At present, 45 million people live in the southwestern United States, which also contains some of the most productive agricultural lands in the Nation. Because most of the surface water in the region has been appropriated, the search for new water supplies emphasizes development of ground water resources and new types of conjunctive use related to artificial recharge and effluent discharges. Ground water, used with surface water, is of paramount importance to communities and agriculture in this rapidly growing region. Availability and sustainability of ground-water resources are measured in terms of changes in water levels and water budgets, changes in streamflow discharge from ground-water systems, and stability of riparian ecosystems dependent on shallow ground water. Because of potential diminution of riparian areas, management of ground-water dependent riparian ecosystems is assuming critical importance in the management of southwestern water supplies. The Southwestern Ground-water Resources Project is a 5-year study of ground-water and surface-water interactions and their effects on the availability and sustainability of ground-water supplies in the Southwest. The project chief is Stanley A. Leake, Tucson office of the Arizona District, U.S. Geological Survey. The study area includes aquifer systems in the arid to semiarid basins in southwestern states of California, Nevada, Utah, Arizona, New Mexico. The main focus topics of the project are: Regional synthesis of information on the interaction of ground water and surface water. Development of improved methods of quantifying inflow to ground-water systems from streams and application of these recharge methods in the Southwest. Assessments of the effects of ground-water development on riparian systems. Assessments of the effects of climate variations on recharge to and discharge from ground-water systems. Development of improved methods of simulating interaction of surface water and ground water.   This study will use existing and developing techniques to provide fundamental information about the relations between ground water and surface water in the Southwest. Study results will augment the current capacity to understand the impacts of past ground-water development, current use, and climatic change on associated surface-water systems, and will provide criteria that will enable estimation of the effects of future hydrogeologic impacts. Synthesis of information will provide the basis for future monitoring. New and existing methodologies used will have immediate transfer value both within and outside of the Southwest, particularly in areas having similar climatic and hydrogeologic conditions. Integration of the products of this study will provide a basis on which to examine the impacts of historic and present ground-water development on riparian systems in arid to semiarid climates.

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