In Partial Fulfillment of the RequirementsFor the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
1974
THE UNIVERSITY OF ARIZONA
GRADUATE COLLEGE
I hereby recommend that this dissertation prepared under my
direction by JOSE MATIAS FILHO
entitled WATER RESOURCES MANAGEMENT FOR PART OF THE LOWER
GILA VALLEY
be accepted as fulfilling the dissertation requirement of the
degree of DOCTOR OF PHILOSOPHY
Date
After inspection of the final copy of the dissertation, the
following members of the Final Examination Committee concur in
its approval and recommend its acceptance:*
Oct /7 /9'74
4,1 /7/ /72z/
&i-19.-t. /7 0'74'
CIO( -7X-
-
This approval and acceptance is contingent on the candidate's
adequate performance and defense of this dissertation at the
final oral examination. The inclusion of this sheet bound into
the library copy of the dissertation is evidence of satisfactory
performance at the final examination.
STATEMENT BY AUTHOR
This dissertation has been submitted in partial fulfillment ofrequirements for an advanced degree at The University of Arizona andis deposited in the University Library to be made available to bor-rowers under rules of the Library.
Brief quotations from this dissertation are allowable withoutspecial permission, provided that accurate acknowledgment of sourceis made. Requests for permission for extended quotation from or re-production of this manuscript in whole or in part may be granted bythe head of the major department or the Dean of the Graduate Collegewhen in his judgment the proposed use of the material is in the in-terests of scholarship. In all other instances, however, permissionmust be obtained from the author.
SIGNED:72-'
ACKNOWLEDGMENTS
It gives me great pleasure to express my sincere appreciation to
my advisor, Professor W. Gerald Matlock, who spared no effort in his
guidance and suggestions throughout this study.
I am particularly thankful to my major professor, Dr. M. M.
Fogel, and the other members of my graduate committee, Dr. D. D.
Fangmeier, Dr. M. J. Zwolinski, and Dr. G. S. Lehman, for their
assistance and time they profferred so generously.
Grateful acknowledgments are expressed to Dr. LeMoyne Hogan for
his interest and cooperation as chief-of-party of the Federal University
of Ceara/University of Arizona/AID Contract LA-145 at the time of my de-
parture to the United States.
I am specially grateful to my wife, Zuleica, and my children,
for their patience, understanding, and assistance.
The author was supported by Ford Foundation and United States
Agency for International Development (USAID), to whom he is deeply
grateful.
TABLE OF CONTENTS
Page
LIST OF TABLES vii
LIST OF ILLUSTRATIONS
ix
ABSTRACT
INTRODUCTION 1
Water Resources Management 1Purpose and Objectives 5
THE LOWER GILA RIVER 7
History 7Physical Features 11
Location and Extent 11Topography 14Vegetation 15Hydrogeology 16Natural Drainage System and Flow Characteristics 20
Description of the Modeling Process 65Phase I Model 65Phase II Model 69
Validation of the Model 79Alternative Management Strategies 79
General 79Change in Irrigation Method 80Increase in Irrigated Area 80Change in Crop Allotment 81Increase in Water Use Efficiency 81Additional Drainage Facilities 82Conjunctive Use of Water Resources 83Phreatophyte Control 84Desalinization 85Summary 86
Model Operation with Alternative Strategies 87Alternative Strategies 88
Strategy I 88Strategies II, III, and IV 89Strategies V and VI 89Strategy VII 89Strategy VIII 89
RESULTS AND DISCUSSION 90
Phase I Strategy I
Phase II Strategy I Strategy II Strategy III Strategy IV Strategy V Strategy VI
9090929497101103106106
vi
TABLE OF CONTENTS--Continued
Page
Strategy VII 108Strategy VIII 111
CONCLUSIONS AND RECOMMENDATIONS 114
Conclusions 114Recommendations 119
APPENDIX A: DESCRIPTION AND LISTING OF THE COMPUTER PROGRAM . 121
SELECTED BIBLIOGRAPHY 126
LIST OF TABLES
Table Page
1. Wellton-Mohawk Irrigation and Drainage District: IrrigatedArea, in Acres 12
2. Wellton-Mohawk Irrigation and Drainage District: CropAcreage 17
3. Mean Annual Precipitation for Representative Stations inthe Lower Gila River Basin 24
4. Lower Gila River Surface Flow, in Acre Feet 28
5. Lower Gila River: Estimated Painted Rock ReservoirReleases Distribution Between the Dam and Dome, Duringthe Spring 1966, in Acre Feet 29
6. Wellton-Mohawk Area: Land Classification, Acreage, andSoil Characteristics 39
8. Wellton-Mohawk Irrigation and Drainage District:Estimated Water Supply Delivered to the Mesa, inAcre Feet 46
9. Wellton-Mohawk Irrigation and Drainage District:Estimated Water Supply Delivered to the Valley, inAcre Feet 47
10. Wellton-Mohawk Irrigation and Drainage District:Estimated Annual Salt Input, in Tons 52
11. Wellton-Mohawk Irrigation and Drainage District:Estimated Salt Output, in Tons 60
12. Yuma Projects: Water Delivered to Farms Per IrrigatedAcre, in Acre Feet 61
13. Yuma Projects: Operation and Maintenance Costs PerIrrigated Acre, in Dollars 62
vi i
LIST OF TABLES—Continued
Table
14. Yuma Projects: Gross Crop Values Per Irrigated Acre,in Dollars
15. Wellton-Mohawk Irrigation and Drainage District:Estimated Land Use 72
16. Wellton-Mohawk Irrigation and Drainage District--Strategy I: No Modification of the Present ManagementPolicy (Phase I) 91
17. Wellton-Mohawk Irrigation and Drainage District:Groundwater Depths for Various Alternative ManagementStrategies, in Feet
93
18. Wellton-Mohawk Irrigation and Drainage District--Strategy I: No Modification of the Present Manage-ment Policy (Phase II) 95
19. Wellton-Mohawk Irrigation and Drainage District--Strategy II: Sprinkler Irrigation on 25 Percent ofthe Mesa Area
98
20. Wellton-Mohawk Irrigation and Drainage District--Strategy III: Sprinkler Irrigation on 50 Percent ofthe Mesa Area
102
21. Wellton-Mohawk Irrigation and Drainage District--Strategy IV: Sprinkler Irrigation on 100 Percent ofthe Mesa Area
105
22. Wellton-Mohawk Irrigation and Drainage District--Strategy V: Reduction by 50 Percent of thePhreatophytes
107
23. Wellton-Mohawk Irrigation and Drainage District--Strategy VI: Reduction by 100 Percent of thePhreatophytes
109
24. Wellton-Mohawk Irrigation and Drainage District--Strategy VII: Sprinkler Irrigation on 50 Percent ofthe Mesa Area and Reduction by 50 Percent of thePhreatophytes
110
25. Wellton-Mohawk Irrigation and Drainage District--
Strategy VIII: Sprinkler Irrigation on 100 Percent of theMesa Area and Reduction by 100 Percent of the Phreatophytes 112
viii
Page
64
LIST OF ILLUSTRATIONS
Figure Page
1. Gila River Downstream from Painted Rock Dam 13
2. Wellton-Mohawk Irrigation and Drainage District 38
3. Flow Diagram for the Hydrologic Model Used in Phase I 70
4. Flow Diagram for Typical Sectors of the HydrologicModel Used in Phase II 78
ix
ABSTRACT
The Wellton-Mohawk Irrigation and Drainage District occupies a
valley and adjoining mesa along the lower Gila River, in the southwestern
part of the State of Arizona. The area has been irrigated for centuries,
and now shows problems which reflect past and present water management.
First, the water supplies came from the Gila River; later, the ground-
water reservoir was used and within about 30 years, groundwater levels
declined and salt accumulation, as a consequence of water recirculation,
put a limit on attempts to maintain irrigated agriculture. Recently,
Colorado River water was brought into the area as the solution to assure
permanent large-scale irrigation development. The application of water
for crops and leaching of salts caused serious drainage problems.
Salinity also caused a problem out of the District as drainage water
from the aquifer with high salt content reached the Colorado River and
became a source of friction between the United States and Mexico.
The water conveyance system in the District is unique in that
irrigation water is pumped up the valley into the distribution system.
During flood flows along the lower Gila River, this leads to the situa-
tion where water is going down the River with little chance to be used,
and goes up the valley through a sophisticated conveyance system.
Flood flows along the lower Gila River are dependent on in-
frequent releases from Painted Rock Reservoir, at the upstream boundary
of the lower Gila River. The few times they have occurred (2 in 15
xi
years), they created high groundwater levels which were damaging to
crop production.
The water problems in the District could have short-run solu-
tions through technically possible and economically feasible management
practices.
The objectives of the study are focused on better use of the
water resources, reduction of risks of flood damages, and decrease of
salt content of water being diverted to Mexico. A mathematical model
was developed to analyze the impact of selected alternatives which could
meet these objectives upon the hydrologic system of the District.
