlier concerning the problems of averaging data not nor-
mally distributed are present here as well. Finally,
there is some evidence that the soil chemistry itself is
highly dependent on the characteristics of the irrigation
water and how it is applied.
It is anticipated that return flow models in the future
will improve in reliability and accuracy, but the answers
they give will remain basically the same from a management
standpoint. Thus, future investigations should begin
encompassing the detailed simulation models within the frame-
work of an optimization analysis which may permit the
expansion of evaluations from, "What will be the effect of
an alternative policy for controlling the quality of return
flows?" to "What is the best alternative?" The plans for
managing water quality must be made with available modeling
results because the legislative bodies have so demanded.
However, without the added analysis of optimization, it
will be difficult to identify the most cost-effective alter-
native.
As the return flow programs evolve from the investiga-
tion to implementation stage,there are several problems to
be solved. First, funding and time limitations will
require that the most relevant parameters, which vary from
one site to another,- be defined for the operation of these
models. This will necessitate performing many of the
sensitivity and library suggestions made during this
workshop. Secondly, the continual changes in certain con-
cepts, which occur from a better understanding of the
system, or in the physical elements themselves, require
some level of basic support. And finally, the science
and art of irrigation return flow modeling is probably not
completely bridged by any single individual. Thus, the
models and analyses developed and reported can easily be
used inadequately. In each of these three areas, it
would be helpful if a text or manual were available which
considered the various guidelines relevant to data collec-
tion and model analysis of field conditions. A proposed
table of contents was prepared by the Project Officer
and discussed by the workshop participants. Most agreed
33
-------
to consider the outline, make suggestions for its improve-
ment, and cooperate at such time as such a document may be
written.
34
-------
SECTION V
WORKSHOP SUMMARY
Although a certain amount of summarizing of the work-
shop has already been given, there are two additional
areas that should be discussed in order to complete this
report. At the conclusion of the workshop, the partici-
pants were given a take-home questionnaire designed to
assess the contents and operation of the workshop. These
results are presented in Appendix G. And after working
with the USSR models, conducting the workshop, and observing
its results, the writer feels several comments are in
order.
PARTICIPANT ASSESSMENT OF WORKSHOP
The first question on the take-home questionnaire
invited the participants to evaluate how well the workshop
accomplished its goals. All of the participants felt the
workshop objectives concerning the USER models were met.
Specifically, two of the project objectives were to intro-
duce these models to the group and provide the contact with
USBR personnel to explain their philosophy, theory, and
applications. The goal of providing a vehicle for inter-
change of concepts, methods, and ideas useful in the
modeling of irrigation return flow systems was generally
viewed as an important accomplishment of the workshop.
But, a couple of the participants felt this objective was
not fully met because of a high degree of commonality
among the participants. Another stated objective—the
stimulation of an interdisciplinary awareness of the
problems which must be addressed in irrigation return flow
analysis—was considered met by most of the participants.
The one objective receiving the widest variety of responses
concerned providing a forum for assessing the present
and future needs in this area of modeling. Most agreed
this goal should have been given more time and emphasis
during the workshop.
The second question noted that topics were covered
which were outside the scope of most individual interests.
The group was invited to point out the degree to which the
association with people of varied interests expanded their
own perceptions. All of the responses indicated that the
workshop had facilitated this to some degree. In general,
35
-------
most found they had learned possible new approaches to
problems they had encountered in their previous work.
There were those who benefited by the broad dimension of
interest represented at the workshop and thereby gained a
better understanding of the various philosophies being
applied to modeling irrigation return flows. Of course,
the people present were a good source of references for
potential problems that may occur in some future work, and
the participants were pleased with the opportunity to
associate with the USER personnel.
The time, depth, and organization of the workshop were
difficult factors to plan since there were no applicable
workshops of a similar nature from which to judge. Most
of the responses to a question asking for comments on
these factors were positive. Some areas of the workshop
that could have been given more emphasis include:
(1) Needs for future research. The discussion of
research needs and future emphasis occurred at
the end of the workshop when several of the par-
ticipants were absent. The discussion, however.-
should have been more explicit.
(2) The discussions relative to a central program
library could have been extended to investigate
the structure more fully. Also, this discussion
should have been at the end of the workshop
after the USER models had been introduced.
(3) Some participants could have benefited if more
practical use of the models had been discussed,
and in particular, the type of results generated.
Questions 4, 5, and 6 dealt with potential research topics
(or problem areas) and comments regarding the USER work,
both of which were discussed previously.
Finally, the participants were asked for their frank
opinions of the workshop. The responses in question 7 speak
for themselves, but in general, most felt the workshop was
worthwhile and productive.
36
-------
A PERSONAL ASSESSMENT
If the past twenty or thirty years of irrigation
research and application are viewed closely, one finds an
extremely rapid development of the technology in all its
aspects. Each discipline has been so excited by the
discoveries in their topic area and the questions remaining
to be solved that little concern for the other discipline
areas were considered in design or operational decisions.
When the quality of return flows began imposing on the
utility of the available water resources in a region, it
soon became apparent that the problem and its solutions
were very complex. At first, investigators studied within
their particular scope, but as officials charged with
remedying these problems insisted on suggestions for
management and control, an integration of existing knowl-
edge had to occur. An example of this integration is the
modeling effort of the USER and this interdisciplinary
assessment. State and Federal agencies strive to coordi-
nate research. A point is reached, however, when the tech-
nology generated by research shifts into the hands of
state, local, and private groups charged with detailing and
implementing a policy for pollution control. Obviously,
analytical models which consolidate the concepts gener-
ated by research are going to be useful to those who
define policy, and workshops are necessary steps in model
development for these purposes.
Another important problem with regard to computer
models is that they remain a deck of coded cards written
in the style of its author and arranged to manipulate the
computer as efficiently as possible. There are two
problems that inevitably result. The first is the vast
differences in the coding skills of individuals writing
the programs versus those who use them, and the second is
the variance among computers. In the latter respect, a
good example is the USER conjunctive use model. When it
was submitted to the Colorado State University computer,
basic fortran errors dealing with redefinition of loop
parameters were encountered as well as indefinite operands,
yet the USER and CSU computers are made by the same
company.
If most research and development budgets are examined,
computer costs are a small fraction of the totals. When
a great deal of time is spent on adding sophistication to
the computer codes in the name of saving money, the
37
-------
perspective of using models as a computational tool has been
lost. In general, the cost of time spent to save a few
dollars in computational time are poorly invested by those
other than expert programmers.
The concept of an irrigation return flow model library
will be unworkable unless the entries are coded in a manner
that minimizes the alterations necessary to make the pro-
grams operational on other computers. In addition, the use
of program logic must be straightforward if the many indi-
viduals from different schools of computer training can
successfully implement the program in their own work.
In examining these points and the others raised during
the workshop, the inclusion of other noted individuals to
the list of participants would have been helpful. The
available funding and facilities precluded additional
modeling specialists, several of which indicated a desire
to participate. Consequently.- those who were invited were
selected to sample the principal research and administra-
tive areas. While the writer personally believes a better
group of people could not be assembled, no offense to the
many other people interested in irrigation return flow
modeling was intended.
38
-------
SECTION VI
REFERENCES
Childs, S.W. and R.J. Hanks. 1975. Model to predict the
effect of soil salinity on crop growth. Soil Sci. Soc.
Amer. Proc. (In press)
Dutt, G.R. and M.I. Shaffer. 1972. Computer simulation
model of dynamic bio-physicochemical processes in
soils. Technical Bulletin 196. Agricultural Experi-
ment Station. The University of Arizona, Tucson, Arizona.
Hanks, R.J. 1974. Model for predicting plant growth as
influenced by evapotranspiration and soil water.
Agronomy Journal 66 (5) :660-665.
