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
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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. When ground water is with- pipelines, canals, drainage ditches, roads, drawn by humans, the immediate effect railroads, dams, and bridges. Some is the removal of water from storage in locations in the Southwest having signi- the area of the withdrawal. However, ficant amounts of land subsidence are: after continued withdrawal, the effects in an aquifer spread from the area of Location Subsidence, in feet withdrawal and decrease the amount of Lancaster, California 6 discharge from the aquifer (Winter and Near Mendota, California 29 others, 1998). Ground-water with- Las Vegas, Nevada 6 drawals in excess of the amount of Eloy, Arizona 15 discharge can result in the loss of an West of Phoenix, Arizona 18 entire riparian system. For example, a mesquite bosque was present along the Degradation of Water Quality Santa Cruz River south of Tucson, Arizona, in 1942 (fig. 8 on following Many basins in the Southwest con- page, the photograph to the left). By tain the highest quality water at 1989, the large riparian trees were shallower depths. Initial development gone (fig. 8, the photograph to the of ground-water resources commonly right). Ground-water levels in the area focuses on first pumping the water of declined by more than 100 feet in that highest quality from the shallowest part period, resulting in a loss of the of the aquifer. Continued pumping then ground water that had sustained the can result in degradation of water riparian plants. quality over time as more water of lower quality is pumped from deeper Figure 7. Earth fissure related to land subsidence, south-central Arizona. parts of aquifers. Figure 8. Views of the Santa Cruz River looking south from the same location on Martinez Hill, south of Tucson, Arizona. In the 1942 photograph to the left, the flood plain is covered with a mesquite bosque, valued for its wild bird population. In the 1989 photograph to the right, the bosque no longer exists.
Southwest Ground-Water Climate Variability and Ground Water Improved Methods of Simulating Resources Project Interaction of Ground Water and This study is quantifying the Surface Water The U.S. Geological Survey has a responses of different aquifers to program for continuing evaluation of the climate variations at various time Existing computer programs can be ground-water resources of the United scales. For selected areas in the used to simulate interaction of ground States. Part of this program is the study of Southwest, time series of precipitation, water and surface water, but the methods major ground-water issues in different streamflow, ground-water levels and used are not ideal for streams that go dry regions of the country. In the Southwest, discharge, and tree-ring thicknesses frequently or that are underlain by a the major issue under investigation is the will be analyzed by various mathe- thick unsaturated zone. For use in the interaction of ground water and surface matical methods to quantify the desert Southwest and other arid regions, water. The study will provide water relations between climate variations the Southwest Ground-Water Resources managers with better information on and ground-water resource availability. Project is developing better methods of interaction of ground water and surface simulating loss of water from ephemeral water and relations of that interaction to Ground-Water Development and Riparian streams and movement of that water to sustainability of ground-water resources. Systems the underlying aquifer. The project is focused on the desert basins of the Southwest shown on figure 1. The effect of ground-water avail- Obtaining More Information on Ground- Major study components include (1) ability on riparian ecosystems is being Water Resources in the Southwest regional synthesis; (2) effects of climate studied at three sites in riparian habitats variability on recharge to and discharge in California, Arizona, and New Mexico. In addition to the Southwest Ground- from ground-water systems; (3) effects of The study is using a multidisciplinary Water Project, District Offices of the development of ground-water resources approach centered on at least six U.S. Geological Survey in California, on sustainability of riparian systems; (4) riparian tree species to determine the Nevada, Utah, Arizona, and New development of improved methods of sustainability of riparian systems with Mexico collect data and conduct studies estimating recharge to desert basins; and respect to ground-water and surface- on ground-water resources in each state. (5) development of improved methods of water development in the Southwest. Links to pages on the World Wide Web simulating interaction of ground water and with information on District programs surface water in arid and semi-arid regions. Improved Methods of Estimating and the Southwest Ground-Water Recharge Resources Project can be found at site Regional Synthesis http://water.usgs.gov/ogw/ The quantity of ground-water Hydrologic and geologic data are recharge to alluvial basins in the —S.A. Leake, A.D. Konieczki, J.A.H. Rees collected by federal, state, and local Southwest is one of the least known agencies to monitor water supply and factors in the water budget. Several References hydrologic conditions. Compilation of studies are currently being conducted to that information and evaluation of long- assist in quantifying recharge estimates. Alley, W.M., Reilly, T.E., and Franke, O.L., 1999, term data sets are assisting in charac- A network of four sites along streams in Sustainability of ground-water resources: U.S. terization of areas in terms of the California, Nevada, Arizona, and New Geological Survey Circular 1186, 79 p. interactions of ground water and surface Mexico has been established to evaluate Galloway, D.L. Jones, D.R., and Ingebritsen, S.E., water, the potential for future overdraft, methods to estimate recharge. The 1999, Land subsidence in the United States: and the impact on riparian systems in the methods include: geochemical analysis, U.S. Geological Survey Circular 1182, 177 p. Winter, T.C., Harvey, J.W., Franke, O.L., and Southwest. Land-use data also will be thermal tracing, Darcian flow Alley, W.M., 1998, Ground water and surface used to analyze the relation between land- measurements, geophysical surveys, and water— a single resource: U.S. Geological use changes and hydrologic conditions. channel loss. Survey Circular 1139, 79 p.