The application of Strategy I, which proposes the increase of
the irrigated acreage by about 5,000 acres, proved to be impracticable
under present management conditions since the amount of drainage water
to be disposed would be greater than the capacity of the disposal system.
Strategies II, III, and IV, which propose increasing levels of
change from flood to sprinkler irrigation (25, 50 and 100 rcent)
showed results that although not economically encouraging, provide,
however, for solution of the internal water problem of the area, and
substantial decrease of drainage flow of high salt content delivered to
the Colorado River.
Strategies V and VI, which proposed reduction by 50 percent or
complete elimination of riparian vegetation also proved to be im-
practicable. Under present management conditions in the District,
phreatophytes are an important auxiliary of the water discharge system
of the area.
xii
Strategies VII and VIII showed that the combination of changes
to sprinkler irrigation and reduction of riparian vegetation at levels
proposed (50 and 100 percent) practically counteract each other in terms
of drainage water to be pumped and does not achieve the proposed
objectives.
Change in the water management system of the Wellton-Mohawk
District would solve its water problems and significantly reduce
salinity of the Colorado River water at Morelos Dam, for which hundreds
of millions of dollars will be expended in a desalting complex.
Drainage from excessive irrigation on the mesa flowing into the valley
aquifer is the main cause of high groundwater levels there. Riparian
vegetation, although increasing flood damages, is indispensable under
the present management system.
INTRODUCTION
Water Resources Management
Man's attitude toward his natural resources was for many
centuries based on the assumption that demand could ever exceed the
supply. This belief led man to consume his natural resources with
little regard to conservation and economy. Population growth and the
consequent need for more food and fiber have proved, however, that man
can no longer retain this selfish and unconcerned attitude toward his
natural resources. Water resources cannot be excluded from the reality
that demand often exceeds supply.
"The possibility of increasing supplies of usable water no
longer exists through the exploitation of new resources but rather an
integration of technological, political, social, and economic factors
will be necessary" (Dvoracek and Peterson 1971, p. 219). The problem of
increasing water supplies becomes, then, more and more a question of
water management to reduce water inefficiency which is characteristic of
high water use developments.
Irrigated agriculture, is and has been, the dominant user in the
allocation of water resources, and has also been accused of being an
inefficient and uneconomical user. A major part of the water used by
irrigated agriculture is lost through the process of evapotranspiration,
in contrast to other uses that do not "consume" water (Jensen 1967).
1
2
In manufacturing, for example, about 98 percent of the water withdrawn
is returned for reuse, but only about 40 percent is returned from irri-
gated agriculture. The remaining 60 percent, termed consumptive use, is
lost through evapotranspiration.
Because irrigated agriculture accounts for a very high percentage
of the water consumed around the world, the greatest opportunity for
saving water through better management is, consequently, through better
management of agricultural water. Management responsibility for an
irrigation system consists essentially of that associated with the
control, movement, and disposal of water or its reuse where possible.
But something more than merely supplying water to the land and removing
the excess as drainage is necessary. An important factor now, and even
more so in the future, will be the impact that the system has on the
quality of the supply and the quality of the drainage. "The most
obvious, or perhaps notorious, examples of man's short-term impact on
the quality of his water resources are found in his manipulation of
water for irrigation" (Orlob and Woods 1967, p. 49).
Reducing evaporation from free water surfaces, controlling
seepage from conveyance channels, artificial replenishment of ground
water, and improvement of structures for water control and management
are some important aspects of water storage and movement which offer
considerable opportunity to increase the availability of our water
resources. Considerable research on evaporation from open water surfaces
and seepage from conveyance channels has been conducted to find
economically feasible techniques which could eliminate or control their
3
effects in depleting the available supplies. Some developed linings
have been shown to be permanent and effective for seepage control.
Research toward controlling evaporation from water surfaces has
centered largely on the use of monomolecular films of long-chain
alkanols. More recently, Myers and Frasier (1970) found that floating
granular materials which cool the water by reflecting incoming shortwave
radiation appear promising as a means of reducing evaporation from water
surfaces. Although substantial evaporation reduction has been observed
in ponds and small reservoirs, the application of these techniques to
larger bodies of water don't produce similar results and much work is
till to be done to justify their use as a practical and economic
measure.
Artificial recharge can be used to store excess water during
flooding periods or through planned "overirrigation," and to dispose of
waste waters. These aspects of artificial recharge have been explored
in theory and in the field, and although many problems are still to be
solved, some techniques have proven both technically and economically
feasible under the conditions where they were developed. Artificial
recharge can also be an important practice in the integrated development
and conjunctive use of surface and groundwaters. This is a very import-
ant aspect in water deficient regions as it provides opportunity for
augmentation of irrigation supplies and a wide range of alternative
decisions for their utilization.
The increasing demand upon available water resources is reaching
levels where agricultural operations cannot be insensitive to wasteful
4
water-use practices. As the competetive pressures from industrial,
recreational, and urban uses increase, agriculture will increasingly
have to justify its high use of water. Of the water delivered to the
farm, the 40 percent not used by plants through evapotranspiration is
lost by surface runoff and deep percolation. Frequently, more agri-
cultural water is wasted by not knowing when to irrigate than by poor
application efficiency (Erie 1968). In many agricultural areas, in-
cluding many modern irrigation projects, farmers continue to use
traditional irrigation practices little influenced by modern science
and technology (Committee on Research of the Irrigation and Drainage
Division, ASCE 1974). Irrigation management services, provided by an
irrigation district to member farmers are being used to supply the
farmers with information that will improve on-farm water management.
Based on more reliable estimates and predictions than the farmers
generally can make of evapotranspiration, soil moisture depletion levels,
and plant nutritional status, these services are producing positive
results. The most significant improvements in water use efficiency,
then, will potentially come from improved water management.
Irrigation practices on the farm are also the primary source of
present return flow quality problems. Historically, water management
has dealt with the distribution of water in time and space, and research
has been directed towards controlling such distribution. Agricultural
water management relates to water quality in a number of ways such as
erosion and sedimentation and through return flows that contain plant
nutrients, animal wastes, and salts. Since all waters contain some
5
dissolved salts, salinity may become a problem when inadequate pre-
cautions are taken to prevent a buildup of salts in the soil. Sub-
stantial progress has been made in developing an understanding of the
physiochemical behavior of salt-affected soils and the movement of
water through them. The tolerance of many plants to salinity has been
determined. An important impact of salt occurrence in irrigated areas
is, however, the concentration of salts in the drainage water being
disposed of downstream that is often too high for safe reuse for
agricultural purposes. A wide range of alternative management practices
have been developed which have shown that it is possible to manage
irrigation water in such a manner that the leaching fraction can be
substantially lower than is now customary or recommended. In this way,
the quantity of drainage water is reduced and its quality increased.
Purpose and Objectives
The purpose of the present work is to develop simple modeling
techniques which can be helpful in applying alternative management
strategies to irrigated agricultural projects and thus to contribute to
better use of the water resources in such areas. The study conducted
refers to many aspects of the field of water resources, and attempts to
analyze irrigation systems as they operate at present. It also seeks
feasible techniques for improving the water management practices being
applied to increase water use efficiency in water deficient areas.
The Wellton-Mohawk Irrigation and Drainage District, one of the
most controversial irrigated areas in the American West was chosen as
the problem area. It is a 75,000-acre irrigable area at the lower reach
of the Gila River, Arizona, in which irrigation practices conducted
for centuries brought salinity, drainage and flooding problems, which
are attracting much attention during the last few years.
The possibilities for alternative management solutions which
could reduce the impact of these problems are various, and for the
present study they were directed to the following proposed objectives.
1. Decrease the volume of drainage water delivered into the
Colorado River.
2. Reduce the salt content of the drainage water returned to the
Colorado River.
3. Reduce the risks of flood damages along the lower Gila River.
4. Make more efficient use of the Gila River flood waters.
6
THE LOWER GILA RIVER
History
The knowledge of man's intense effort for existence in this part
of the southwestern American deserts dates from the Hohokam civilization.
Hohokam means "vanished ones," and their culture extended for a period
of time from about the beginning of the Christian era to A.D. 1400
(Ligner et al. 1969). Although earlier civilizations had been estab-
lished in the lower Gila River basin, the Hohokam are thought to be the
first to learn to manage the meager supply of water and to develop a
productive culture. They built several hundred miles of irrigation
canals that diverted water from the rivers to the cultivated fields
(Ligner et al. 1969). The reasons why the Hohokam vanished are not
well known, but most authorities believe that it was because of severe
drought that lasted for several years. The dependency on a scanty and
sometimes missing supply from the lower Gila River surface flow and the
absence of means to store water during the flow periods left the
Hohokam unable to face such emergencies. From the Hohokam civilization
comes the first evidence of the need to understand and properly manage
the water resources available in this desert region.
The progress of agricultural history in the Lower Gila River
basin follows the Pima Indians, who were irrigating the Gila River
flood plain in the vicinity of the Wellton-Mohawk area in the early
1500's (U.S. Bureau of Reclamation 1950).
