GROUND-WATER RESOURCES FOR THE FUTURE Desert Basins of the Southwest Ground water is among the Nation’s most important natural resources. It provides causing unacceptable environmental, drinking water to urban and rural communities, supports irrigation and industry, economic, or social consequences sustains the flow of streams and rivers, and maintains riparian and wetland (Alley and others, 1999). ecosystems. In many areas of the Nation, the future sustainability of ground-water resources is at risk from over use and contamination. Because ground-water systems typically respond slowly to human actions, a long-term perspective is needed to Occurrence of Ground Water in the manage this valuable resource. This publication is one in a series of fact sheets that Southwestern Basins describe ground-water-resource issues across the United States, as well as some of the activities of the U.S. Geological Survey that provide information to help Much of the landscape in the south- others develop, manage, and protect ground-water resources in a sustainable manner. western United States (fig. 1) consists of valleys separated by mountain ranges. The mountain ranges are composed of a round-water resources in the South- water resources while looking for variety of rocks, many of which contain Gwest are among the most overused supplemental sources of water. water-filled fractures, but do not yield in the United States. Natural recharge to This fact sheet reviews basic informa- large quantities of water to wells. Most aquifers is low and pumping in many tion on ground water in the desert basins of the valleys, however, are basins areas has resulted in lowering of water of the Southwest. Also described are containing water-filled layers of gravel, tables. The consequences of large-scale some activities of the U.S. Geological sand, silt, and clay. Though these removal of water from storage are Survey (USGS) that are providing sediments are as much as 10,000 feet becoming increasingly evident. These scientific information for sustainable thick, commonly only the upper few consequences include land subsidence; management of ground-water resources thousand feet of sediments are aquifers loss of springs, streams, wetlands and in the Southwest. Ground-water sustain- that contain water of good quality that associated habitat; and degradation of ability is defined as developing and can be readily extracted by wells. Water water quality. Water managers are now using ground water in a way that can be in these aquifers occurs in small pores seeking better ways of managing ground- maintained for an indefinite time without between the grains of sediment (fig. 2). The pores are completely saturated below the water table, but are only 120° 115° partially saturated above the water table where they cannot readily release water. 40° 110° CALIFORNIA NEVADA Area of Map UTAH 105° Water table ARIZONA 35° NEW MEXICO 0 100 200 MILES 0 100 200 300 KILOMETERS Figure 2. Ground water in the basins of the Southwest occurs in pore spaces Figure 1. The desert basins in the Southwest, shown in the darker brown, extend between grains of gravel, sand, and clay over parts of California, Nevada, Utah, Arizona, and New Mexico. sediments. U.S. Department of the Interior USGS Fact Sheet 086-00 U.S. Geological Survey August 2000 to import surface water from distant sources have been implemented. In many areas of the Southwest, however, surface-water supplies are limited and ground water remains a critical e g g resource. n r i a p e Evapotranspiration h g m c r u e a P h R r Consequences of Ground-Water s e c at d m e e w n a c R rfa Use a e u l s r t m t ro e f S w nflo W I Ground-water use has the positive aspect of providing water for human Aquifer needs including public supply, agri- culture, and industry. On the other hand, ground-water use in areas where Outflow to Bedrock surface water recharge is limited can have negative consequences. Concerns where with- drawals exceed recharge include loss of Figure 3. Hydrology of a desert basin drained by a stream. Most recharge to the available ground-water supply, land aquifer occurs at higher elevations, which receive more rainfall than lower elevations. Ground water flows to the central part of the basin where it may discharge to springs, subsidence, degradation of water wetlands, streams, or evapotranspiration. Ground-water pumping has the local effect quality, and loss of riparian habitat. of removing water from storage in the aquifer and the eventual effect of reducing outflow to surface-water features and riparian plants. 