This study is developing an aquifer database for an improved computer model of groundwater flow in the San Antonio segment of the Edwards aquifer. The Edwards aquifer, the major source of water for more than 1.5 million people in the San Antonio area, provides nearly all of the water used in the region for industrial, military, irrigational, and public supplies. If withdrawals of groundwater are accelerated, furthermore, spring flow at Comal and San Marcos Springs will be threatened. Both springs supply water to meet downstream needs, sustain Federally listed endangered species, and support local economies by attracting tourists. The U.S. Geological Survey and the Bureau are collaborating to develop the database and computer model on behalf of the Edwards Aquifer Authority. The study, which began late in 2000, will be completed in 2003. The model being developed will enable water managers to test the effects of alternative, potentially costly management scenarios before enactment. Input simplifications and output enhancements will make the model user-friendly for trained personnel, as well as ensure that the graphics-rich output is understandable to nonscientists.