7
8
Father Kino, the Jesuit missionary, noted agriculture in the
lower Gila River area in 1700. Among his many references to the Gila
River, and talking about the reach near the present town of Wellton
he wrote that all its inhabitants are fishermen and have many nets and
other tackle with which they fish all the year, sustaining themselves
with abundant fish and with their maize, beans, and calabashes.
The earliest agricultural development by white settlers in the
lower Gila River basin began with the establishment of a garrison of
United States troops at Fort Yuma in 1856. Supplies for settlers and
troops were brought in boats that travelled up the Colorado River from
the Gulf of California. With the establishment of the first stage
coach line in 1857, these supplies were distributed by mule team along
the lower Gila River basin, from Fort Yuma to Sacaton, a distance of
about 190 miles.
With the arrival of the white settlers, irrigation farming had
its origin in the Lower Gila River basin. By 1875, a number of homestead
filings had been made in the Wellton-Mohawk area. By 1891, a canal
some ten miles in length and having a concrete heading structure was
located at a narrow section of the valley in the extreme eastern end
of the present Wellton-Mohawk District. A brush dam was constructed
across the river channel to divert water to irrigate a few hundred
acres of land. In this same year, however, a disastrous flood destroyed
everything except the concrete heading structure and ended this attempt
to irrigate that area (Moser 1967). During the following years other
irrigation systems were developed by the people moving to the west,
9
but the severe droughts of 1897, 1898, and 1899 terminated everything
again (Ligner et al. 1969).
In 1908 a new diversion structure was built in another river
section to the west to irrigate 1200 acres of land on the south side of
the valley (Moser 1967). By this time, with the growing development
upstream, the Gila River flow at the Wellton-Mohawk area became even
less dependable. Roosevelt Dam, the first of the storage dams in the
Gila River drainage basin, was completed in 1911 to store flow from the
Salt River, the most important of its tributaries. As other reservoirs
were constructed and additional diversions were being made for agri-
cultural and other purposes, the flow in the lower Gila River basin
became practically nonexistent. This undependable water supply situation
continued for the next few years until about 1920, when groundwater
began to be developed as a new source of irrigation supply. About that
time several small districts were organized to furnish power and to
drill wells that would be pumped for irrigation (Moser 1967). The
Mohawk Municipal Water Conservation District, was formed in 1923 to
supply lands on the north side of the Gila River between the towns of
Wellton and Mohawk. The Gila River Power District was organized at this
same time to serve and develop the area.
This cooperative effort allowed a rapid expansion of irrigation
until a maximum of approximately 11,000 acres were in cultivation by the
early 1930's (Moser 1967). During the ensuing years, however, the rapid
lowering of the water levels, and the deterioration of the water quality
by evapotranspiration and continued recirculation led the farmers to
10
another crossroads in their attempt to expand the irrigation and in-
crease the progress of the lower Gila River agriculture.
By 1934, excessive salts were being pumped from many wells and
the water table decreased to an alarming extent. This situation came
as a result of the growing development in upper Gila River for which
additional reservoirs and wells were constructed, and extensive diver-
sions were being conducted. Spring floods in 1941 gave a short-lived
reprieve from the situation by bringing an additional supply of low
salt flood water that partially replenished the depleted aquifer.
Agriculture flourished again for a period of two years but after that
previous conditions of high salinity levels of soil and water and deep
lifting of the pumped irrigation water returned. One after another,
farms were abandoned as soil and water become too saline for profitable
farming. At this time the transfer of Colorado River water appeared as
the only reasonable solution of continued irrigation water supply which
the Bureau of Reclamation was already seeking to bring to the area.
In 1947, the U.S. Congress re-authorized the Wellton-Mohawk
Division of the Gila Project and provided for the construction of an
irrigation system that would bring water from the Colorado River to
75,000 irrigable acres within the project.
On April 16, 1951, the old power and water districts were
dissolved and their functions were taken over by the newly organized
Wellton-Mohawk Irrigation and Drainage District. The District entered
into a contract with the United States under the Reclamation Act for the
construction of a modern concrete-lined canal and distribution system
11
as well as for a supply of water from the Colorado River (Arizona
Interstate Stream Commission 1967).
The construction of the Wellton-Mohawk Division facilities was
started August, 1949, and by April 21, 1952, Colorado River water was
applied to the Wellton-Mohawk fields for the first time.
With the introduction of water from the Colorado River, the
expanding irrigated acreage (Table 1), the application of large amounts
of surface water, and very little pumping of groundwater, water levels
rose rapidly, and about 1960 a general drainage problem existed in the
area.
The construction of many drainage wells and a conveyance channel
in 1961, and the utilization of wells previously used for irrigation
purposes as drainage wells had by 1964 considerably alleviated the
severity of the drainage problem (University of Arizona 1970).
Later, additional wells were constructed for improving the
drainage conditions and to permit selective well pumping, and at the
present time a condition of practical equilibrium exists.
Physical Features
Location and Extent
The lower Gila River drainage basin is located in the south-
western corner of the State of Arizona and occupies portions of Yuma,
Maricopa, and Pima Counties (Figure 1). It comprises about 7,300 square
miles of which about 2,700 square miles are between Painted Rock Dam
(126 river miles) and Texas Hill (66.5 river miles) (U.S. Army Corps
12
Table 1. Wellton-Mohawk Irrigation and Drainage District: IrrigatedArea, in Acres.*
Year Irrigated Area
1952 14,134
1953 22,396
1954 24,657
1955 30,547
1956 34,939
1957 42,739
1958 45,114
1959 52,813
1960 54,127
1961 52,995
1962 51,735
1963 56,289
1964 58,100
1965 58,040
1966 60,062
1967 61,190
1968 60,758
1969 60,124
1970 60,756
1971 61,152
* Source: Wellton-Mohawk Irrigation and Drainage District, 1952-72.
"••••n
•
L EGEND
••••••11.—.• Boundary of Drainage Area
? Existing Reservoir
5
0
5
10
15flffil
MILES
13
Figure 1. Gila River Downstream from Painted Rock Dam. -- Adapted
from U.S. Army Corps of Engineers, 1962, Plate 1.
14
of Engineers 1962), From Texas Hill to the Gila siphon (8.4 river miles)
where the Gila River flood plain joins the Colorado River Valley, the
river flows for a length of about 58 miles. The Gila River flood plain
throughout this reach is referred to as the Wellton-Mohawk Valley.
The lower Gila River basin consists of broad, flat, low-lying
desert valleys, interrupted by many rugged, but comparatively low and
narrow mountain ridges. The flood plain ranges from less than a mile
to about five miles in width and the river flow meanders over the
generally flat bottom of a shallow channel about 1,000 to 3,000 feet
wide.
Topography
The moderate slopes of the Gila River flood plain have permitted
the development of many irrigation systems which are responsible for
practically all of the agricultural production in Arizona.
The Gila River drainage basin downstream from Painted Rock Dam
consists mostly of gently rolling desert plains ranging in elevation
from 130 to 1500 feet, with elevations of 150 and 325 feet at the Gila
siphon and Texas Hill, respectively. The slope of the river is estimated
to be 3.3 feet per mile from Painted Rock Dam to Texas Hill and 3.0 feet
per mile from Texas Hill to Dome.
The flood plain along the Gila River is only a few feet above
the river bottom. The southern Gila River terrace, known as the Wellton-
Mohawk Mesa constitutes the Mesa section of the Wellton-Mohawk Division.
It has an average elevation of about 70 feet above the adjoining valley.
15
The few minor rugged desert mountains that border part of the
flood plain reach elevations of 3,000 to 4,000 feet (U.S. Army Corps of
Engineers 1962).
Vegetation
The type, density, and distribution of vegetation in the Gila
River basin downstream from Painted Rock Dam reflect the effect of
elevation, temperature, and precipitation. In general the vegetation
between the dam and Texas Hill is cacti, creosote brush, and sagebrush.
Mesquite, saltcedar, and arrowweed grow in dense thickets in most of
the river bottom and in areas where the water table is near the ground
surface. Agricultural development in this area has been limited to a
few isolated ranches (U.S. Army Corps of Engineers 1962).
In the area along the Gila River from Texas Hill to the mouth,
most of the present channel bottom is covered with a heavy mantle of
phreatophytes. This vegetative growth has increased the aggradation of
the river and restricted the channel to such an extent that flows in
excess of 2,500 cubic feet per second would overflow and inundate the
adjoining land (U.S. Army Corps of Engineers 1962).
The 75,000-acre Wellton-Mohawk Division of the Gila Project
occupies most of the irrigable land from Texas Hill to the Colorado
River Valley. That area has been transformed from desert waste to
highly productive farmland. Native vegetation still persists in the
areas not suited to farming.
From a general inventory of the flora in the lower Gila River
conducted by the School of Earth Sciences of The University of Arizona,
16
an approximate estimate of about 16,000 acres of phreatophyte species
stand on the flood plain area between Texas Hill and Dome (University
of Arizona 1970).
