Geohydrology of Arid Lands
(Arizona - A Case Study)

John W. Harshbarger
United States Geological Survey
Department of the Interior

Introduction

In recent years one of the great questions of occupation of arid lands by man is: Can the established and proposed economy caused by large population increase be sustained in the arid zones? This question is particularly applicable in the Southwest; and Arizona, because it is typical of the Southwest, serves as an excellent laboratory in which to analyze the effects of such occupation.

Arid lands are characterized by two dominant factors: (1) an abundance of sunshine, and (2) a shortage of water. Part of the water shortage is due to scant precipitation; in addition, the abundance of sunshine causes an evaporation potential about ten times the amount of the annual rainfall. However, man's success in arid zones is dependent upon the availability of ample water supply. He needs large amounts for agriculture, moderate amounts for industrial enterprises, and an increasing amount per capita for municipal and domestic needs. In addition, man has more leisure time than in past years and is demanding more places with water for recreation.

The basic problem confronted by modern man in the arid zone is what happens when he exceeds the natural, readily available supply of the resources of the region--the one of most importance is water. The amount of water that is available perennially for easy capture by man is the water in surface streams and springs. In Arizona the demand for water has exceeded the amount available in streams and the demand has been met by the development of ground water resources. Indeed, the annual use of water in Arizona is more than three times that available from the surface streams. The use of water from the ground water reservoirs has caused a depletion of water reserves, and such mining of water may eventually cause exhaustion. How, then, can man continue to occupy arid zones unless he takes steps to conserve water?

The major problem today is not one of locating new ground water, but, rather, one of determining the ultimate amount of water that can be withdrawn and managed according to scientific principles and to conditions that prevail in the arid zone. Although the hydrologic systems in the arid zone are controlled by the same physical laws as those in nonarid zones, there are particular conditions and phenomena which are unique in arid lands. It is necessary that man understand these conditions and manipulate them to his best advantage for continued occupation. The problem can be solved by regional scientific investigations that obtain geologic facts as related to surface water, ground water, quality of water, and by understanding their interrelationships. The ultimate critical water problem in Arizona is in the ground water supplies, which have met the demand in excess of the available perennial water supply and undoubtedly will meet even greater demand in the future. Only when the quantitative facts are determined will it be. possible for man to appraise the water resources and to manage them properly.

During the past fifty years water supplies have been developed intensively in Arizona. This development has yielded a wealth of hydrologic data which is invaluable for analyzing and understanding man's activities in arid-zone environment. The pattern of water management and achievements in Arizona may well establish the way of life and economic development in other arid zones throughout the world. In order to appraise the effects of Arizona's water development it is necessary to know something about its "water provinces"--such as their physical setting, environment, occurrence of ground and surface water, and related geologic conditions.

Water Provinces and Their Geologic Framework

The State of Arizona has been divided into three major water provinces (Figure 1): (1) Plateau Uplands, embracing the northern part of the state; (2) Central Highlands, a mountainous area that stretches diagonally across the state; and (3) Basin and Range Lowlands, occupying the southwestern part of the state. These provinces have been determined by topography, geology, climate, and the occurrence of surface water and ground water. The Plateau Uplands, although sometimes called a "cool desert," are nevertheless an arid zone; the Basin and Range Lowlands are, for the most part, hot and arid.

Figure 1: Water Provinces in Arizona (138 KB)

Plateau Uplands

The Plateau Uplands include high tablelands and several gently sloping mountains. The altitude ranges from 4,000 to 10,000 feet above sea level. A wide variety of land-forms occur in this area, commonly as buttes, mesas, and deep, narrow canyons. The Plateau Uplands are a country of vast expanse and magnificent scenery. It is the country of the Little Colorado River, the land of the Navajo, and a region of inadequate water supply.