Hornsby, A.G. 1973. Prediction modeling for salinity
control in irrigation return flows. Report EPA-R2-73-
168. U.S. Environmental Protection Agency. March.
Jensen, M.E. Ed. 1974. Consumptive use of water and irri-
gation water requirements. American Society of Civil
Engineers. New York, N.Y.
Nimah, M.N. and R.J. Hanks. 1973a. Model for estimating
soil water and atmospheric interrelations: I. Descrip-
tion and sensitivity. Soil Sci. Soc. Amer. Proc. 37:522-
527.
Nimah, M.N. and R.J. Hanks. 1973b. Model for estimating
soil water and atmospheric interrelations: II. Field
test of the model. Soil Sci. Soc. Amer. Proc. 37:528-
532.
Second International Drip Irrigation Congress. 1974.
Proceedings. San Diego, California. July-
Skogerboe, G.V. and J.P- Law. 1971. Research needs for
irrigation return flow quality control. Report 13030-
11/17. U.S. Environmental Protection Agency.
U.S. Department of Interior, Bureau of Reclamation. 1976.
Prediction of mineral quality of irrigation return flow.
Vol. I Summary report and verification
Vol.Ill Simulation model of conjunctive use and water
quality for a raver basin system
39
-------
REFERENCES (Cont.)
Vol. V Detailed Return Flow Salinity and Nutrient
Simulation Model
Reports currently under review by the U.S. Environmental
Protection Agency.- Ada, Oklahoma.
40
-------
SECTION VII
APPENDICES
A. List of Workshop Participants
B. Abstract, Conclusions, and Recommendations
from USER Summary Report
C. Workshop Agenda
D. Results of Workshop Questionnaire
E. Bibliography — Plant Root Extraction of
Water and Solutes
F. Bibliography — Groundwater Quality
G. Results of Take-Home Questionnaire
41
-------
APPENDIX A
WORKSHOP PARTICIPANTS
Mr. James E. Ayars
Graduate Research Assistant
Agricultural Engineering Dept,
Colorado State University
Fort Collins, Colorado 80523
(303)491-8362
Dr. James M. Davidson
Professor
Soil Science Dept.
2169 McCarty Hall
University of Florida
Gainesville, Florida 32611
(904)392-1951
Dr. R. John Hanks
Professor and Head
Dept. of Soil Science &
Biometeorology
Utah State University
Logan, Utah 84322
(801)752-4100
Dr. M. Leon Hyatt
Deputy Chief, Review and
Evaluation Branch
U.S. Environmental Pro-
tection Agency
Denver Federal Ctr., Bldg. 53
Denver, Colorado 80225
(303)234-2122
Dr. James D. Oster
Soil Scientist
U.S. Salinity Laboratory
P.O. Box 672
Riverside, California 92502
(714)683-0170
Mr. Eugene Christafano
Hydraulic Engineer
E&R Center Bldg. 67, Rm. 1370
Denver Federal Center
Denver, Colorado 80225
(303)234-2100
Dr. Lynn W. Gelhar
Assistant Professor
Dept. of Geo-Science
New Mexico Tech.
Soccoro, New Mexico 87801
(505)835-5307
Dr. Robert W. Hill
Assistant Professor
Dept. of Agricultural and
Irrigation Engineering
UMC 41
Utah State University
Logan, Utah 84322
(801)752-4100
Dr. David B. McWhorter
Associate Professor
Agricultural Engineering
Dept.
Colorado State University
Fort Collins, Colorado 80523
(303)491-8358
Mr. Richard W. Ribbens
Hydraulic Engineer
Bureau of Reclamation
E&R Center, Bldg. 67
Denver Federal Center
Denver, Colorado 80215
(303)234-2027
42
-------
APPENDIX A
WORKSHOP PARTICIPANTS (Cont.)
Dr. Marvin Shaffer
Soil Scientist
Bureau of Reclamation
E&R Centerf Bldg. 67
Denver Federal Center
Denver, Colorado 80215
(303)234-4081
Dr. A.W. Warrick
Associate Professor
Dept. of Soils, Water
and Engineering
507 Agricultural Sciences
Bldg.
University of Arizona
Tucson, Arizona 85721
(602)884-1516
Dr. Wynn R. Walker
Assistant Professor
Agricultural Engineering
Dept.
Colorado State University
Fort Collins, Colorado 80523
(303)491-5252
Mr. Kenneth K. Tanji
Lecturer in Water Science
Dept. of Water Science and
Engineering
University of California
Davis, California 95616
(916)752-0683
Dr. Arthur G. Hornsby
Soil Scientist
U.S. Environmental Protection
Agency, Robert S. Kerr
Environmental Research
Laboratory
P.O. Box 1198
Ada, Oklahoma 74820
(405)322-8800
43
-------
APPENDIX B
ABSTRACT, CONCLUSIONS, AND RECOMMENDATIONS
FROM USER SUMMARY REPORT*
ABSTRACT
This volume of the report outlines the purpose and
scope of the return flow research and specifically explains
the capabilities of the conjunctive use model for predicting
the mineral quality of irrigation return flow. The purpose
of the research was to develop a conjunctive use model which
would (1) predict the salinity contribution from new irri-
gation projects and (2) predict the change in return flow
salinity that would result from operational changes on exis-
ting projects.
The model developed and described herein describes the
chemical quality in terms of eight ionic constituents and
total dissolved solids. A nodal concept has been used to
facilitate subdividing the project area along physical or
hydrologic boundaries as desired. The study may be limited
to 1 or as many as 20 nodes.
A description of the Vernal Field Study which describes
the physical setting for the model testing is included. A
narrative describing the problems encountered with the orig-
inal data is included. A data collection program was
initiated to fill the gaps. The model satisfactorily simu-
lated the new 2-year data base. Tables and figures showing
the computed-observed comparisons from the verification are
included. Results of model operations for the Cedar Bluff
and Grand Valley areas are also described.
It is concluded that the model can satisfactorily be
used to simulate irrigation return flows if sufficient data
are available, especially groundwater hydrology and
chemistry.
U.S. Department of the Interior, Bureau of Reclamation, 1976,
Prediction of Mineral Quality of Irrigation Return Flow -
Summary Report and Verification. Reproduced for this
Workshop by Special Permission from the U.S. Environ-
mental Protection Agency. The complete set of five
volumes may be acquired from the U.S. Government Printing
Office or National Technical Information Service at the
completion of the USBR-EPA contract.
44
-------
The report was submitted in fulfillment of Project
EPA-IAG-D4-0371 by the U.S. Bureau of Reclamation,
Engineering and Research Center, under the sponsorship of
the Environmental Protection Agency.
45
-------
CONCLUSIONS
This research was concerned with the development of pro-
cedures for predicting the mineral and nutrient quality of
return flows from irrigation. Actual field conditions that
typify irrigation development in the western United States
were studied. In each study area, the research involved
characterizing field conditions, applying computer models to
predict quality of the percolating irrigation water, deter-
mining the effect of the percolating water on drainage
effluent, and evaluating system changes on quality of return
flow.
The percolation of water through soil in the process
of irrigating crops results in very complex chemical rela-
tionships. Both the mineral and nutrient content and the
quantity of return flow are difficult to predict under con-
ditions found in irrigated agriculture.
In developing a predictive mathematical model to simu-
late the effect of irrigation on water quality, it was
relatively simple to duplicate surface conditions. The
complexity of the problem stems from not having sufficient
knowledge of subsurface conditions such as soil chemistry,
volume of groundwater, aquifer capacity, depth to barrier,
and drainage characteristics. Variation in soil types within
short horizontal distances makes the acquisition of this
type of data costly, and it is not always available as needed.
This study dealt with three irrigated areas in attempting
to verify the predictive conjunctive use model - the Vernal,
Utah area; the Grand Valley, Colorado area; and the Cedar
Bluff, Kansas area.