The cropping pattern in the irrigated area of the Wellton-
Mohawk District has been practically invariable, although the acreage
for the various growing crops has varied, probably according to market
trends. Alfalfa hay, irrigated pasture, and grasses (all kinds)
accounted for 35 percent of the total crop acreage cultivated in the
District during the period from 1967 to 1972 (Table 2). Alfalfa, a
very high water user, is by far the dominant crop in the area and
accounted for 22 percent of the total crop acreage under irrigation
rotation during the same period.
Although practically all the crops are grown in the valley and
on the mesa, about 50 percent of the irrigated acreage on the mesa in
1972 was cultivated with citrus.
Hydrogeology
The geology of the watershed of the lower Gila River is typically
characteristic of the Basin and Range Physiographic Province, the western
most of the drainage provinces into which the State of Arizona is divided.
The study region consists of three sub-provinces: the dissected block
mountain, the intermountain bajadas and alluvial surfaces, and the Gila
River flood plain.
Because of its much greater importance as a repository to store
and transmit water and because much more information is available within
the limits of the Wellton-Mohawk District, only the flood plain area and
17
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18
adjoining mesa below Texas Hill, referred to as the "Wellton-Mohawk
area," will be considered.
The Wellton-Mohawk area is an irregular alluviated structural
basin bordered in part by north-west trending mountain ranges. It in-
cludes material of Tertiary and Quaternary age. The older of this
series has been designated as "older alluvium" and the younger part as
"Recent alluvium." The older alluvium is composed of two general
lithologic units. The upper unit is about 200 feet thick and is composed
of lenses of silt, sand, and gravel. The lower unit is of much greater
thickness and predominantly clay (Metzger 1952).
The older alluvium underlies the Gila River alluvium and its
thickness is estimated to be as much as 2,000 feet in the center of the
basin. It is a body of earth materials of varied sizes, ranging through
cobbles, coarse gravels, sand, silt, and clay, but it is predominantly
a clay alluvial fill. While part of the unit is considered permeable,
it is said to form the "impermeable" boundary between the river alluvium
and the bedrock floor of the basin.
The geologic unit referred to as Recent alluvium is formed by
the materials filling the broad and shallow trench dug by the Gila River
flow through many erosion cycles. It is composed of clay, silt, sand,
and gravel arranged into many and varied combinations to form a more or
less layered configuration in which a wide range of permeability values
can be found.
The Recent alluvium underlies the flood plain of the Gila River
and the ground surface of the mesa, and contains the principal aquifers
19
in the area. The alluvial deposits in this geologic unit contain highly
permeable gravel horizons with intervening beds of less permeable sand,
silt, and clay. The finer graded materials seriously retard the move-
ment of ground water and are partially responsible for the mesa drainage
problem (dellton-Mohawk Division 1970).
At least three gravel horizons have been recognized in the
Recent alluvial fill. The highest horizon, entirely above the valley
floor is 20 to 40 feet thick but does not extend as a continuous layer
throughout the whole area. It is highly permeable except where cemented,
and where it underlies irrigated areas it can be tapped to provide
drainage.
The second gravel layer in depth contains typical Gila River
gravel and cobbles. Its occurrence is scattered but where present its
thickness ranges from 10 to 20 feet. By its relatively small dimensions
and erratic occurrence this layer has only localized importance as a
pumped aquifer although it can play an important task in the underground
flow from the mesa to the valley. Its elevation is at or near the Gila
River grade.
The third and deeper gravel horizon underlies the mesa and
valley alluvial fill but does not extend to all parts of the mesa. It
contains typical Gila River gravels, and its thickness ranges from 10 to
85 feet. It is tapped by drainage wells in the valley and provides good
hydraulic connection between the mesa and the valley.
The capacity of the various geologic units of the alluvium to
transmit and store water depends on the quality of the alluvial beds and
their thickness, continuity, and location.
20
Measurements of the hydraulic characteristics of the deeper
gravel layer in the Wellton-Mohawk aquifer were obtained by selecting
some wells representatively located, and measuring the water-level
drawdown during field pumping tests. Transmissivity values ranged from
0.26 to 1.29 cubic feet per second per foot of head, and the specific
yield ranged from 10 to 18.5 percent and averaged 15 percent (U.S.
Bureau of Reclamation 1963). It is interesting to note an average
transmissivity value of about 0.33 cubic feet per second was derived from
studies conducted along the Gila River aquifer upstream from San Carlos
Reservoir in a reach of about 15 miles long and about 60 feet deep
(Hanson 1972).
On the basis of an average aquifer depth of about 85 feet, the
effective aquifer underlying the Wellton-Mohawk District could be esti-
mated at about 10,000,000 acre feet. By assuming a porosity of 30
percent (U.S. Bureau of Reclamation 1963), the volume of water in storage
could be three million acre feet, or about 35,300 acre feet per foot
depth of the aquifer.
Natural Drainage System andFlow Characteristics
The Gila River main stream enters the lower Gila River basin at
Painted Rock Dam and flows in a southwesterly direction for a distance
of about 114 miles to a narrow constriction below Dome where it joins
the Colorado River basin. Along this reach the main stream receives
many secondary tributaries the most important of which are:
21
Watershed Drainage Area (square mile)
Palomas Wash 1,360
San Cristobal Wash 1,810
Kofa Wash 575
Rio Cornez Wash 243
Mohawk Wash 710
Coyote Wash 450
Ligurta Wash 30
Castle Dome Wash 410
Fortuna Wash 45
The Gila River, even before the many impoundments and diversions
realized during the present century, was reported to have undependable
flow in its lower portion (Ross 1923). Flow in the lower Gila River
has probably always been seasonal in character and subject to infrequent
and abnormal occurrences. At the present, flow in the lower Gila River
basin is limited to infrequent releases from Painted Rock Reservoir and
flood runoff from the contributing watersheds below the dam, during
periods of heavy precipitation. The channel aggrades during summer
runoff from local washes, and tends toward scouring and meandering when
relatively sediment-free waters are released from Painted Rock Reservoir
during winter months (University of Arizona 1970).
All the tributary washes flowing into the main stream are
ephemeral and discharge both runoff and sediment to the Gila flood plain.
Watersheds such as the San Cristobal, Kofa, and Rio Cornez have poorly
defined channels, and their tributaries disappear into alluvium. Sandy
22
soils are characteristic of these watersheds and contribute to the
apparent lack of heavy runoff in the last several decades.
In contrast to the above, the Coyote and Ligurta washes drain
watersheds with fairly high runoff characteristics. Their combined
areas, however, are considerably less than the tributary watersheds with
low runoff characteristics (University of Arizona 1970).
Climatologic Factors
Precipitation
The climate of the Gila River basin downstream from Painted Rock
Dam is subtropical and arid, characterized by a low annual rainfall, low
humidity, high evaporation, high summer temperatures from June to
September, and high percent of possible sunshine.
The rainfall over the area is biseasonal (July through September
and December through March) in distribution, slightly unbalanced in
favor of winter. Its average annual value has been estimated as less
than four inches (U.S. Army Corps of Engineers 1962).
Practically all moisture for summer rainfall is drawn into the
area from the Gulf of Mexico and Atlantic Ocean, although most of the
recorded summer rains in the past century have been associated with deep
surges of tropical air into Arizona from the Gulf of California and
Pacific Ocean (Green and Sellers 1964). These rains, which occur most
frequently in July, August, and September, are sporadic and likely to
occur at almost any hour, although a nighttime peak has been recorded
at most of the stations. The rains are of high intensity and short
23
duration contributing, in infrequent years, to high peaks of runoff
with devastating effects.
Water from sudden summer thunderstorms does not penetrate deeply
into the soil. Even when the storms are heavy enough to cause surface
runoff, much of the water that is retained in the stream channels either
evaporates or is transpired by river bank vegetation.
Most flood-producing precipitation in the area downstream from
Painted Rock Dam results from showers of short duration and small areal
extent or from general summer storms centering in the area downstream
from the dam. Storms of the thunderstorm type occur separately or in
conjunction with general storms (U.S. Army Corps of Engineers 1962).
Precipitation during the winter usually results from general
winter storms associated with extratropical cyclones of North Pacific
origin. During the months from November to April, such storms move
south over Mexico and result in precipitation over areas of up to
thousands of square miles. Precipitation during general winter storms
may be more or less continuous for several days. Relatively intense
showers near the end of such storms are common. Most precipitation from
these general winter storms would normally occur upstream from Painted
Rock Dam (U.S. Army Corps of Engineers 1962).
Although rainfall amounts may be similar for summer and winter
storms, the summer storms are of short duration with higher intensities
than winter storms.
Precipitation data for representative stations downstream from
Painted Rock Dam are given in Table 3.
24
Table 3. Mean Annual Precipitation for Representative Stations in theLower Gila River Basin.*
Elevation Length of RecordStation(Feet) (Years)
Mean AnnualPrecipitation
(Inches)
Yuma Citrus 191 30 3.47
Yuma 138 90 3.43
Wellton 225 26 4.13
Mohawk 538 64 3.70
Aztec 492 17 4.48
Agua Caliente 516 6 3.50
Sentinel 685 20 4.76
Ajo 1,763 44 8.20
Gila Bend 737 48 5.79
* Source: U.S. Army Corps of Engineers 1962, p. 19.
