This report is phase 1 of a 3-phase study and includes information needed to build a digital hydrologic model of Borrego Valley. It includes sources and amounts of recharge water to the basin, areas of water withdrawal, total ground water in storage at steady-state conditions (1945), net ground-water depletion, grid network used to determine storage and depletion, total depth of alluvial fill, transmissivity, hydraulic conductivity and specific yield of aquifers, thickness of individual aquifers, and chemical quality of water. (USGS)