Adequate data were available for the Vernal area and
the verification effort was minimal. Less data were avail-
able from the Cedar Bluff area with respect to the groundwater
body, and the verification proved to be much more difficult.
The chemistry of the return flow water is dependent to a large
degree on the volume and chemistry of the subsurface water.
Although the quality of the groundwater was well established,
considerable adjustment of the groundwater volume was neces-
sary in order to simulate existing conditions. This suggests
that the primary requirement in the simulation process is
to have a good knowledge of hydrologic conditions, including
the groundwater body, and particularly to establish a hydro-
logic balance in the system.
46
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RECOMMENDATIONS
The complexity of the salinity problem as it relates
to irrigated agriculture is borne out by these studies.
The need for a mathematical prediction model to assist in
understanding the salinity and nutrient problem is now
more clearly evident. The usefulness of the model in
simulating project conditions has been demonstrated, and
its ability to forecast changes resulting from improved
management has also been demonstrated in a limited way.
Although model development and testing have been
hampered by the lack of sufficient data, confidence could
be extended by the collection of additional data and
testing the model under a variety of conditions. The two
primary functions of the model are (1) to predict the
salinity effect from new irrigation projects and (2) to
predict the change in salinity that might result from
operational changes on existing projects. Some further
work should be undertaken, particularly on Item 2, since
the results could be quickly monitored and since there is
very little development of new irrigation projects under-
way.
Model development has demonstrated that data of good
quality and quantity are the primary requirements in
achieving a good simulation of irrigation project condi-
tions . Another requirement would be a good basic knowledge
of hydrologic conditions in the study area. If these
elements are lacking, difficulty can be expected in simu-
lation results.
A comparison of month-by-month observed and predicted
values with the annual values in the various studies indi-
cates that the predictions are more reliable on an annual
basis than a monthly basis. Since a great many factors
influence salinity levels on a monthly time frame, decisions
related to salinity projections based on model studies
should be limited to annual values.
47
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APPENDIX C
MODELING WORKSHOP AGENDA
March 19, 1975
Session 1. Modeling Philosophy
a. Objectives of Workshop
b. Workshop Format
c. History Leading to Existing USER Models
d. Position Paper on Structure of Irrigation
Return Flow Models
Session 2.
Formulation of Irrigation Return Flow Model
Systems
a. Modeling Irrigation Efficiency by Robert Hill,
Utah State University.
b. Modeling Root Extraction and Evapotranspira-
tion by John Hanks, Utah State University.
c. Modeling Root Zone Chemistry and Nitrogen
Movement and Transformation by Marvin Shaffer,
Bureau of Reclamation.
d. Management of Root Zone Salinity by James
Oster, U.S. Salinity Laboratory.
e. Modeling Groundwater and Drainage Flows
Associated with Irrigation Systems by Arthur
Warrick, University of Arizona.
f. Scaling Research Models for Application to
Large Irrigation Systems by Kenneth Tanji,
University of California at Davis.
g. Systems Approach to Managing Basin-wide
Salinity by M. Leon Hyatt, U.S. Environmental
Protection Agency.
Session Summary by Arthur Hornsby
48
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March 20, 1975
Session 3. Irrigation Return Flow Modeling in the Bureau
of Reclamation
a. Simulation of Conjunctive Use and Water
Quality in a River System. A presentation of
the USBR Conjunctive Use Model by Eugene
Christafano, Hydraulic Engineer, Bureau of
Reclamation.
b. Modeling Chemistry of the Partially Saturated
and Saturated Zones. A presentation of the
USBR unsaturated chemistry model by Marvin
Shaffer, Soil Scientist, Bureau of Reclama-
tion.
c. A presentation of the USBR unsaturated flow,
saturated flow, and drainage models by
Richard Ribbens, Hydraulic Engineer, Bureau
of Reclamation.
Session 4. Discussion of Field Verification Trials
a. Case Study 1 - Vernal by Eugene Christafano
b. Case Study 2 - Cedar Bluff by Eugene Christafano
c. Case Study 3 - Grand Valley by Wynn Walker
d. Case Study 4 - South Montezuma Valley by
Marvin Shaffer
Session 5.
March 21, 1975
Workshop Summary and Review of Research Needs
Present and Future
a. Open group discussion.
49
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APPENDIX D
WORKSHOP QUESTIONNAIRE RESPONSES
The following questions and statements were formulated to solicit
comments concerning where we now stand in the topic area of irrigation
return flow quantity and quality modeling technology. Responses were
rated on a scale of 1 to 5, if the feeling that the question or state-
ment was completely correct or if strong agreement was indicated, a 5
was written in the box at the right. If the reaction varied from quite
negative to mildly positive, responses ranged between 2 and 4. This
summary of the feelings of the workshop participants excludes those of
the writer and project officer. The numbers presented for questions or
statements 1-10 equal the percentage of responses falling into one of
the five choices. In questions 11, 12, and 13, written comments
were transcribed directly.
1. Numerous models have been developed relating
to individual or varied aspects of irrigation
return flow systems.
(a) How would you rate the general availa-
bility of information regarding these
models?
(b) How much would you benefit from having
information describing such models sum-
marized into one or a series of reports
or users manuals?
(c) How much would you benefit from a docu-
mented user's manual for these models
describing input requirements, methods
of solution, operational characteris-
tics, and format of results?
(d) How much would you benefit from an
assessment of the sensitivity of model
parameters and results as affected by
various kinds and qualities of input
data?
(e) How would you rate the general accep-
tance and use of these models by those
other than the author, especially
those in other disciplines?
5
8
25
33
67
0
4
8
58
42
25
8
3
33
17
25
8
42
2
42
0
0
0
42
1
9
0
0
0
8
50
-------
2. The components of irrigation return flow
models have been identified. How would
you rate these components for adequacy
and degree of development for general
applicability?
(a) irrigation efficiency models
(b) evapotranspiration models
(c) unsaturated flow models
(d) root extraction models
(e) root zone chemistry
(f) plant chemistry models
(g) saturated flow and drainage models
(h) saturated zone chemistry models
(i) surface hydrology and routing models
(j) management and optimizational models
5
8
0
8
0
0
0
0
0
17
0
4
8
67
42
25
25
0
50
0
42
25
3
33
17
42
50
50
33
50
67
25
25
2
33
8
8
25
25
33
0
33
16
25
1
8
8
0
0
0
0
34
0
0
25
3. Models of irrigation return flow systems
have been formulated in a number of dif-
ferent ways depending on their use and
the areas to which they are applied.
Below are several ideas for increasing
the use of existing models and concepts.
What are your preferences?
(a) Development of one generalized, all
inclusive irrigation return flow
model.
(b) Development of a set of generalized
subsystem models such as surface,
root zone, and groundwater models, etc.
(c) Library of subroutines and functions
simulating individual components of
the irrigation return flow system,
i.e. in the root zone having separ-
ate but standardized models for root
extraction, infiltration, chemistry
(soil and plant), percolation and
evapotranspiration.
(d) Irregardless of your choice above, how
do you rate the importance of docu-
mented field verification?
5
0
_3_3
50
75
4
8
33
25
25
3
8
17
17
0
2
25
9
8
0
1
59
8
0
0
51
-------
The model of conjunctive use and
regional water quality simulation
consists of the following functions.
How would you rate your interest in
these functions?
(a) Reservoir operation and quality
(b) Hydro-power generation
(c) Irrigation related consumptive use
(d) Irrigation return flow
(e) Soil water movement and chemistry
(f) Groundwater movement and chemistry
5
12
0
25
63
63
50
4
12
0
63
25
25
25
3
39
38
12
12
12
13
2
12
12
0
0
0
12
1
25
50
0
0
0
0
Because the conjunctive use model inte-
grates the interests of several disci-
plines, how would you assess the model's
usefulness to you with regard to the
following statements?
(a) Indicating the impact of an irri-
gated area on the total flow
resource.