Temperature
Temperature varies widely over the year from a few degrees below
freezing to a value as high as 120 F (Wishart and Nelson 1963).
During the summer temperature frequently exceeds 100 F. In July
and August the average daily temperature exceeds 90 F with variation from
the middle 70's in the early morning to well over 100 degrees in the
25
early afternoon. Readings above 110 F are quite common during summer
months (University of Arizona 1970).
Temperatures during the coldest part of the year normally rise
into the high sixties in the afternoon, and usually stay above freezing
at night. The average annual frost-free period in the basin is 311 days.
Although the area has a 12-month growing season, temperatures
may drop low enough to damage tender crops any time from November to
February and occasional frosts occur that cause substantial losses.
Hydrologic Characteristics
Surface Water Hydrology
The sources of surface water flowing in the lower Gila River
basin are infrequent releases or uncontrolled flows from Painted Rock
Reservoir, sporadic storm runoff from rainfall over the 7,300 square
mile watershed below Painted Rock Dam, irrigation water diverted from
the Colorado River, and drainage water flowing into the Gila River
channel as return flow from irrigation in the Wellton-Mohawk District
or flowing into the Wellton-Mohawk Main Outlet Drain as groundwater
pumped from the Wellton-Mohawk aquifer.
Runoff records for the Gila River main stream are available for
only two gaging stations: (1) the water-stage recorder gaging station
referred to as "Gila River below Pointed Rock Dam" on the left bank
0.3 mile downstream from the dam, and (2) the water-stage recorder gaging
station ultimately referred to as "Gila River near Dome" on the right
bank three miles west of Dome. The gaging station just below Painted
26
Rock dam was established in October, 1959, and continuous records are
available from that time. The records for the gaging station near Dome
are continuous since 1903.
There are only a few stream-flow records available on the Gila
River tributaries. In the lower reaches their streambeds are dry and
any flow which does occur is very infrequent and erratic (Arizona
Interstate Stream Commission 1967).
Previous to the many impoundments and diversions for agricultur-
al developments and other beneficial uses upstream, the Gila River
surface flow at Painted Rock Dam site was dependent on precipitation
falling over the contributing watershed. Nearly all the major floods
that occurred in the past were winter floods caused by prolonged general
precipitation centering on the Gila River basin upstream from the dam.
The greatest of these floods, in recent years, occurred in January,
1916, with a discharge, adjusted to reflect control at Painted Rock Dam
and other upstream dams constructed since 1916, of about 200,000 cubic
feet per second at Dome (U.S. Army Corps of Engineers 1962).
Flows occurring in the past at the Dome gaging station were
caused in part by winter precipitation falling over the watershed up-
stream from Painted Rock Dam but mostly they are summer runoff from the
contributing watershed below the dam (University of Arizona 1970).
The largest summer flood recorded at Dame occurred in August,
1921, and had a mean daily discharge of 25,000 cubic feet per second.
At the present time, Gila River flow from runoff contributions
upstream from Painted Rock Dam is partially controlled by upstream dams
27
and diversions, and depleted groundwater reservoirs. The lower Gila
River developments are, however, subject to floods caused by rainstorms
falling over the watershed below the dam.
Painted Rock Dam was constructed at the end of the 1950's to
reduce the potential threat of flood damages in the Wellton-Mohawk area,
in the lower Gila River basin, and some areas in the lower Colorado River.
The storage capacity of the reservoir is 2,492,000 acre feet and it was
designed to take a maximum inflow of 300,000 cubic feet per second and
release a maximum outflow of 22,500 cubic feet per second (University
of Arizona 1970).
During the years from 1960 to 1971, about 290,000 acre feet of
water (Table 4) were released from Painted Rock Reservoir. Of this
total, 258,100 acre feet were released in the spring of 1966. This was
the first water stored since the completion of the dam in December,
1959 (Irelan 1971). The flood water was released for a period of three
months--January, February, and March--with a mean daily discharge of
1,430 cubic feet per second. Much of this released flow, about 61
percent, was estimated to dissipate in the dry streambed between the dam
and the east boundary of the Wellton-Mohawk District.
From the estimated 101,300acre feet of released flow reaching
the Wellton-Mohawk area (Table 5), 47,400 acre feet were estimated by
the Bureau of Reclamation Engineers (Irelan 1971) to have infiltrated
into the Wellton-Mohawk aquifer; 16,500 acre feet were pumped into the
Wellton-Mohawk irrigation system, and approximately 37,400 acre feet
were estimated to have reached the gaging station near Dome, near the
28
Table 4. Lower Gila River Surface Flow, in Acre Feet.*
Year Below Painted Rock Dam Near Dome
1960 3,660 17,770
1961 244 11,790
1962 0 3,290
1963 77 7,210
1964 1,320 102
1965 524 323
1966 258,100 39,840
1967 1,590 526
1968 17,370 797
1969 652 1,200
1970 3,050 2,660
1971 3,130 3,760
Total 289,717 89,268
* Source: U.S. Geological Survey, 1960-71.
29
Table 5. Lower Gila River: Estimated Painted Rock Reservoir ReleasesDistribution Between the Dam and Dome, During the Spring1966, in Acre Feet.*
A. Releases from Painted Rock Reservoir
B. Water pumped from the Gila River channel
C. Estimated infiltration volume into theWellton-Mohawk aquifer
D. Estimated volume reaching Dome
E. Estimated volume reaching the Wellton-Mohawkarea (B + C + D)
F. Estimated infiltration volume between PaintedRock Dam and the Wellton-Mohawk area
258,100
16,500
47,400
37,400
101,300
156,800
* Sources: U.S. Geological Survey (1960-71); Wellton-Mohawk Division(1961-71); Irelan (1971).
dividing point between the Wellton-Mohawk and Yuma areas. The precipita-
tion falling over the lower Gila River basin from January to March, 1966,
as usual, probably did not contribute any runoff to the Gila River
surface flow during those months.
A second period of water releases from Painted Rock Reservoir
occurred in 1973, and lasted for more than six months. About 400,000
acre feet of water were released during that period (American Water
Resources Association 1973). The releases began about the middle of
March and by April 12, when about 50,000 acre feet of water had been re-
leased, water flow reached Avenue 51 E, four and one-half miles down-
stream from the east boundary of the Wellton-Mohawk.
30
From April 12 to July 31, 288,000 acre feet reached Avenue 51 E.
About 128,000 acre feet were dissipated through the processes of infil-
tration and evapotranspiration along about 60 miles of river.
During this 112-day period water release rates averaged 1,287
cubic feet per second. During 64 of the 112 days water release rates
were greater than 1,000 cubic feet per second, and for 23 days they
averaged 2,344 cubic feet per second. The maximum release rate was
2,520 cubic feet per second.
Water from Painted Rock Reservoir reached the Colorado River
on May 5 and continued into September. A total of about 100,000 acre
feet of released water reached the Colorado River (American Water
Resources Association 1973). Thus about 300,000 acre feet went into
the ground, was pumped into the Wellton-Mohawk irrigation system, or
was lost to evapotranspiration between Painted Rock Dam and the Colorado
River along about 126 miles of river channel.
Gila River surface flow originating downstream from Painted Rock
Dam could be derived from sporadic storm runoff from the contributing
watershed and from return flow from irrigation in the Wellton-Mohawk
area. Storm runoff can occur in the winter or summer, but predominantly
during the summer.
The flow of the Gila River at its mouth includes the flood water
passing Dome and the normal drainage from irrigation entering the river
between Dome and its mouth.
The annual flow of the Gila River near Dome, in the gap where
the Gila River enters the lower Colorado River Basin has shown
31
considerable decline caused by developments of irrigation within the
basin. Excluding 1966, when releases from Painted Rock Reservoir passed
the gaging station near Dome, the only flow during the period from 1960
to 1971 (Table 4), was surface drainage return flow from irrigation in
the Wellton-Mohawk area or runoff from occasional local storms. During
that period 89,268 acre feet were recorded at Dome, and from this total
37,400 (Table 5) were estimated to be flood water released from Painted
Rock Reservoir.
The difference between these two values averaged for the 12
year record gives an average annual flow of about 4,325 acre feet. This
means that the contribution from precipitation to the Gila River surface
flow in the lower Gila River basin based on these data could be con-
sidered insignificant, since most of this annual flow is drainage return
flow as can be seen from monthly Gila River flows measured at Dome.
Since the confluence of the Gila River and Colorado River is
below all storage reservoirs and below all but one of the main diver-
sions (Alamo Canal), there is little opportunity to use any flood flow
that may occur in the lower Gila River basin.
Irrigation water supply and pumped drainage water flowing in the
Wellton-Mohawk District system account for practically the whole surface
waters flowing in the lower Gila River basin since the introduction of
the Colorado River water into that area.
