The Edwards-Trinity aquifer system is currently (1993) being studied as part of the Regional Aquifer-Systems Analysis program. A major goal of the project is to understand and describe the regional ground-water flow system. A finite-element model for simulating two-dimensional steady-state ground-water flow was applied to the major aquifers of the Edwards-Trinity aquifer system and contiguous hydraulically connected units for the winter of 1974-75 and for the predevelopment conditions. The major aquifers are the Edwards-Trinity in the western semiarid part of the study area, the Trinity in the eastern subhumid part of the study area, and the Edwards in the southeastern part of the study area. The Edwards-Trinity, upper part of the Trinity, and part of the Edwards aquifers are shallow and unconfined over most of the study area. Regional ground-water flow is toward the perennial streams for the Edwards-Trinity and Trinity aquifers, as indicated by the observed and simulated potentiometric surfaces.

The transmissivity values used in the simulations were within estimated ranges and generally are: 1,000 to 10,000 ft²/d (feet squared per day) for the Edwards-Trinity and Trinity aquifers; 100,000 to greater than 1 million ft²/d for the Edwards aquifer; and less than 500 to 10,000 ft²/d in contiguous hydraulically connected units. Simulated flow through the Edwards-Trinity aquifer system and contiguous hydraulically connected units is about 3 million acre-feet per year. Estimates of areally distributed recharge from the simulations range from 0.1 to 1 inch per year for the Edwards-Trinity aquifer and increase to 4 inches per year for the Trinity aquifer. Recharge to the Edwards aquifer occurs along streambeds that cross outcropped high-permeability rocks of the Edwards Group through joints and faults. Many of the streams are diverted completely underground during periods of no precipitation. The movement of a substantial quantity of water (about 400 cubic feet per second) from the Trinity and Edwards-Trinity aquifers into the Edwards aquifer was simulated. Results of the simulations indicate that anisotropy strongly influences flow in the Edwards aquifer. In the San Antonio and Austin areas, the Edwards aquifer is the most active part of the ground-water flow system with one-third of ground-water discharge occurring in 5 percent of the modeled area for both simulations.