Figure 2: Plateau Uplands Water Province (835 KB)

The arid nature of these uplands is accentuated by the flat desert plains in most of the area. The meager rainfall has every possibility of returning to the atmosphere by the process of capillary recirculation. This is particularly evident, as there is a paucity of vegetation to protect the moisture from evaporation. The major point of natural discharge of water from the ground water system is near the junction of the Colorado and Little Colorado Rivers. Here Blue Springs (Figure 2) discharges 160,000 acre-feet annually.

Occurrence of Water

In the northeastern part of the state the ground water reservoirs occur in rock sequences of Permian age and younger. Owing to the lithologic character of the rocks, a series of water tables are perched throughout the stratigraphic sequence (Figure 3). For the most part, ground water occurs in sandstone and limestone that are separated by relatively impermeable beds of siltstone and claystone. The occurrence of water in these rock units in specific areas is related not only to their lithologic character but to the geologic structural conditions which constitute definite controls. Ground water does not saturate the rock units uniformly because of wavy uplifts and downwarps.

Figure 3: Rock Formations in the Navajo Country (219 KB)

Some rocks older than Permian contain ground water, and several large springs discharge from the Redwall limestone of Mississippian age in the Grand Canyon area and along the Mogollon Rim escarpment. This formation is so far beneath the land surface that it is not within practical reach of the drill or within economic limit for pumping water. Perhaps in the future these sources might constitute potential supplies, depending upon the need for ground water.

The Coconino sandstone of Permian age is one of the major waterbearing formations in the northern part of the state (Figure 2). It is a fine-grained sandstone, and it attains a maximum thickness of about 900 feet. Thus, where fully saturated, it constitutes a large reservoir. Unfortunately, however, the sandstone is relatively impermeable and does not yield large quantities of water to wells except in areas where it is highly fractured.

In Apache and Navajo Counties the Coconino sandstone yields moderate amounts of water to irrigation wells, and some wells topping this aquifer in the St. Johns- Springerville area yield from several hundred to 2,000 gallons per minute. Further studies may indicate that similar yields may be available in a larger area. The short growing season and the absence of a nearby market may account for the lack of development, but future demands may accelerate additional exploration.

Several aquifers of Triassic and Jurassic age are capable of yielding small supplies of ground water to wells (Figure 3). The most important of these is the Navajo sandstone which crops out in the Navajo and Hopi Indian Reservations. The Navajo yields only moderate quantities of water to wells because the sandstone if fine-grained.

The beds of sandstone in the Mesaverde group of Cretaceous age comprise aquifers that yield small amounts of water for institutional and domestic supplies. There is little likelihood that sufficient water could be developed from these sands for irrigation purposes. Several small industries have been supplied by ground water from these formations; per gallon of water produced, however, constructidn and well development in this area are more costly than they are in the southern part of the state.

Generally, wells yield only small amounts of water and use is limited mostly to domestic and municipal supplies. There are exceptions where geologic factors have rendered local areas favorable for large production. Although the aquifers are widespread, are several hundreds of feet thick, and contain large amounts of water in storage, they are not capable of yielding large quantities of water except locally. The natural character of the rocks makes it impossible to withdraw large quantities, and the movement of water within the rocks is extremely slow--perhaps a few inches a day.

Central Highlands

The Central Highlands form a topographic high across the central part of the state. One of the distinctive features of grandeur is the Mogollon Rim escarpment (Figure 4). It extends more than 200 miles and ranges in height from several hundred to more than 2,000 feet. As the escarpment breaches one of the main aquifers at its base, springs discharge into the Gila, Salt, and Verde Rivers.

Figure 4: Central Highlands Water Province (748 KB)

Precipitation ranges from about 10 to 35 inches annually, accounting for the perennial streams in the central part of the state. Water from the Central Highlands drains into Tonto Creek, Salt, Black, White, Verde, and Gila Rivers, and a lesser amount into the Little Colorado River. The Salt River and Tonto Creek join in the highlands area and discharge into the Basin and Range Lowlands through a single outlet (Figure 4). In this gorge are impounded surface waters that provide the perennial replenishment for agriculture, industry, and municipalities in the Salt River Valley and the Phoenix metropolitan area.

