The Rio Grande is an interstate and international stream which begins in high mountains of Colorado, flows across New Mexico, and forms the boundary between Texas and Mexico. Precipitation ranges from 8 inches (20 em) to more than 30 inches (76 em), but irrigation is required for growing crops throughout the region. The population of the region has been increasing rapidly, from 750,000 in 1929 to 1, 700,000 in 1970, and it is expected to increase to 2,500,000 by 2020. The basic economy of the region was agricultural until recent years. Since 1950, the mining and petroleum industries have increased much more rapidly than agriculture. Annual precipitation on the region is about 86 million acre-feet (110,000 hm 3); however, all but 4 million acre-feet (4,900 hm 3) is returned to the atmosphere by evapotranspiration. The groundwater reservoirs contain an aggregate of 5,800 million acre-feet (7,200,000 hm 3) of fresh and slightly saline water in storage, which could be withdrawn through wells. In contrast, the surface reservoirs have a combined storage capacity of only 18 million acre-feet (22,000 hm 3). Thick deposits of valley fill in stream and intermontane valleys comprise the principal groundwater reservoirs. In most areas they are capable of yielding large supplies of water to wells. In some areas, limestone constitutes major aquifers. Withdrawal of ground water in the region in 1970 was 2.7 million acre-feet (3,300 hm 3), of which 88 percent was used for irrigation. About 53 percent of the water withdrawn was consumed. Ground water has been "mined" in some areas, and severe declines in water levels have resulted. The loss of water by evapotranspiration in wetlands and phreatophyte areas is 2.5 million acre-feet (3,100 hm 3) per year. In comparison, about 3. 7 million acre-feet (4,600 hm 3) per year of surface water and ground water is consumed by man's activities. Salvage of water lost to noneconomic evapotranspiration in wet and phreatophyte-infested areas offers the greatest possibility of improving the effective water supply in the region. Salvage of half the water lost would increase the effective supply by 1.2 million acre-feet (1,500 hm 3) per year. The usable water supply could be increased tremendously by drawing on the large reserve of ground water in storage, but this withdrawal could affect the flow of streams in some areas. The region appears to offer several possibilities for utilizing underground space for purposes other than the withdrawal of water, such as waste disposal, artificial recharge, water-quality control, and development of geothermal energy. Planners for groundwater management should have detailed information on the physical parameters that affect ground water, so improved management would be possible. D1 D2 SUMMARY APPRAISALS OF THE NATION'S GROUNDWATER RESOURCES management. Although ground water comprised 21 percent of the water used in the United States in 1970, planning for groundwater development has been limited to the municipal, water district, or private level, except for a fe...