Groundwater Hydrology
Groundwater occurring in the lower Gila River basin is from
three main sources:(1)Gila River and mountain area underground inflow;
32
(2) Gila River and tributary flood waters that infiltrate and percolate
to the groundwater table; and (3) seepage from excessive irrigation
water applications.
Average annual values for the Gila River underground inflow and
outflow at the boundaries of the Wellton-Mohawk District have been
estimated as 5,000 and 1,000 acre feet respectively (Babcock, Brown and
Hem 1947).
Underground flow from mountain areas could be considered of less
importance.
Storm runoff from the Gila River tributaries could probably
provide at times a considerable volume of recharge to the Wellton-Mohawk
aquifer. Most of the runoff flowing in the smaller washes infiltrates
readily into the coarse, gravel stream bed materials, and the water
either percolates to the water table or returns to the atmosphere by
evaporation and transpiration, but seldom reaches the Gila River channel
(Babcock, Brown and Hem 1947).
Test holes drilled along several tributaries of the Gila River
in April 1968 to see the groundwater recharge effect of winter pre-
cipitation found no water in the alluvium which means that the amount of
underflow along these tributaries is small and probably occurs only
during wet periods (Weist 1971).
On the basis of long-range studies of rainfall and runoff, it is
estimated that 8 to 15 percent of the total precipitation in the mountain
areas becomes runoff, and as much as 50 percent of this runoff may reach
the groundwater reservoir in the pervious zones immediately adjacent to
33
the mountain front and through the recent alluvial fill underlying the
stream channels downstream (Metzger 1952).
In general, more than 95 percent of the precipitation water over
the lower Gila River basin is estimated to be lost by evaporation before
reaching the stream channel (Arizona Interstate Stream Colmuission 1967).
How much of the remaining five percent reaching the stream channel
becomes groundwater recharge is very difficult to estimate in areas such
as the lower Gila River basin, where runoff is of infrequent occurrence.
Groundwater recharge investigations conducted in the Queen Creek
area, Arizona, show that about one-half of the total streamflow occurring
in that area is recharged to the groundwater reservoir (Babcock and
Cushing 1942).
Substantial contributions to groundwater storage occur when re-
leases from Painted Rock Reservoir flow through the lower Gila River
basin. From 658,100 acre feet of water released in 1966 and 1973, more
than 500,000 acre feet were estimated to be lost through infiltration
and deep percolation to the groundwater table, and evapotranspiration
between Painted Rock Dam and the Colorado River. About 70 percent of
this amount was dissipated between the dam and the Wellton-Mohawk area.
During the years from 1940 to 1971, about 1,367,000 acre feet (Arizona
Water Commission 1973) were estimated to have been pumped from this
lower Gila River reach providing for a high storage capacity for in-
frequent releases from Painted Rock Reservoir.
Seepage from excessive irrigation water applications is, however,
the main source of groundwater recharge in the Wellton-Mohawk District.
34
From 1962 to 1971, 2,100,000 acre feet of drainage water was pumped from
the Wellton-Mohawk aquifer, and about 75 percent of this volume, or
1,600,000 acre feet, can be estimated as seepage from excessive irriga-
tion water applications.
Since groundwater levels in the Wellton-Mohawk District during
recent years showed only small average variation, the recharge from
irrigation and other sources to the groundwater reservoir has practically
been counter-balanced by the amount pumped from drainage wells.
The most important source of groundwater in the desert region
of southern Arizona is the alluvial fill. The principal aquifers of the
alluvial fill are permeable lenses of sand and gravel interfingered with
relatively impermeable lenses of silt and clay. Although the alluvial
fill has been separated into older alluvial and Recent alluvial fill,
they are interconnected and the groundwater reservoir is continuous.
Wells in the older alluvial fill are reported to yield 500 to
1,000 gallons per minute from the sand and gravel of the upper 200 feet.
Wells in the Recent alluvial fill yield from 600 to 4,000 gallons per
minute (Babcock and Sourdry 1948). At the present time, the 107 wells
in operation in the Wellton-Mohawk District have an average discharge
of 1,634 gallons per minute with a range from 450 to 4,400 gallons per
minute (U.S. Bureau of Reclamation 1972b).
The general groundwater movement in the lower Gila River basin
is directed from the mountain fronts or the borders of the flood plain
to the Gila River bottom, and westward down the valley gradient. In the
mesa section of the Wellton-Mohawk District some water moves toward the
35
desert area, but predominantly the groundwater movement there is also
directed to the valley following the steep gradients caused by topo-
graphic differences and drainage water pumping at the foot of the mesa.
The continuity and extent of the deeper gravel layer in the Recent
alluvial fill underlying the mesa and the valley is another important
factor in the groundwater movement from the mesa to the valley.
Pumpage of groundwater for irrigation began in the early 1900's
and increased steadily until 1952, when surface water from the Colorado
River entered the Wellton-Mohawk area and groundwater pumping progres-
sively began to decrease.
As a result of the heavy withdrawal of groundwater in the area,
the water table in the farmed areas of the Wellton-Mohawk area had been
declining at an average rate of 0.9 feet a year for the period 1928,-48
(Babcock and Sourdry 1948). By 1950 the Wellton-Mohawk District of the
Gila Project had been reauthorized and a modern irrigation system was
under construction. In April, 1952, Colorado River water entered the
Wellton-Mohawk area.
The application of large amounts of water to cultivated lands
and the decline in pumping of groundwater for irrigation caused the
water levels to rise rapidly. Pumping of groundwater for irrigation
nearly ceased by 1957.
Subsequent recharge by return flow of Colorado River water
caused water levels to rise to within a few feet of the ground surface
necessitating extensive drainage facilities
36
Groundwater is discharged from the lower Gila River basin by
pumping for drainage and by natural means. Natural discharge includes
transpiration and evaporation of groundwater in the areas of cultivated
and natural vegetation along the flood plain, and underflow through the
narrow section of the Gila River valley near Dome.
THE WELLTON-MOHAWK IRRIGATION AND DRAINAGE DISTRICT
Description
The Wellton-Mohawk District occupies an area of about 113,500
acres that extends for a distance of about 45 miles from Avenue 55E,
just upstream from Texas Hill to Avenue 10 E, a few miles below Dome
(Figure 2). The area is bounded on the east by the Mohawk Mountains,
on the west by the Gila Mountains, on the north by the Muggins and
Castle Dome Mountains, and on the south by the Wellton Hills and the
Copper Mountains (Metzger 1952). Its boundaries are established by
Public Law which restricted the irrigable area to be supplied with
Colorado River water to a maximum of 75,000 acres of which approximately
60,000 acres are located in the valley of the Gila River and the remain-
ing 15,000 acres are on the adjacent mesa south of the valley land.
The valley section of the Wellton-Mohawk District includes
practically the whole Gila River flood plain between those avenues.
The mesa section extends for about 21 miles along the river as a strip
of land with an average width of 2.15 miles, which is, on the average,
70 feet above the valley floor.
Soils
One hundred and forty-eight thousand five hundred acres were
classified to determine the 75,000 acres best suited for irrigation
37
39
purposes in the Wellton-Mohawk area. Table 6 gives the soil character-
istics and acreage of the different classes of land.
Table 6. Wellton-Mohawk Area: Land Classification, Acreage, and SoilCharacteristics.*
Mesa Lands Valley Lands
**W.H.C. 3-6 acre-inches/acre 8 acre-inches or more/acre
Gila River surface outflow.Gila River underground inflow.Gila River underground outflow.Wellton-Mohawk Canal inflow diverted to the mesa.(VODOTFL UNBALAN)--Drainage water pumping to balancethe annual water budget.Total underground flow leaving the mesa area to thedesert.Total crop evapotranspiration from the mesa.Total Wellton-Mohawk Main Outlet Drain flow leavingthe mesa area.
96
Table 1, Continued.
TMUINFL = Total underground flow leaving the mesa area to thevalley.
TOVINFL = Total water inflow reaching the valley.TOVOTFL = Total water outflow leaving the valley.TPEVPTR = Total phreatophytic evapotranspiration from the valley.TPRECIP = Total precipitation contribution to groundwater recharge
in the valley.TVEVPTR = Total crop evapotranspiration from the valley.TWMINFL = (MWMINFL + VWMINFL)--Total Wellton-Mohawk Canal inflow
diverted to the District.UNBALAN = (TOVINFL - TOVOTFL)--Amount of water to balance the
annual water budget in the valley.VODOTFL = Total Wellton-Mohawk Main Outlet Drain flow leaving the
District.VWMINFL = Wellton-Mohawk Canal inflow diverted to the valley.WATCUSE = (TWMINFL - VODOTFL)--The amount of water consumptively
used in the District.
97
Strategy I is by itself then an impracticable management alterna-
tive since the recharge is greater than the withdrawal and a progressive
rise of the groundwater levels will occur. No protection is provided
against floods occurring along the area since groundwater levels will
stay high and could assume critical elevations even with small addi-
tional recharge from infrequent flood flows along the lower Gila River.