(b) Scaling the results of a small
study area to an entire irriga-
ted area.
(c) Producing regional water quality
planning information.
(d) Assessing the environmental impact
of proposed development of change.
(e) Verification of other irrigation
return flow models.
5
12
0
12
0
0
4
50
12
38
63
25
3
38
63
50
25
50
2
0
25
0
12
25
1
0
0
0
0
0
52
-------
6. The data necessary for the conjunctive use
model comes from different kinds of monitor-
ing systems or studies all of which have
varied objectives, accuracy, and time inter-
val characteristics. In terms of the model's
purpose and scope, how would you rate the
consistency of the data with their use and
interpretation in the following areas.
(a) Soil water movement
(b) Soil chemistry
(c) Groundwater movement
(d) Groundwater chemistry
(e) Surface distribution of irrigation
diversions
(f) Consumptive use
(g) Reservoir operation and chemistry
7. The flow of water, salts, and nutrients in
the unsaturated soil profile has been
modeled more extensively by Bureau per-
sonnel with improvements and updating of
the so-called "Dutt Model." As more
research is completed and better under-
standing of the basic principles of the
unsaturated system is gained, where do
you feel the most significant improve-
ments should be made?
(a) Distribution of water under
different irrigation regimes
(b) Nitrogen chemistry
(c) Fertilizer movement
(d) Water, salt, and nutrient
root extraction patterns
(e) Estimation of evapotranspiration
(f) Salt movement, exchange and precip-
itation
5
0
0
0
0
0
0
0
4
25
37
12
12
38
38
38
3^
25
38
38
12
62
62
38
2
50
25
38
50
0
0
12
1
0 .
0
0
26
0
0
12
5
25
12
12
38
25
0
4
37
38
38
12
25
63
3
0
25
25
38
38
37
2
38
25
25
12
12
0
1
0
0
0
0
0
53
-------
10.
11.
How interested would you be in exploring
the interfacing of the unsaturated chemistry
model with others you may know of?
To what extent can you visualize "
this unsaturated chemistry being of
use to you in your on-going or anticipated
studies?
Bureau models of saturated flow and
chemistry have been oriented primarily
towards drainage design and evaluation.
In those situations where a drainage
system does not exist, these models
impose a hypothetical system to
evaluate return flow. How would you
rate the model's utility to your work?
5
50
50
12
4
0
25
38
3
38
12
38
2
12
13
12
1
0
0
0
The USER models have been structured from subroutines which can be
used as required to simulate the particular system in question.
Do you feel that sufficient detail has been given for a user to deter-
mine when it is appropriate to use or not use a particular subroutine?
- No
- Yes, providing considerable study or effort is expended to get
on top of actual workings of the model.
- No, however, I'm not certain that the USER personnel should be
expected to consider all potential problems. Many problems will
only be obvious to those with experience in a given area.
- Yes, certainly with planned manuals.
- Yes, for some, no for others. Should have math models and method
of solution. Not everyone has access to Bulletin 196 and other
USER citations.
- Yes, but it is not clear that the information is sufficient for
immediate implementation. Should look at dispersion effects
more explicitly; general approach should transfer to natural cir-
culation .
- Yes
- Yes
54
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12. The impetus in developing these models has been the need to assess
the effect of irrigating large acreages on the quality of the return
flow. This requires compromises in detail and assumptions regar-
ding homogeneity of properties in soil units. What changes
would you suggest to improve the models for use in modeling large
areas?
- Sensitivity studies and determination of field variability.
- Movement of water and associated chemistry through aquifers or
other portions of the groundwater system.
- How do we average across various soils? I'm not certain that
we can just assume an average soil profile.
- In some cases, may be able to break nodes down by soils, rather
than by contiguous areas.
- Groundwater and its chemistry; pickup of salts, sediments,
phosphorus, etc. in return flows; consider organic N and sus-
pended N in surface irrigation return flows; time dependent
root activity and uptake; dynamic reservoir quality modeling.
- There may be a need for models covering an intermediate level
of detail.
- Simplify.
13. General Comments?
- Don't feel that the USBR conjunctive use model is that good.
Know of others that can produce as good or better results.
Feel that concept is great, however.
55
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APPENDIX E
BIBLIOGRAPHY—PLANT ROOT EXTRACTION
OF WATER AND SOLUTES
1. Bernstein, L., L.E. Francois, and R.A. Clark. 1975.
Minimal leaching with varying root depths of alfalfa.
Soil Sci. Soc. Amer. Proc. 39:112-115.
2. Bloodworth, M.E., C.A. Burleson, and W.R. Cowley. 1958.
Root distribution of some irrigated crops using undis-
rupted soil cores. Agron. J. 50:317-320.
3. Bower, C.A., G. Ogata, and J.M. Tucker. Root zone salt
profiles and alfalfa growth as influenced by irrigation
water salinity and leaching fraction. Agron. J. 61:
783-785.
4. Bower, C.A., G. Ogata, and J.M. Tucker. 1970. Growth of
sudan and tall fescue grasses as influenced by irrigation
water salinity and leaching fraction. Agron. J. 62:793-
794.
5. Breazeale, J.F. 1930. Maintenance of Moisture Equilib-
rium and Nutrition of Plants at and Below the Wilting
Percentage. Arizona Agr. Exp. Sta. Tech. Bull. 29:137-177
6. Brust, K.J., C.H.M. van Bavel, and G.B. Stirk. 1968.
Hydraulic properties of clay loam soil and the field
measurement of water uptake by roots: III. Comparison
of field and laboratory data on retention and of measured
and calculated conductivities. Soil Sci. Soc. Amer. Proc.
32:322-326.
7. Ehlig, C.F. and Gardner, W.R. 1964. Relationship be-
tween transpiration and the internal water relations of
plants. Agron. J. 56:127-130.
8. Feddes, R.A., E. Bresler, and S.P. Neuman. 1974. Field
test of a modified numerical model for water uptake by
root systems. Water Resour. Res. 10:1199-1206.
9. Feddes, R.A. and P.E. Ritjema. 1972. Water withdrawal
by plant roots. J. Hydrology 17:33-59.
10. Gardner, W.R. 1960. Dynamic aspects of water avail-
ability to plants. Soil Sci. 89:63-73.
56
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11. Gardner, W.R. 1964. Relation of root distribution to
water uptake and availability. Agron. J. 56:35-41.
12. Gardner, W.R. and C.F- Ehlig. 1962. Some observations
on the movement of water to plant roots. Agron. J. 54:
453-456.
13. Ginsburg, H. and B.Z. Ginsburg. 1970. Radial water and
solute flows in roots of Zea mays: I. Water flow.
J. Exp. Botany 21:580-592.
14. Hanks, R.J. 1974. Model for predicting plant yield as
influenced by water use. Agron. J. 66:660-664.
15. Hunter, A.S. and O.J. Kelley. 1946. The extension of
plant roots into dry soil. Plant Phys. 21:445-451.
16. Klute, A. and D.B..Peters. 1969. Water uptake and root
growth. In Root Growth. ed. by W.J. Whittington. pp.
105-134, Butterworth, London.
17. Molz, F.J. 1971. Interaction of water uptake and root
distribution. Agron. J. 63:608-610.
18. Molz, F.J. and G.M. Hornberger. 1974. Water transport
through plant tissue in the presence of diffusable
solute. Soil Sci. Soc. Amer. Proc. 37:833-837.
19. Molz, F.J. and I. Remson. 1970. Extraction term models
of soil moisture use by transpiring plants. Water
Resour. Res. 6:1346-1356.
20. Molz, F.J. and I. Remson. 1971. Application of an ex-
traction term model to the study of moisture flow to
plant roots. Agron. J. 63:72-77.
21. Nakayama, F.S. and C.H.M. van Bavel. 1963. Root activity
distribution patterns of sorghum and soil moisture con-
ditions. Agron. J. 55:271-274.