The Central Highlands consist for the most part, of hard, dense rocks which have been faulted and fractured. Water occurs in the fractures, and accounts for some of the large springs in the area. These springs, and those at the base of the Mogollon Rim escarpment, discharge annually about 130,000 acre-feet of water, which drains toward the lowlands province. Several small alluvial-basin deposits containing small amounts of water occur in the Verde Valley area and to a lesser extent in the Tonto Basin area. Surface water is much more plentiful than ground water in this province. The average run-off for the Salt River above Roosevelt Lake is more than 600,000 acre-feet per year. The average flow at the mouth of the Verde River is about 500,000 acre-feet per year. Thus a little more than 1,000,000 acre-feet of water is available perennially to supply the agricultural, industrial, and municipal needs in the Phoenix area. Without this perennial supply of water it is doubtful that man could have attained the foothold he has today in the arid lands of Arizona. How demands in excess of this perennial supply will be met remains a problem.

Basin and Range Lowlands

The fact that more than 80 per cent of the state's population is located in the Basin and Range Province attests to the desirability of this area as a place to live, since the climate is perennially favorable to outdoor work and recreation. The explosive population increase provides an adequate labor supply and the lowlands provide an ideal setting for agriculture, industry, mining, and living.

In the lowlands there are still large areas of fertile land that remain desert. These probably will someday provide additional space for occupation when adequate water supplies are developed. Man's activities in Arizona are limited because there is a deficiency of available water. Without this natural resource he cannot progress in agriculture and industry, or enjoy life; thus, it becomes paramount that he learn all there is about the water supply in this province, or he may find that it will revert to desert wasteland.

Occurrence of Water

The southern part of Arizona consists of large alluvial basins separated by mountain ranges (Figure 5). The mountain ranges, for the most part, trend northwestward. The mountains and basins are about equal in areal extent and the alluvial slopes are rather steep in the southeastern part of the state. The basins are larger and their slopes are gentler in the central part. The basins have been filled with alluvium and lake deposits, the upper part of the fill consists mainly of unconsolidated gravel, sand, silt, and clay. The material of late Tertiary and Quaternary age, that constitutes the younger or upper part of the fill in the basins, probably was eroded from the adjacent mountain blocks. At the time of their deposition, the climate was wetter, allowing the basins to be filled with ground water. The deeper deposits of the basin undoubtedly are upper Tertiary rocks, and commonly are called "lake beds." Although the total thickness of lake beds and alluvial sediments in the major basins is unknown, it is believed that it ranges from less than 3,000 to more than 5,000 feet.

Figure 5: Alluvial Basins in Central Arizona (682 KB)

The water level in the alluvial fill of the basins ranges in depth from a few feet to several hundred feet below land surface. Ground water occurs in two ways: (1) under artesian pressure in aquifers overlain by relatively impervious beds of silt and clay, and (2) unconfined in water table or nonartesian aquifers.

Generally the water table has a greater permeability than the artesian aquifers and yields larger quantities of water. The artesian aquifers commonly lie at depths ranging from about 700 to 1,500 feet below land surface.

The alluvium contains perched water where clay lenses retard movement downward to the main water table. Water supplies from this source, however, are meager and sufficient only for domestic or stock wells.

Movement

The movement of ground water in the various basins in the state is controlled by: (1) permeability of the aquifers, (2) the cross sectional area of saturated sediments, (3) the hydraulic gradient, and (4) the replenishment.

Water in saturated sediments moves down gradient under the force of gravity toward an area of lower hydraulic head. A water table contour map of an area indicates the gradient of the water table, and ground water moves at right angles to the contour lines. When a well is pumped the water table is lowered in the immediate vicinity of the well, and the local gradient that is created, allows the water to move toward the well. The rate at which it moves, however, will depend upon the permeability of the material through which it moves and the magnitude of the gradient created.

