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Ground Water <br />The amount of ground water in storage in the United States is changing in response to climate <br />variability, ground -water withdrawals, and other stresses. Data from several ground -water <br />basins and aquifers already show significant depletions in freshwater availability over several <br />decades, but knowledge is quite limited about these changes nationwide. <br />The program will develop several indices of ground- water -level changes; some will represent <br />composite indices for the Nation, whereas others will be analogous to "sector" indices in the <br />stock market and will reflect specific geographic regions or specific types of aquifers, terrains, <br />environments, or land -use settings. The indicator wells will include wells open to water -table <br />aquifers and deep artesian aquifers, wells located both near and distant from pumping centers, <br />and wells that are widely distributed geographically. The various indices will provide water <br />managers, major water users, and the public with quick summaries of the magnitudes and <br />significance of trends in water -level changes. <br />In addition to the ground- water -level indices, the program will conduct periodic assessments of <br />changes in aquifer storage due to ground -water withdrawals, saltwater intrusion, mine <br />dewatering, land drainage, and other mechanisms that affect ground -water availability. These <br />assessments will be based on nationwide summaries of observed water -level changes and <br />ancillary data describing the aquifers and their changing storage conditions. These assessments <br />will require a greater level of effort than that required for development of the ground- water- <br />level indices because estimates of changes in ground -water storage require knowledge of <br />aquifer storage properties and spatial interpolation of ground - water -level measurements. The <br />program also could provide measures of the nationwide status of ground- water - supply <br />infrastructure, such as changes in the number and capacity of water- supply wells and artificial <br />recharge facilities; these measures are analogous to those reported for surface -water reservoirs. <br />Changes in ground -water use and the effect of development are not usually as variable year -to- <br />year as with surface water and, therefore, the periodic assessments of ground -water storage will <br />be made at 5- to 10 -year intervals. Ground -water storage changes will be evaluated by major <br />aquifer and then aggregated into regional and national assessments. In the absence of <br />systematic water -level monitoring for many aquifers, synoptic surveys during which many <br />water -level measurements are made over a short period of time will be made at 5- to 10 -year <br />intervals. In some cases, historical changes in ground -water storage may need to be estimated <br />by use of ground -water simulation models that account for all ground -water storage processes, <br />including storage changes in confining units. This modeling will build, in part, on work from <br />the USGS Regional Aquifer- Systems Analysis (RASA) Program, which studied and modeled <br />the Nation's regional aquifers from 1978 to 1995. <br />Water Use <br />Existing USGS water -use estimation efforts need strengthening to reflect the increased <br />importance of, and demands for, national water -use data and analyses. The USGS has compiled <br />and disseminated estimates of water use for the Nation at 5 -year intervals since 1950 (Solley <br />and others, 1998). Water -use information is compiled in collaboration with the States through <br />