22. Newman, E.I. 1969. Resistance to water flow in the soil
and plant. I. Soil resistance in relation to amounts
of root: Theoretical estimates. J. Appl. Ecol. 6:1.
23. Nimah, M.N. and R.J. Hanks. 1973a. Model for estimating
soil water, plant, and atmospheric interrelations, 1,
Description and sensitivity- Soil Sci. Soc. Amer. Proc.
37:522-527.
57
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24. Nimah, M.N. and R.J. Hanks. 1973b. Model for estimating
soil, plant, and atmospheric interrelations, 2, Field
test of model. Soil Sci. Soc. Amer. Proc. 37:528-532.
25. Ogata, Gen., L.A. Richards and W.R. Gardner. 1960.
Transpiration of alfalfa determined from soil water
content changes. Soil Sci. 89:179-182.
26. Raats, P.A.C. 1974. Steady flows of water and salt
in uniform soil profiles with plant roots. Soil Sci. Soc,
Amer. Proc. 38:717-722.
27. Rapier, C.D., Jr. and S.A. Barber. 1970. Rooting sys-
tems of soybeans, 1, Differences in root morphology
among varieties. Agron. J- 62:581-584.
28. Rawitz, E. 1970. The dependence of growth rate and
transpiration on plant and soil physical parameters
under controlled conditions. Soil Sci. 110:172-182.
29. Reicosky, D.C., R.J. Millington, A. Klute, and D.B.
Peters. 1972. Patterns of water uptake and root dis-
tributions of soybeans in the presence of water table.
Agron. J. 64:292-297.
30. Robins, J.S. and H.R. Haise. 1961. Determination of
consumptive use of water by irrigated crops in the
western United States. Soil Sci. Soc. Amer. Proc.
25:150-154.
31. Rose, C.W. and W.R. Stern. 1967. Determination of
withdrawal of water from a soil by crop roots as a
function of depth and time. Aust. J. Soil Sci. 5:11-19.
32. Rose, C.W., W.R. Stern, and J.E. Drumond. 1965. Deter-
mination of hydraulic conductivity as a function of
depth and water content for soil in situ. Aust. J.
Soil Sci. 3:1-9.
33. Schuurman, J.J. and M.A.J. Goedwaagen. 1971. Methods
for the Examination of Root Systems and Roots. 2nd ed.
26 pp. Center for Agricultural Publications and Docu-
ments, Wageningen, Netherlands.
34. Slatyer, R.O. 1957. The significance of the permanent
wilting percentage in studies of plant and soil water
relations. Botanical Rev. 23:585-636. '
58
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35. Stone, L.R., M.L. Horton, and T.C. Olson. 1973. Water
loss from an irrigated sorghum field, 2, Evaporation
and root extraction. Agron. J. 65:495-497.
36. van Bavel, C.H.M. 1967. Changes in canopy resistance
to water loss from alfalfa induced by soil water deple-
tion. Agr. Meteorol. 4:165-176.
37. van Bavel, C.H.M. and G.B. Stirk. 1967. Soil water
measurement with 2'*-'-Am-Be source and an application to
evaporimetry. J. Hydrol. 5:40-46.
38. van Bavel, C.H.M., G.B. Stirk, and K.J. Brust. 1968.
Hydraulic properties of a clay loam soil and the field
measurement of water uptake by roots: I. Interpreta-
tion of water content and pressure profiles. Soil
Sci. Soc. Amer. Proc. 32:310-317.
39. van Bavel, C.H.M., K.J. Brust, and G.B. Stirk. 1968.
Hydraulic properties of a clay loam soil and the field
measurement of water uptake by roots: II. The water
balance of the root zone. Soil Sci. Soc. Amer. Proc.
32:317-321.
40. Vazques, R. and S.A. Taylor. 1958. Simulated root
distribution and water removal rates from moist soil.
Soil Sci. Soc. Amer. Proc. 22:106-110.
41. Whisler, F.D., A. Klute, and R.J. Millington. 1968.
Analysis of steady state evaporation from a soil
column. Soil Sci. Soc. Amer. Proc. 32:167-174.
42. Whisler, F.D. A. Klute, and R.J. Millington. 1970.
Analysis of radial steady state solution and solute
flow. Soil Sci. Soc. Amer. Proc. 34:382-387.
43. Wilcox, J.C. 1959. Rate of soil drainage following an
irrigation. I. Nature of soil drainage curves. Can.
J. Soil Sci. 39:107-119.
44. Wilcox, J.C. 1960. Rate of soil drainage following an
irrigation. II. Effects on determination of rate of
consumptive use. Can. J. Soil Sci. 40:15-27.
45. Willardson, L.S. and W.L. Pope. 1963. Separation of
evaporation and deep percolation. J. Irr. Drain. Div.
ASCE 89-77-88.
59
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APPENDIX F
BIBLIOGRAPHY—GROUNDWATER QUALITY
1. Benoit, G.R. 1973. Effects of agricultural manage-
ment of wet sloping soil on nitrate and phosphorus in
surface and subsurface water,- Water Resour. Res. 9(5):
1296-1303.
2. Bigbee, P.D. and R.G. Taylor. 1972. Pollution studies
of the regional Ogallala aquifer at Portales, New Mexico,
Biological Sciences Department, Eastern New Mexico
University, New Mexico Water Resources Research Insti-
tute Report No. 005, New Mexico State University. 30 p.
3. Bouwer, H. 1970. Ground water recharge design for
renovation of waste water, J. Sanitary Engineering
Division, Proc. of ASCE, 96, No. SA 1, 59-74.
4. California Department of Water Resources. 1968. Delano
Nitrate Investigation, Bulletin No. 143-6.
5. Campbell, G.G. 1974. A simple method for determining
unsaturated conductivity from moisture retention data.
Soil Sci. 117:311-314.
6. Cartwright, K. 1974. Tracing shallow groundwater
systems by soil temperatures. Water Resour. Res. 10:
847-855.
7. Cearlock, D.B. 1971. A systems approach to management
of the Hanford Ground Water Basin," National Symposium
on Ground Water Quality, EPA, Project 16060 GRB, 182-191.
8. Coehlo, M.A. 1974. Spatial variability of water
related soil physical parameters. Unpublished Ph.D.
Dissertation. The University of Arizona, Tucson.
9. Crabtree, K.T. 1970. Nitrate variation in ground
water, Technical Completion Report OWRR B-044-WIS.
10. Domeico, P.A. 1972. Concepts and Models in Ground
Water Hydrology. McGraw-Hill Book Co., New York.
60
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11. Dooge, J.C.I. 1960. The routing of groundwater
recharge through typical elements of linear storage.
International Association of Scientific Hydrology,
publication No. 52, General Assembly of Helsinki,
August 25 - September 6. pp. 286-300.
12. Eliasson, J. 1971. Mechanism of ground water reser-
voirs, Nordic Hydrology, II, 266-277.
13. Environmental Protection Agency. 1971. Proceedings
of the National Ground Water Quality Symposium, Proj-
ect #16060 GRB.
14. Field, R., E.J. Struzeski, H.E. Masters, and A.N.
Tafuri. Water pollution and associated effects from
street salting, Environmental Protection Agency,
Report EPA-R2-73-257.
15. Freeze, R.A. 1959. The mechanism of natural ground-
water recharge and discharge 1. Water Resour. Res.
6:138-155.
16. Freeze, R.A. 1971. Three-dimensional, transient
saturated-unsaturated flow in a groundwater basin.
Water Resour. Res. 7:347-366.
17. Gelhar, L.W., et al. 1973. Groundwater Pollution, a
re-port prepared as part of the IAP mini-course, Depart-
ment of Civil Engineering, MIT.
18. Gelhar, L.W. 1974. Stochastic analysis of phreatic
aquifer, Water Resour. Res., 10, in press.