Permeability is a measure of the amount of water that passes through a unit cross section of saturated material in a unit time under a hydraulic gradient of 100 per cent. It is dependent upon the size, shape, and degree of compaction of the grains. The "coefficient of permeability" was defined by Meinzer (Stearns 1928) as the rate of flow of water in gallons per day through a cross sectional area of one square foot under a hydraulic gradient of one foot per foot at a temperature of 60°F. The field coefficient of permeability is the same, except that it is measured at the prevailing temperature of the water in the aquifer. The coefficient of permeability of a water-bearing material gives the hydrologist more information as to how much water the material will yield to the well than does the porosity of the material. Cementation is important in regard to permeability because it can retard the rate of movement in various areas throughout the formation, which may have high porosity in some places and low porosity in others, and yet have an overall low permeability.

In general, fine-grained materials have low permeabilities. Some shale, siltstone, and claystone have permeabilities of less than one gallon per day per square foot; permeabilities of fine-grained sandstones generally range from 10 to 100. Aquifers composed of the larger sizes of medium and coarsegrained materials have permeabilities in the magnitude of 1,000. Aquifers consisting of unconsolidated gravel with intermixtures of sand, may have permeabilities that range up to 5,000 or 10,000.

Currently, "coefficient of transmissibility" is more widely used than is the permeability coefficient. The coefficient of transmissibility is defined (Theis, 1935) as the rate of flow of water in gallons per day through a vertical strip of an aquifer one foot wide and extending throughout the saturated thickness, under a hydraulic gradient of 100 per cent at the prevailing temperature. The relation between the two coefficients is T = Pfm, where T is the coefficient of transmissibility, Pf is the field coefficient of permeability, and m is the total saturated thickness of the aquifer, in feet.

The determination of either the coefficient of permeability or the coefficient of transmissibility of an aquifer presents many problems owing to the lenticularity of the sediments. In the southern part of the state the basins are made up of irregular lenses of gravel, sand, silt, and clay; thus the coefficient of permeability ranges widely throughout any one basin. With sufficient data, however, an estimate of the average coefficient of permeability for an area can be ascertained. Thus, in the alluvial basins it is important to know the detailed lithologic character, both vertically and laterally, as it is related directly to the permeability of the aquifer and the movement of ground water.

The rate of ground water movement ranges greatly throughout the aquifers of the state. In some of the fine-grained aquifers in the northern part of the state, movement may be only a few inches or a few feet a year, and well yields are small. In a few of the alluvial basins in the southern part of the state, ground water moves as much as several hundred feet per year, and the yield of wells is many times as great.

Storage

Storage of ground water in the alluvial-filled basins of southern Arizona and in the consolidated rocks of the northern part of the state is related directly to the porosity of the materials. The porosity of a rock is defined as its property of containing interstices or void space. It is expressed quantitatively as the percentage of void space to the total volumetric content. A rock having a porosity of 40 per cent can store water up to 40 per cent of its own volume. The unconsolidated alluvial materials in the basin s, have a greater porosity than rocks that are consolidated and cemented. The porosity of the alluvial material ranges from about 10 to 35 per cent and averages about 25 per cent.

Specific retention and specific yield must be considered in order to determine the effective storage. The specific retention of an aquifer or storage reservoir is a measure of its capacity to retain water against the force of gravity during pumping or other forms of discharge. It is expressed as the percentage of water retained to the total volume of material dewatered. Specific yield is the amount of water that can be drained by gravity from a unit volume.

The determination of the effective storage or the specific yield of a basin is difficult, because of the variables that control these factors. It is impossible to arrive at any reasonable estimate of storage without
detailed knowledge of the geology, including the interrelationships of the various strata and their structural setting (Figure 5). The total storage capacity of a basin includes its entire volume, parts of which may be beyond the economic pumping lift. Adequate knowledge of geologic conditions would make it possible to determine the amount of water that could be withdrawn from the aquifer within certain depths. Current economic factors would be used to determine whether pumping the available water from such depths would be practical.