The drainage flow would have to be increased by about 16,000 acre feet
per year from the present values, increasing by about 90,000 tons the
salt load to the Colorado River.
Strategy II
Column (II), Table 17, shows the average groundwater depths
expected to be reached in the valley from the application of Strategy II
to the model. Most of the sectors show a decline in their groundwater
levels compared to the present levels, but the average annual drop is
only about 0.70 feet. If the annual pumping rate of 231,000 acre feet
were maintained for ten years, it would result in an average decline of
about seven feet, if no flood flow occurs along the lower Gila River.
Sectors 3 and 6, however, would have their groundwater levels progres-
sively raised and would require an intensification of the pumping rates
now being applied there.
The amount of water delivered to the District is about 522,000
acre feet (Table 19), a little higher than that presently used (Table 10).
The amount of water lost by evapotranspiration is practically the same
since no change in crop allotment occurred, and the management practice
change proposed will not substantially affect the evapotranspiration
98
Table 19. Wellton-Mohawk Irrigation and Drainage District--StrategyII: Sprinkler Irrigation on 25 Percent of the Mesa Area.
Water Component* Inflow OutflowAcre Feet Acre Feet
Table 24. Wellton-Mohawk Irrigation and Drainage District--StrategyVII: Sprinkler Irrigation on 50 Percent of the Mesa Areaand Reduction by 50 Percent of the Phreatophytes.
According to the economic analysis for Strategy III, the total
annual gain from the change in irrigation method would be $37,115 and
according to Strategy V, the cost for pumping the 24,000 acre feet of
water saved from the reduction in the riparian vegetation would amount to
$24,000. A total annual gain of $13,115 would then accrue from the
application of Strategy VII.
Strategy VII provides limited opportunity for meeting the pro-
posed objectives in the present work since the drainage flow is about
the same as now being pumped and, consequently, the salt load to the
Colorado River is the same. The opportunity for flood and groundwater
recharge control could only be achieved in the long term.
Strategy VIII
Column (VIII), Table 17, shows the average groundwater depths in
the valley expected from the application of Strategy VIII to the model.
Five of the nine sectors showed substantial decline in their groundwater
levels, but sectors 1, 2, 8, and 9, which are not influenced by the re-
duction in water application to the mesa area, show a substantial rise
in their groundwater levels from the elimination of the riparian vege-
tation. As an average for the whole valley an annual decline of about
0.70 feet in the groundwater levels is observed. As in Strategy VII,
the continued use of Strategy VIII could be possible only by increasing
the present pumping rates in sectors 1, 2, 8, and 9.
The amount of water delivered to the District is about 451,000
(Table 25) or about 65,000 acre feet lower than the amount now being
used. The evapotranspiration from the area occupied by the riparian
112
Table 25. Wellton-Mohawk Irrigation and Drainage District--StrategyVIII: Sprinkler Irrigation on 100 Percent of the Mesa Areaand Reduction by 100 Percent of the Phreatophytes.
Water Component* Inflow OutflowAcre Feet Acre Feet
vegetation was considered to be zero with the assumption that it would
be proportional to the reduction in the phreatophytes. The amount of
water used consumptively is much lower than the limit established by
law which means a large amount of water could be allocated for other
beneficial uses. The amount of drainage water to be pumped to balance
the annual water budget is about 10,000 acre feet less than the present
pumping rate.
According to the economic analysis for Strategy IV, the total
amount gained from the change in irrigation would be $74,230 and accord-
ing to Strategy VI, the cost of pumping the 48,000 acre feet of water
saved from the eradication of the riparian vegetation would amount to
$48,000. A total annual net gain of $26,230 would then accrue from
application of Strategy VII.
Although it provides for the saving of large amount of water,
Strategy VIII reduces the drainage flow from the present rates by only
an insignificant amount which means that the reduction in the salt load
to the Colorado River will also be insignificant. Improvement of the
present drainage conditions and increase of the storage space for
groundwater recharge from eventual flood flows occurring in the lower
Gila River could be obtained only in the long term.
CONCLUSIONS AND RECOMMENDATIONS
Conclusions
In this study, the past and present water management practices
used in the Wellton-Mohawk District and their problems were outlined.
The widespread drainage well pumping and the need for more information
on the groundwater movement from the mesa to the valley and the pre-
cipitation contribution to the groundwater recharge made the understand-
ing of the dynamic behavior of the hydrology system of the area a diffi-
cult matter. Crop maps would have been useful in allocating the applied
water supplies and estimating the drainage withdrawal from each sector.
A mathematical model which represents as closely as possible
the physical hydrology of the area was developed. The objective of
using the model was to generate information which would reflect the
dynamic hydrologic behavior of the area from the application of selected
alternative management strategies. Following are the major conclusions
drawn from the study.
1. New irrigation developments in the District, as proposed in
Strategy I, are legally and technically impracticable under the
present management conditions and drainage disposal facilities
available.
2. Strategy II is shown to be beneficial to farmers and to the
District, but only in the long term would it provide desired
114
115
levels of flood protection and enough storage space for ground-
water recharge from occasional flood flows occurring along the
lower Gila River. The drainage flow and, consequently, the salt
load to the Colorado River would be always higher than the
present levels.
3. Strategy III is shown to be beneficial to farmers and to the
District, and after a few years of full drainage water pumping,
desired levels of flood protection and enough storage space for
groundwater recharge could be obtained. After that drainage
water pumping could drop to about 200,000 acre feet per year,
which would reduce the present annual pumping costs by about
$15,000. The annual salt load to the Colorado River would be
reduced by about 83,250 (15,000 x 5.55) tons.
4. Strategy IV is shown to be beneficial to farmers and to the
District. In a short period of two or three years of full
drainage water pumping desired levels of flood protection and
enough storage space for groundwater recharge could be obtained.
After that, drainage pumping could drop to about 160,000 acre
feet per year, which could reduce the present annual pumping
costs by about $55,000. The annual salt load to the Colorado
River would be reduced by about 305,250 (55,000 x 5.55) tons.
5. Strategy V would bring a net gain to the District resulting from
the difference in the price of 24,000 acre feet of water which
would not be charged against the District because it returns to
the Colorado River, and the costs for pumping this amount of
116
water, which would be $24,000. However, this strategy is im-
practicable as it requires an annual drainage pumping of about
265,000 acre feet, much greater than the design capacity of the
drainage disposal system.
6. Strategy VI would provide a net gain to the District resulting
from the difference in the price of 48,000 acre feet of water
which would not be charged against the District because it re-
turns to the Colorado River, and the costs for pumping this
amount of water, which would be $48,000. As in Strategy V,
Strategy VI is an impracticable management alternative since it
requires an annual drainage pumping of about 290,000 acre feet,
much greater than the capacity of the drainage disposal system.
7. Strategy VII would bring a total annual gain of $37,115 from the
change in irrigation method plus the difference between the
price of 24,000 acre feet of water not charged against the
District, and the costs to pump it into the conveyance channel,
which would be $24,000. Since the drainage flow requirement
would be about the same as now being pumped, this strategy
would not provide any opportunity for achieving the objectives
proposed in the present study.
8. Strategy VIII would provide a total gain of $74,230 from the
change in irrigation method, plus the difference between the
price of 48,000 acre feet of water not charged against the
District, and the costs to pump it into the conveyance channel.
Since the drainage flow to balance the annual water budget is
117
only a little less than the present rate, little opportunity to
meet the objectives proposed in the present study could be found.
9. The present phreatophytic coverage on the Gila River bottom
along the Wellton-Mohawk Valley is an important auxiliary of the
drainage disposal system of the area, although it increases the
opportunity for flood damages along the developed area. Its
substantial reduction or elimination would considerably aggra-
vate the drainage problem now existing there.
10. The amount of water applied to the soil for irrigation on the
mesa area exceeds by large amounts the requirements for crops,
including the amount needed for leaching requirements. Although
it has only about 16 percent of the present irrigated area in
the District, the mesa is the main area responsible for the
serious drainage problem in the valley and, consequently, the
water quality problem in the lower reach of the Colorado River.
It receives about 28 percent of the total water delivered to the
District and about 70 percent of this water reaches the ground-
water reservoir and moves to the valley.
11. The infrequent releases of stored flood water from Painted Rock
Reservoir and sporadic runoff from contributing watersheds below
the dam can, at times, provide good quality water which can be
used to supply part of the irrigation requirements in the Wellton-
Mohawk District. Since Painted Rock Reservoir is a flood con-
trol reservoir with its method of operation directed to this only
purpose, the largest possible utilization in the District of
118
water released from the reservoir could be accomplished through
the scheme of simultaneous pumping from the river channel, as
already has been done, and storage as groundwater recharge of
some part of the flow volume. The assumption of considering as
zero the surface flow contribution from the upstream reach of
the Gila River did not cause any impact on the water balance
defined in the model. It is an abnormal event whose effect on
the groundwater levels can be counterbalanced by increasing the
drainage pumping rates and the increased opportunity for natural
discharge through evaporation and transpiration.