19. Gelhar, L.W. and J.L. Wilson. 1974. Ground water
quality modeling. Ground Water 12:399-408.
20. Gillham, R.W. and R.N. Farrolden. 1974. Sensitivity
analysis of input parameters in numerical modeling of
steady state regional groundwater flow. Water Resour.
Res. 10:529-538.
21. Harmeson, R.H., F.W. Sollo, and T.E. Larson. 1971.
The nitrate situation in Illinois, J. American Water
Works Assoc. 63(5);303-310.
61
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22. Hassan, A.A., B.C. Kleinecke, S.J. Johanson, and C.F.
Pierchala. 1974. Mathematical Modeling of water qual-
ity for water resources management. Volume I. Develop-
ment of the ground water quality model. Completion
Report. OWRR. Agreement number 14:31-0001-3733.
23. Hassan, A.A., B.C. Kleinecke, S.J. Johanson, and C.F.
Pierchala. 1974. Mathematical modeling of water qual-
ity for water resources management. Volume II. Develop-
ment of historical data for the verification of ground
water quality model of the Santa Clara - Callenguas
area, Ventura County. Completion Report. OWRR.
Agreement number 14-31-0001-3733.
24. Highway Research Board. 1970. Effects of deicing salts
on water quality and biota. Nat. Coop. Hwy- Research
Prog. Rept. 91.
25. Holtan, H.N. and N.C. Lopez. 1971. USDAHL-70 Model
of watershed hydrology, Technical Bulletin No. 1435,
Agricultural Research Service, USDA.
26. Hornsby, A.G. 1973. Prediction modeling for salinity
control in irrigation return flows, Environmental
Protection Agency, Report EPA-R2-73-168.
27. Hughes, G.M. , R.A. Landon, and R. N. Farvolden. 1971.
Hydrogeology of solid waste disposal sites in north-
eastern Illinois, U.S. Environmental Protection Agency
publication in the solid waste management series (SW-
12d) .
28. Hurley, P.A. 1968. Predicting return flows from irri-
gation. J. Irr. Br., ASCE(IR1)94:41-48.
29. Konikow, L.F. and J.P. Bredehoeft. 1974. Modeling
flow and chemical quality in an irrigated stream-
aquifer system. Water Resour. Res. 10:546-562.
30. Kraijenhoff Van de Leur, B.A. 1958. A study of non-
steady groundwater flow with special reference to a
reservoir-coefficient, Be Ingenieur, No. 19, 87-94.
31. Lau, L. 1972. Water recycling of sewage effluent by
irrigation: A field study on Oahu, University of
Hawaii, Water Resources Research Center, Technical
Report No. 62.
62
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32. Lin, S. 1972. Nonpoint rural sources of water pollu-
tion, Illinois State Water Survey Circular 111.
33. Lyons, T.C. and J.D. Stewart. 1973. Ground water
model development and verification for the San Jacinto
Ground Water Basin. Water Resources Engineers, Inc.,
Final Report on Task XVI(S)-1.
34. Maddaus, W.O. and M.A. Aarouson. 1972. A regional
groundwater resource management model. Water Resour.
Res., 8(1) :231-237.
35. Marino, M.A. 1974. Water-table fluctuation in response
to recharge. J. Irr. Dr., ASCE (IR2)100:117-125.
36. McQueen, I.S. and R.F. Miller. 1974. Approximating
soil moisture characteristics from limited data. Water
Resour. Res. 10:521-527.
37- Morrill, G.B. and L.G. Tolder. 1973. Effects of septic
tank wastes on quality of waterr Ipswich and Shawsheen
River Basins, Massachusetts. J. Research U.S.G.S. 1_(1) ,
117-120.
38. .Morris, W.J., N.W. Morgan, B. Wang and J.P. Riley. 1972.
Combined surface water-groundwater analysis of hydrologi-
cal systems with the aid of the hybrid computer. Water
Resour. Bull. 8:63.
39. Neumann, S.P., R.A. Feddes, and E. Bresler. 1974.
Finite element simulation of flow in saturated unsat-
urated Soils considering water uptake by plants. Third
Annual Report (Part 1) Project No. ALO-SWC-77.
Technion, Haifa.
40. Nielsen, D.R., J.W. Biggar, and K.T. Erh. 1973. Spatial
variability of field-measured soil-water properties.
Hilgardia 42:215-259.
41. Nimah, M.N. and R.J. Hanks. 1973. Model for estimating
soil, water, plant and atmospheric interrelations: I.
Description and sensitivity. Soil Sci. Soc. Amer. Proc.
37:522-527.
42. Olson, R.A., C. E. Seim, J. Muir. 1973. Influence
of agricultural practices on water quality of Nebraska:
a survey of streams, groundwater, and precipitation.
Water Resour. Bull. 9 (2) : 301-311.
63
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43. Parizek and Meyer. 1968. Recharge of ground water
from renovated sewage effluent by spray irrigation.
Pennsylvania State University, Institute for Research
on Land & Water Resources, Reprint Series No. 2, from
Proceedings of the Fourth American Water Resources
Conference, New York.
44. Perez, A.I., W.C. Huber, J.P. Heaney and E.E. Pyatt.
1972. A water quality model for a conjunctive surface-
groundwater system: an overview. Water Resour. Bull.
8:900-908.
45. Perez, A.I., W.C. Huber, J.P. Heaney, and E.E. Pyatt.
1974. A water quality model for a conjunctive surface-
groundwater system. EPA-600/5-74-013. USEPA, Office
of Research and Development, Washington, D.C.
46. Perlmutter, N.M. and A.A. Guerrera. 1970. Detergents
and associated contaminants in ground water at three
public supply well fields in Southwestern Suffolk
County, Long Isalnd, New York. USGS Water Supply
Paper 2001-B.
47. Perlmutter, N.M. and E. Koch. 1972. Preliminary hydro-
geologic appraisal of nitrate in ground water and
streams, Southern Nassau County, Long Island, New York.
U.S. Geological Survey Professional Paper 800-B, B225-235.
48. Pikul, M.F., R.L. Street and I. Remson. 1974. A numer-
ical model based on coupled one-dimensional Richards and
Boussinesq equations. Water Resour. Res. 10:295-304.
49. Pinder, G.F. 1973. A galerkin-finite element simulation
of groundwater contamination on Long Island, New York
Water Resour. Res. 9 (6): 1657-1669.
50. Quasim, S.R. and J.C. Burchinal. 1970. Leaching from
simulated landfills, Journal Water Poll. Control Fed.,
£2(3):371-379. March.
51. Rogowski, A.S. 1971. Watershed physics: Model of the
soil moisture characteristic. Water Resour. Res.
7:1575-1582.
52. Rogowski, A.S. 1972. Watershed physics: soil vari-
ability criteria. Water Resour. Res. 8:1015-1023.
64
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53. Rubin, J. and R.V. James. 1973. Dispersion-affected
transport of reacting solutes in saturated porous
media: Galerkin method applied to equilibrium-con-
trolled exchange in unidirectional steady water flow.
Water Resour. Res., 9(5):1332-1356.
54. Schmidt, K.D. 1973. Groundwater quality in the
Cortaro area northwest of Tucson, Arizona. Water
Resour. Bull., 9 (3) :598-606.
55. Schwartz, F.W. and P.A. Domenico. 1973. Simulation of
hydrochemical patterns in regional groundwater flow.
Water Resour. Res. 9:707-720.
56. Smith, S.O. and J.H. Baier. 1969. Report on nitrate
pollution of Groundwater-Nassau Country.- Long Island,
Nassau County Department of Health.
57. Snyder, W.M. and L.E. Asmussen. 1972. Subsurface
hydrograph analysis by convolution. J. Irr. Dr.,
ASCE(IR3) 98=405-418.
58. Soil Conservation Service. 1972. National Engineering
Handbook, Section 4, Hydrology, USDA.