Figure 6: Decline of Water Table in Lower Santa Cruz Basin (600 KB)

Water table declines indicate that the amount of water in storage is being decreased in some areas (Figure 6). This does not mean that the entire ground water basin will be depleted within a few years, but it indicates a reduction of storage in part of the basin.

Withdrawing ground water from the storage basins in Arizona is largely a mining process. This natural resource, which has been laid down during geologic time, is being removed in amounts in excess of replenishment. A complete knowledge of the geologic conditions that control the ground water movement, and of the differences in permeability throughout the basins is necessary in order to make sound decisions regarding the depletion or mining of this important resource.

Use of Water In Arizona

The major beneficial use of water in Arizona is for irrigation. During the period 1936-58, an average of 1,300,000 acre-feet was diverted each year from streams, mostly from the Central Highlands. During the period 1948-58, the annual gross diversion from the Colorado River ranged from 720,000 to 1,270,000 acre-feet. Return surface flows from the Yuma area averaged more than 300,000 acre-feet, so that the net diversion from the Colorado River has not exceeded 970,000 acre-feet annually. The total net diversions of surface water for irrigation in Arizona are about 2,300,000 acre-feet per year. The greatest use of surface water for municipal supply is diversion of 30,000 acre-feet per year from the Verde River for the Phoenix area. About 10,000 acre-feet are diverted from the Black and San Francisco Rivers to supply water for the Morenci mining enterprise.

More than 95 per cent of the precipitation falling on Arizona (about 80 million acre-feet per year) is consumed by evaporation and used by natural vegetation. Percolation in the soil from rains is not deep and after the storm the surface soil moisture is soon evaporated. Much of the run-off that does occur is used to saturate the porous materials in the stream bed. Evaporation from flowing streams is usually very large in arid lands; also much water is lost by evaporation from surface reservoirs. During 1957, 90 inches of water were evaporated from Lake Mead alone--a water loss of 800,000 acre-feet. The total loss of water by evaporation from all the reservoirs within the state is more than one million acre-feet each year.

For the past five years the annual ground water withdrawal for agriculture has been more than 4-1/2 million acre-feet. Eighty per cent of the ground water is pumped in Maricopa and Pinal Counties. In 1957, industry, including mining activities, used more than 175,000 acre-feet from the ground water reservoir, principally for cooling and sanitation purposes. This is less than five per cent of the total ground water pumped, but represents a 400 per cent increase over the amount used in 1949 for this purpose. Because of the increase in population in recent years, the amount of water used by municipalities has become a significant part of the water used in the state. Municipalities used 40,000 acre-feet in 1949; their use increased to about 130,000 acre-feet in 1958.

In summation, Arizona uses more than seven million acre-feet annually for agriculture and industry and for providing man with modern comforts. About five million come from the ground water reservoirs and the other two million come from surface water supplies. Half a million acre-feet are used for industry and municipalities, and 6-1/2 million are used for irrigation. Although surface water supplies were originally adequate, many communities, such as Flagstaff, Winslow, Safford, and Phoenix, now use ground water as a supplementary supply.

How Can Water Supplies Be Increased In Arid Lands?

The problem of increasing available water supplies to meet the future demands in arid lands is of utmost importance. How can this be done? Many possibilities have been suggested to increase available water, but an analysis of these indicates that three are realistic: (1) conversion of saline or brackish water, (2) capture of water that is otherwise lost to the atmosphere, and (3) scientific exploitation of the existing ground water reserves.