6 S N O I 5 Most recharge to the aquifers occurs at springs and in stream channels where L T L I at higher elevations from rainfall and it seeps to the surface. Later access was E M E 4 F snowmelt runoff in the mountains. Water through shallow dug wells (fig. 4), low- N - I , seeps into the aquifers through sand and capacity drilled wells, and finally, E L R A 3 gravel near the edges of the basins under high-capacity large-diameter drilled C A W A normally dry washes that flow during wells with turbine pumps. The high- F R O 2 infrequent periods of runoff and by sub- capacity deep-well turbine pumps came D H surface flow from water-filled fractures into widespread use in the mid 1940’s T I beneath the mountains (fig. 3). and resulted in large increases in W 1 Ground water flows from basin edges ground-water use in desert basins. For toward lower elevations in the central example, ground-water use in Arizona 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 9 parts of the basins. Movement of ground more than doubled from 1945–53, 0 9 9 9 9 9 9 9 9 9 0 1 1 1 1 1 1 1 1 1 water is slow, with typical velocities when turbine pumps were installed for 2 ranging from tens to thousands of feet irrigation and public supply (fig. 5). Figure 5. Annual ground-water with- per year. Water eventually discharges Ground-water use in some parts of drawals in Arizona, 1915–90. from aquifers to springs, streams, the Southwest has declined as projects wetlands, playas, plants, and adjacent basins. Because of the slow movement, ground water can be in basins for tens of thousands of years before discharging. Many of the basin aquifers contain large quantities of water. For example, in a basin in which the upper 1,000 feet of saturated sediments contain 15 percent removable water and having an average width and length of 15 miles and 25 miles respectively, the volume of water is 36 million acre-feet. That volume of water, if it could be prac- tically pumped, would amount to a 2,000-year supply for a population of 100,000 having a per capita water use of 160 gallons per day. Ground-Water Use in the Desert Southwest Ground water has long been a critical resource to humans in the Southwest. Figure 4. Gonzales' well, Dome Rock Mountains, Yuma County Arizona, 1917. Ground- water resources were important to travelers in the Southwest in the early 1900's. The earliest access to ground water was Loss of Available Supply 120° 115° EXPLANATION Any withdrawal of ground water WATER-LEVEL DECLINE, 110 IN FEET: results in removal of water from storage. 40° ° More than 200 Large-scale withdrawals in many areas CALIFORNIA in the Southwest have resulted in wide- 100 to 200 spread lowering of water tables (fig. 6). 50 to 100 The amount of the lowering reflects a NEVADA quantity of ground water that is no UTAH longer available. Several other aspects of 105° lowered water tables affect future water availability. First, lowering of water tables results in increased costs to lift ARIZONA water a greater distance. For some water 35° uses such as agriculture, pumping costs from deep aquifers could be NEW MEXICO prohibitively high. Second, lowered water tables can result in loss of well productivity. In most basins, shallower 0 100 200 MILES sediments are less compacted and more 0 100 200 300 KILOMETERS readily release water to wells than deeper sediments. Lowered water tables can result in the need to drill more and deeper wells to maintain a desired rate of Figure 6. Locations in the basins of southern California, Nevada, Utah, Arizona, and ground-water withdrawal. New Mexico where significant ground-water level declines have been measured. In some areas, water levels have recovered in response to reduction in pumping and increased recharge efforts. Land Subsidence Land subsidence is the sinking of the Loss of Riparian Habitat land surface. A major cause of land subsidence in the Southwest is slow As shown in figure 3, under natural drainage of water from the clay and silt conditions most recharge in basins sediments in or next to aquifers occurs at higher elevations, resulting in (Galloway and others, 1999). As water ground-water flow toward discharge levels in the aquifers decline, the slow points near the center of the basins. drainage from the clay and silt layers Many of the discharge points are cause them to compact, and the land springs, streams, and wetlands. The surface to be lowered. Uneven land discharge of ground water to the subsidence can change the slope of the surface sustains abundant plant and land surface and can cause earth fissures animal life in otherwise harsh environ- (fig. 7), resulting in damage to buildings, ments.