59. Thomas, J.L., J.P. Riley and E.K. Israelsen. 1972.
A hybrid computer program for predicting the chemical
quality of irrigation return flows. Water Resour.
Bull., 8 (5):922-934.
60. Tyson, H.N., Jr. and E.M. Weber. 1964. Computer
simulation of ground water basins. J. Hydro. Div.
ASCE 90(4).
61. Water Resources Engineers, Inc. 1969. An Investiga-
tion of Salt Balance in the Upper Santa Ana River
Basin, Final Report to State Water Resources Control
Board.
62. Weber, E.M. and A.A. Hassan. 1972. Role of models
in ground water management. Water Resour. Bull. 8(1).
63. Weber, J.E., C.C. Kisiel and L. Duckstein. 1973. On
the mismatch between data and models of hydrologic and
water resource systems. Water Resour. Bull. 9:1076-1088
65
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APPENDIX G
RESULTS OF TAKE-HOME QUESTIONNAIRE
Included below are the questions and individual responses
of the ten participants who sent back their take-home
questionnaire.
1. During the first session of this workshop, a list of five
objectives we had hoped to accomplish were passed out and
discussed. In your view, how well has this workshop met
these goals?
a. The workshop has met these goals very well. With
regard to objective 4, the present and future needs
did not seem to come out as explicitly as might have
been hoped.
b. The workshop met the goals fairly well.
c. Very well!
d. Objectives 1, 2, 3 and 5 met, objective 4 addressed
only indirectly and not to sufficient breadth and
scope.
e. I believe the objectives were met, in the main.
Objective 4 was not dealt with in a great deal of
detail, except insofar as the discussion of a library
of subroutines applied to this objective. Objective
5 may pretty well have been already met by the people
attending because of their own background in modeling.
f. I believe the general intent and goals of the IRF
workshop were met, especially with regard to exposure
of the USER model and modelers. The group from out-
side the Bureau should be of help in exposing the
USER model to other research personnel and users.
I believe the goals as given in objectives 2 and 3
were fully accomplished. The Bureau personnel did
not raise probing questions about research needs
identified by others at the conference. However,
they did give us a good insight into their philosophy
of modeling and of verification. I may have a biased
66
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view but I believe the degree of interdisciplinary
awareness of the problems associated with irrigation
return flows quality is highly developed within the
Western U.S., particularly among scientists and exten-
sion personnel interested in the soil-plant-water
system. Consequently, I view objectives 1, 4 and 5
more from the perspective of how well we, the par-
ticipants, exchanged comments and ideas. I have the
feeling we tried, but, because of the high degree of
commonality among us, some comments and ideas were
traded back and forth rather casually. For example,
did we really attempt to prove Ken's call to
simplicity?
i. 1 met partially, provided an opportunity not "the"
vehicle
2 met
3 met
4 opportunity again provided
5 most present already had such an awareness
j. Has accomplished them in a very satisfactory manner.
Workshop initiators can feel proud of the job they
have done.
2. The workshop hopefully covered topics outside of most
individual interests. To what extent do you feel the
introduction to people of varied interests and work of
new dimensions have expanded your own perceptions?
a. Several specific items were new to me: (a) Finite
elements, (b) mass balance problems associated with
saturated flow to tile drains, (c) Bresler's treat-
ment of dispersion coefficients, and (d) the possi-
bility of obtaining the root to 3rd and 4th order
equation in closed form. Another stimulating ques-
tion was how to average spatial variability in soil
chemical and physical properties of soil. Also, I
perceive with much greater clarity the advantage of
nonsteady-state modeling in elucidating the long
times associated with chemical changes in IRF!
b. I believe this is one of the principle benefits of
the workshop - especially to meet with the USSR
people and see how they think and how they talk
about their work. Most of the rest of the group I had
met before - or met people that have similar ideas.
67
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c. I appreciated the opportunity to visit with individ-
uals that had a different interest and philosophy
in the area of soil-water and solute modeling. I
picked up several good approaches to problems of
interest to me and our research group. The broad
dimension of interest was of value to me.
d. A great deal. I now have a better idea of the
feasible approaches to problems of varying scale
and required detail.
e. A great deal.
f. Generally good, but somewhat variable.
g. The workshop produced a good source of references for
problems I've encountered in my limited modeling
experience. It expanded considerably my perceptions
of the intricacies of modeling irrigation return flow.
h. Very much, I feel my time was well spent.
i. Slightly
j. Very much. Have a much better idea of the scope of
other disciplines in regards to irrigation return flow.
3. Could you comment on the time, depth, and organization of
the workshop and point out where more or less emphasis
should have been given?
a. Allocation of time was all right. Perhaps we could
have made more concrete suggestions as to future
needs and efforts. Also, maybe could have elucidated
schemes and objectives for computer program library.
b. The first session on modeling philosophy should have
spent more time investigating the concept of levels
of model (process modeling) and the development of a
subroutine library. The seven presentations in the
afternoon seemed to be hurried and somewhat dis-
organized. I would have preferred few discussions
with possibly more detail given in each session
about the subject being covered. The case studies
might have been better if they were included in the
original discussion of the USER models. Actual use
of the models required a previous knowledge of data
and format.
c. Good!
68
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d. Practical use of modeling efforts, particularly with
regards to estimates on what kinds of changes can be
effected with differing management.
e. I believe the workshop was well organized and
appropriately balanced. I think somewhat more time
could have been spent on summarizing the state-of-
the-art and assessing future needs.
f. The facilities and physical operation of the workshop
were excellent. Out of personal interest, I would
like to have had more discussion on the actual models
being used to describe chemical processes. Also,
numerical solution techniques for the various models
and methods of holding computer costs to a minimum.
As we go to larger, more complex problems, the cost
of using the computer is going to become an important
issue.
g. I believe it was about right - at least the two days
that I was there. I believe the philosophical
discussions the first half day were of least value.
h. Discussion of a computer library should have occurred
toward the end of the workshop after we had a chance
to evaluate the usefulness of models. A great oppor-
tunity to evaluate the various inputs was missed on
Friday, partially because of poor attendance.
i. Organization was excellent, timing - O.K. Depth as
good as expected for the time. Less emphasis on
actual operation of a computer.
j. Generally and overall was very good. I personally
would like to have gone into more detail on Wynn's
idea of different levels of modeling efforts.
4. After having had the association of other individuals
interested in irrigation return flows and hearing their
comments or questions, what do you feel are the major
modeling problems we must begin resolving?
a. 1) Groundwater - unsaturated flow linkage; 2) yield -
n.anagement models including salt tolerance - irrigation
methods interactions; and 3) methods of model verifica-
tion need study. Would hydrologic and chloride balance
offer a tool to simplify the process? Chloride ratios
could be used to estimate mass emissions and concentra-
tion of individual ions based on soil chemistry models.
There should at least be a reduction in the amount of
data required without, I believe, a loss in accuracy.
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b. I feel we are far from understanding the soil-water
chemistry of return flow. There are many problems
that come up in the field that are surprises - not
according to the salinity handbook. I believe we
are still far from being able to tell irrigators how
to manage irrigation to minimize flows nor am I sure
we could really accomplish much on a large river
salt load if we could. However, the impact back on
the farm may be very serious.
c. How do we handle variability as we go from small to
large land areas? Also, many of our chemical models
are detailed and others are rather simple in their
approach; therefore, how sensitive are our models
over long time periods? We need to make sensitivity
analyses on these models.
d. Eliminate the inefficiency of "reinventing the wheel"
as much as possible. This is a tough problem but
some of the ideas concerning libraries of well docu-
mented sub-programs might go a long way.
e. Obtaining adequate data base and information on
groundwater hydrology.
f. i) May need model with intermediate level of detail
between the overall conjunctive use and the detailed
flow and chemistry models; ii) improve calibration
procedure - systematize; iii) determine real data
requirements for each level; iv) use for management.
g. The proper use of a model in a particular situation.