12. Much more information about crop and water distribution is
needed to allocate water supplies and estimate pumpage with-
drawal more precisely among the various sectors. Groundwater
levels in the mesa, and transmissivity and specific yield esti-
mates for the Wellton-Mohawk aquifer are not sufficient to evalu-
ate the underground flow movement and changes in groundwater
levels resulting from given recharge or withdrawal. More pre-
cipitation data are needed for the mountain areas, where practi-
cally all runoff contributing to groundwater recharge originates.
The establishment of streamflow measuring stations in the lower
reaches of the major runoff-contributing watersheds would be
useful in estimating the precipitation contribution to the sur-
face and underground flows of the Gila River. Evapotranspiration
from phreatophytes is largely influenced by groundwater depth
and will require research for its reliable estimate.
119
13. The proposed desalting complex will treat water for irrigation
at a cost of $114.00 per acre (considering all irrigation
districts involved in the problem) compared to estimates of the
marginal value of water in the Yuma area of $2.00 to $12.00 per
acre foot (Martin 1974). It would then seem much more reasonable
for the United States Government to subsidize the investment
costs, and possibly other costs, of sprinkler or trickle irriga-
tion systems which could conveniently reduce the excessive
drainage flow from the Wellton-Mohawk District and, consequently,
the salt content of the Colorado River water at Morelos Dam.
14. The economic impact of water saving from the application of the
various strategies to the model, based on the expected returns
to farmers and to the District, is insignificant and probably
would not encourage them to change from their present management
practices. However, the social returns of some of these strate-
gies, although not specifically evaluated here, would contribute
to the good will between the United States and Mexico and save
money that would be drawn from taxpayers. Their total value
might amount to millions of dollars.
Recohmlendations
The following recommendations are made for future consideration
in the Wellton-Mohawk District.
1. The prevention of any new irrigation development on the mesa
area until more efficient water management practices are applied.
120
2. The change to sprinkler irrigation on the mesa area at least at
the level proposed in Strategy III (50 percent of area).
3. Studies to define an engineering and management structure for a
better utilization of water from flood flows occurring along
the lower Gila River, and greater protection against flood
damages.
4. Further analysis of the data available to better define the
groundwater movement in the area.
5. Studies for more accurate estimation of evapotranspiration losses.
6. The establishment of a more intensive net for collection of pre-
cipitation and stream flow data.
APPENDIX A
DESCRIPTION AND LISTING OF THE COMPUTER PROGRAM
121
30
35
40
45
50G
55
60
zo
25
5
to
15
122
PROGRAM H2OBAL f INPUT, OUTPUTOUINFL = UNDERGROUND FLOW LEAVTNG EACH SECOTR ON THE MESA TO THE
'DESERTFACTOR = pRoPoRTIoNALITy FACTOR WHICH ALLOCATES THE WATER COMPCNE
ANTS AMONG SECTORSGRSINFL r GILA RIVER SURFACE INFLOW REACHING EACH SECTOR IN THE
1V ALLEYGRSOTFL = GILA RIVER SURFACE OUTFLOW LEAVING EACH SECTOR IN THE
1VALLEYGRUINFL GILA RIVER UNDERGROUND INFLOW REACHING EACH SECTOR IN1THE VALLEYGRUOTFL = GILA RIVER UNDERGROUND OUTFLOW LEAVING EACH SECTOR IN
AVALLEYINFILTR = GILA FLOW INFILTRATEO THROUGH ITS BED IN EACH SECTOR IN
1THE VALLEYHr ESTIMATED AVERAGE SATURATED THICKNESS AT EACH CROSSASECTION LIMITING THE SEcTDRS, IN FTMDELTwL = NET CHANGE IN WATER TABLE LEVEL FOP EACH SECTOR ON THE
1MESAHESAREA = TOTAL AREA FOP EACH SECTOR ON THE MESAmEVPTRN = COP EVAPOTRANSPIRATfCN FROM CACH SECTOR ON THE MESAmNRCHAO = NET RECHARGE TO OR WITHDRAWAL FROm FOR EACH SECTOR ON1THE MESAMODINFL r HELLION-m0HAwK MAIN OUTLET DRAIN. INFACw REACHING EACH1SECTOR ON THE MESAMOOOTFL HELLION-m0HAWK HAIN OUTLET DRAIN OUTFLOW LEAVING EACH1SFCTOR ON THE MESAmpERc0 r PORTION OF THE HATER DELIVERED To THE MESA THAI BECOMESAGPoUNOWATER RECHARGEmUINFL UNDERGROUND FLOW LEAVING EACH SECTOR ON THE MESAMwMINFL = HELLION-m0HAwK CANAL INFLOW REACHING EACH SECTOR ON THE
1MESAMwmOTFL wELLTON-m0HAWK CANAL OUTFLOW LEAVING EACH SECTOR ON THE
AMESANRCHAO = NET RECHARGE TO OD WITHDRAWAL FROM FOR EACH SECTOR IN THE10ISTRICT
= ESTIMATED AVERAcE ALLUVIUM FILL PERMEABILITY, IN SO FTIPEP DAYPEVPTRN = pHREATopHyTES EVAPDTRANtRIRATIOn FROM EACH SECTOR IN THE1VALLEYPPECIP LOWER GILA RIVER BASIN PRECIPITATION CONTRIBUTION TO GROUANOwATZR RECHARGE REACHING SamE SECTORS IN THE VALLEYRETFLOw = RETURN FLOW FROM IRRIGATION AND DIRECT RUNOFF FROM'RAINSTORMS REACHING EACH SECTOR IN THE VALLEY
ESTIMATED AVERAGE HyDRAULIC GFAOIENT FOR EACH SECTOR,UN FT PER FTTnuINFL = TOTAL UNDERGROUND FLOW LEAVING THE MESA TO THE DESERTTINFILT TOTAL GILA RIVER FLOW INFILTRATED THROUGH ITS BED ALONG
1THE WELLTON-HOHAwK DISTRICTTHE VPTR = TOTAL CROP EVAPOTRANSPIRATION ON THE MESATmOnoTF = TOTAL wELLTON-moHARK MAIN OUTLET DRAIN OUTFLOW LEAVING
ITHE MESATmUINFL = TOTAL UNDERGROUND FLOW LEAVING THE MESA TO THE VALLEYTOTINEL r TOTAL INFLOW REACHING EACH SECTOR IN THE DISTRICTTOTOTFL = TOTAL OUTFLOW LEAVING EACH SECTOR IN THE DISTRICTTPEVDTR TOTAL PHREATOPHYTES EVAPOTRANSPIRATION IN THE VALLEYTPRECIP = TOTAL LOWER GILA RIVER PRECIPITATION CONTRIBUTION TO GRO
1UNOwATER RECHARGE REACHING THE HELLION-m0HAwK VALLEYTRETFL TOTAL RETURN FLOW FROM IRRIGATION AND DIRECT RUNOFF FROMiRAINSTOPms REACHING THE GILA RIVER CHANNEL ALONG THE DISTRICTTVEVDTR = TOTAL CROP EvApoT0ANSPIRATION IN THE VALLEY
123C VALAREA = TOTAL AREA FOR EACH SECTOR IN THE VALLEYC " VOELTWL = NET CHANGE IN WATER TABLE LEVEL FOR EACH SECTOR IN THEC ** 'VALLEY
65 C " VEVPTRN = CROP EVAPOTRANSPIRATION FROM EACH SECTOR IN THE VALLEYC ** VFHATO = WATER TABLE DEPTH AT THE ENO OF THE PERIOD FOR EACH SECTOC " 1R IN THE VALLEYC ** VINwATO = INITIAL WATER TABLE DEPTH FOR EACH SECTOR IN THE VALLEYC ** VNRCHAO = NET RECHARGE TO 0-2 wITHORAWAL FROM FOR EACH SECTOR IN
70 C ** lfHE VALLEYC VODINFL = wELLTON-MOHAwK MAIN OUTLET DRAIN INFLOW REACHING EACHC ** ISFCTOR IN THE VALLEYC VOBOTFL = HELLION-m0HAWK MAIN OUTLET DRAIN OUTFLOW LEAVING EACHC ** 1SECTOR IN THE VALLEY
75 C ** VHMINFL = HELLTON-MOHAWK CANAL INFLOW REACHING EACH SECTOR IN THEC ** 1VALLEYC ** VWMOTEL = wELLTON-M0mANK CANAL OUTFLOW LEAVING EACH SECTOR IN THEC ** 1VALLEYC ** X = NUMBER OF YEARS FOR THE STUDY PERIOD
AO C W = WIDTH or THE CROSS SECTIONS LIMITING THE SECTORS, IN FTC ** ALL WATER COMPONENTS ABOVE ARE CONSICERFD IN ACRE FEET
REAL MwmINFL, mODOTEL, mwMOTEL. mEVIDTRN, NRCHAO. MNRCHAO, HOELTWL,1MINWATO, MFwATO, INFILTR, mESAREA, m, mPERCO, MUINELDIMENSION VWMINFL (9), mwmINFL (9). GRSINFL (9), GRUINEL (9), VODI
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