The desalting of sea water for fresh water use seems feasible, and recently it has been shown that the cost of conversion is comparable to some present municipal water costs. One large factor, however, is the cost of transporting the water. To transport to Tucson, which lies 100 miles from the nearest seacoast and 2,500 miles above sea level, would cost several hundred dollars per acre-foot. This contrasts to present-day municipal costs of $50 to $80 per acre-foot. Converting brackish or saline waters in our ground water basins, however, offers a real possibility, since the cost of transportation would probably be small and within the realm of the present-day economic situation.

Figure 7: Artificial Recharge by Wells (324 KB)

One of the most practical measures for increasing the availability of water supplies--and one that has been suggested by many early workers--is capturing liquid water and storing it underground. The newly stored water would not be lost by evaporation and could be recovered when needed, since there is little possibility that the water would leave the basin by underflow. This suggested measure includes: (1) the capture of flood waters in the lowlands and base flow from the highlands, both of which for the most part are now lost by evaporation; (2) transportation of the water in lined canals or conduits to areas where there have been large ground water withdrawals or areas which have the greatest need and highest use; and (3) recharge of the water into dewatered sediments by wells (Figure 7). One advantage of recharging water into dewatered sediments instead of bone-dry material is that the specific retention is already satisfied, and it would be possible to recover about 100 per cent of the recharged water. There are many engineering, geologic, and economic problems, however, that need to be resolved before artificial recharge can be put into operation in Arizona.

Conclusions

The greatest challenge to man occupying arid lands is the development of adequate water supplies. To meet this challenge it is necessary to make
quantitative determinations of all possible water sources and to manage them scientifically. When water demand exceeds supply or perennial replenishment, man has several choices to meet his needs: (1) transporting water into the area, (2) capturing additional water that escapes under natural conditions, and (3) moving into areas blessed with ample water. Information is needed on our total water supply, the magnitude of our reserves, and the rates of depletion. A better understanding of the hydrologic system and the mechanics of ground water movement, particularly of the surface water and ground water interrelationships, is essential. All the disciplines in the water resources field need to be harmonized and merged to achieve a clear understanding of Arizona's hydrology, and to attain the most efficient manner of exploitation.

Bibliography

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Hardt, W. F., Cahill, J. M., and Booher, M. B. Annual Report on Ground Water in Arizona, Spring 1957 to Spring 195 Arizona State Land Department Water Resources Report No. 5' 1958.

Harshbarger, J. W. "Use of Ground Water in Arizona," Climate and Man in the Southwest. ("University of Arizona Bulletin, Volume 2 , Number 49 195 .

Harshbarger, J. W., Repenning, C. A., and Callahan J. T. "The Navajo Country, Arizona-Utah-New Mexico," The Physical and Economic Foundation of Natural Resources, Part IV, Subsurface Facilities of Water Management and Patterns of Supply. "Type Area Studies; House of Representatives, Interior and Insular Affairs Committee") pp. 105-129, 1953.

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Ross, C. P. The Lower Gila Region, Arizona--A Geographic, Geologic, and Hydrologic Reconnaissance, with a Guide to Desert Watering Places. 'United States Geological Survey Water Supply Paper 9F" 1923.

Schwalen, H. C., and Shaw, R. S. Water in the Santa Cruz Valley. ("University of Arizona Agricultural Experimental Station Bulletin 288") 1957.

Stearns, N. D. Laboratory Tests on Physical Properties of Water-Bearing Materials. ("United States Geological Survey Water Supply Paper 596-F") 1928.
Theis., C. V. "The Relation Between the Lowering of the Piezometric Surface and the Rate and Duration of Discharge of a Well Using Ground Water Storage," American Geophysical Union Transaction, pp. 519-524, 1935•

Turner, S. F., and Others. Ground Water Resources of the Santa Cruz Basin, Arizona. ("United States Geological Survey open-File Report" 19 3.

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(Annual). "Water Levels and Artesian Pressures In Observation Wells in the United States, Southwestern States," United States Geological Survey Water Supply Papers.

From:
Arid Lands Colloquia, 1958-1959. Tucson, Ariz.: The University of Arizona Press, 1959.