Dissemination of existing models and testing of same
with written evaluations are two of the most impor-
tant problems. Documentation of existing models is
crucial. The process of plant-water extraction and
movement of soil water are two major physical pro-
cesses to be resolved.
h. Need more of a balance of treating various components
or perhaps guidelines on what parts to include for a
given case. It seems to me the flow under saturated
conditions is crude compared to the rest of the Vol.
3 model. I think the spatial variability problem is
a big one, but unlike the other factors, the theory
is relatively underdeveloped.
i. Adequate data for field truth.
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j. A systematic and uniform approach for integrating
the modeling efforts together where this is useful.
I can see a need for generalized, yet comprehensive,
large-scale models with integrated subroutines that
consist of more specialized models which look in
more detail at specific processes.
5. What would you identify as the most important problems
associated with managing and controlling irrigation
return flows?
a. Defining most relevant parameters—these vary from
one site to the next. Overall wish to minimize
water returning by not over-irrigating and mini-
mizing seepage losses from canals and ditches.
Monitoring systems perhaps need more attention.
b. 1) Managing canal losses; 2) managing irrigation
tailwater - deep percolation.
c. Sound information on applied water and ET. Realistic
information on groundwater quantity and quality.
Feasibility of water control on individual forms.
d. Better fix on salt pickup - salt deposition, natural
sources, etc. Identification and evaluation of con-
trol technologies and their anticipated results.
e.
f. It seems to me that the USER model could be used to
identify the major problems associated with managing
and controlling irrigation return flows. This
technique would also point out problems in the var-
ious submodels as well as the integration of the
submodels over long periods of time.
g. Being able to predict over long periods of time what
the salt buildup will be in the soil with a wide
variety of management schemes.
h. Social and legal restraints and politics. In terms
of irrigation management, I believe the biggest prob-
lem is in the measurement of water at the field gate
and estimates of ET.
i. Splitting out the natural contribution of salts from
the effects of irrigation only.
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j. Control of the water throughout the irrigation system.
It will take considerably more effort and definition
of the interflow and groundwater portions of the
system.
What suggestions and recommendations would you make to
the Bureau of Reclamation for improving the capability of
their models in light of the objectives for which they
were developed and are expected to be utilized?
a.
b. It would appear to me that the USER effort is rather
fragmented and confused - at least I don't clearly
get what they are doing. They don't seem to be
talking to other people as much as they could. They
don't clearly come across with what or how they are
doing things.
c. It was not clear to me how the submodels were called
into the main program. I would like to see a modular
procedure used so that submodel changes could be made
with minimum disturbance to the computer program.
d. The model assumes the presence of a subsurface
drainage system that is not present in many of our
applications. Sub-programs capable of handling
other saturated flow situations would be helpful.
e. Consider case of no underdrain. Improve interfacing
between different models, i.e., conjunctive use and
routing for example. Consider steady-state approach.
f. Include anisotropy in saturated flow model, allow
bare soil evaporation in unsaturated flow model,
clarify ACUMEN subsurface flow structure.
g. 1) More extensive field testing; 2) expand capa-
bilities of unsaturated flow to include layered soil,
hystersis, modify extraction terms.
h. Overall it is a very good effort. Probably stress
two-dimensional subsurface part and put internodal
transfer on a more scientific basis. Check carefully
for errors arising strictly from computational part,
e.g., due to inadequate time or space steps. Accept
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fact that programs will continue to evolve with time.
Perhaps, need more flexibility on running separate
parts alone.
i. Get a more physically based model, better control on
field data, more candor, and more humility in their
claims.
j. More integration into a total or complete package or
model rather than the 3-box separate modeling effort.
7. Would you give us your frank opinion of the workshop
which previous questions did not address?
a. I, personally, gained a lot from the workshop. Real-
istically, the USER models will be of the most use
to the Bureau itself. I'm interested primarily in
small pieces of the overall model and will profit
more from the write-ups and descriptions than from
the programs themselves.
b. —
c. It was worthwhile!
d. I think the workshop was very worthwhile from my
personal standpoint.
e. I thought the workshop was a very worthwhile effort.
The idea of a library of programs should be considered
further as it would help many researchers evaluate
what is available and methods of approaching their
specific problem. However, the various computer
libraries, such as SHARE (Triangle Universities Com-
putation Center, Research Triangle Park, N.C.), should
be considered before a lot of effort is expended.
f. Another conference should be held, but, at this one,
the legal, farming, research, economic, Bureau and
EPA interests should be represented to assess the
social-economic implications associated with
modeling efforts.
g. Although math models and solutions were described
and referred to in presentations, handout materials
did not contain such information for study prior to
workshop. USER made an admirable effort in meeting
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their EPA project objectives, but there seems to be
an imbalance between detailed chemical modeling and
physical modeling. I believe some of the partici-
pants were reluctant in making constructive criti-
cisms and/or comments. Perhaps they will feel more
at ease with this take-home questionnaire. Many of
the participants have not had the experience of
tackling large-scale modeling.
h. The main value of any workshop is to get to exchange
ideas with people. This the workshop did very well.
It allowed me to get together with the USER people
and spend some time with them and really try and
find out what they are doing. It also helped me to
realize where the work I am doing fits in the whole
scheme of things and helps to put it in perspective.
It certainly demonstrated how disorganized we all
are and how poor many of our models are. It also
demonstrated again the great value of keeping in
contact with each other. I suspect,as a result of
the conference, some of us will make some changes
in our research efforts.
i. I enjoyed getting acquainted with the individuals
present and sharing ideas.
j. It was a first-class effort. I can think of no
constructive criticism that applies to the workshop.
Those topics that are pertinent were covered. Time
allowed to discuss each item was appropriate and
reasonable. I have to offer my congratulations on a
fine job.
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TECHNICAL REPORT DATA
.Please react liislructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-76-219
4. TITLE AND SUBTITLE
ASSESSMENT OF IRRIGATION RETURN FLOW MODELS
3. RECIPIENT'S ACCESSION-NO.
5. REPORT DATE
October 1976 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Wynn R. Walker
8. PERFORMING ORGANIZATION REPORT NO.
3. PERFORMING ORGANIZATION NAME AND ADDRESS
Agricultural Engineering Department
Colorado State University
Fort Collins, Colorado 80523
10. PROGRAM ELEMENT NO.
1HB617
11. CONTRACT/GRANT NO.
R-803477
12. SPONSORING AGENCY NAME AND ADDRESS
Robert S. Kerr Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Ada, Oklahoma 74820
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
is. ABSTRACT Throughout the Western United States irrigation return flows
contribute to the problem of water quality degradation. Evaluating the
effectiveness of alternative management strategies involves models which
simulate the processes encompassed by irrigated agriculture. The develop -
nent and application of these models require multidisciplinary expertise.
^ workshop involving fifteen specialists in the varied aspects of irri-
gation return flow modeling was held to review the status of these models
Irrigation return flow and conjunctive use models recently developed by
bhe Bureau of Reclamation served as focal points for the workshop. As
the field verification and potential applications of these models were
iiscussed, several general problems were identified where further inves-
tigation is needed. Particular emphasis was given to the description of
e spatially varied aspects of soil, crop, and aquifer systems, and
the proper alignment of model objectives with available data. The large
lumber and diversity of existing models illustrate the individualistic
lature of irrigation return flow modeling. In order to affect more wide-
spread utilization of existing models, a systematic procedure should be
ieveloped to update and disseminate this modeling technology.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
COSATI Field/Group
Irrigation, Salinity,
Mathematical modeling, Soil
chemistry, Water consumption,
Drainage, Leaching
Irrigation efficiency
Irrigation scheduling
Return flow, Digital
computer model
08/G
02/C
13. DISTRIBUTION STATEMENT
Release to public
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
83
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
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