Located within the Basin and Range Physiographic Province, the Great Basin Carbonate Aquifer Province covers 110,000 mi²  with 167 surface-water basins, predominantly in eastern Nevada and western Utah. The aquifer system, previously investigated during the 1980s under the USGS Regional Aquifer System Analysis (RASA) program, is comprised of 17 interconnected sub-regional ground-water flow systems. While most ground water utilization comes from shallow basin-fill sediments, an integrated fabric of underlying and intervening mountain-block carbonate and volcanic rock aquifers provide local recharge to these basins and can act as conduits for interbasin flow. During the late 20^th century, this area experienced one of the highest rates of both population growth and per capita water use in the nation. These, combined with its arid setting, have levied intensive demand upon existing ground-water resources. Competing demands include municipal supply to urban centers (Las Vegas, Salt Lake City), agriculture, and streamflow to support desert ecosystems. With future climate-change scenarios predicting rising temperatures of 3 to 6º Celsius, there will be less winter snowpack and increased  evapotranspiration that would likely result in reduced ground-water recharge. This reduced recharge, along with increasing demand, has caused the Department of Interior to list the region as having a high probability for future water conflicts and shortages. An ongoing 4-year Ground Water Availability study that is part of the National Water Census, is evaluating water sustainability in the eastern Great Basin. This study will provide an updated description of the regional ground-water flow system utilizing state-of-the-art water-budget analysis techniques including a basin characterization model, construction of an enhanced three-dimensional hydrogeologic framework of the aquifer system, and development of quantitative numerical modeling tools to assess system responses to future human and climatic stresses. Concurrently, coupled thermal/flow models are being developed to independently assess recharge rates along climate gradients.