Abstract

A three-dimensional, numerical groundwater flow model of the Middle Trinity aquifer in the Hill Country area of south-central Texas was developed to help estimate groundwater availability and water levels in response to pumping and potential future droughts. The model includes historical information on the aquifer and incorporates results of new studies on water levels, structure, hydraulic properties, and recharge rates. A steady-state model was calibrated for 1975 hydrologic conditions when water levels in the aquifer were near equilibrium, and a transient model was calibrated for 1996 through 1997 when the climate transitioned from a dry to a wet period. Using the model, values of recharge, hydraulic conductivity, specific storage, and specific yield were calibrated for the aquifer. The model was used to predict future water levels and saturated thickness under drought-of-record conditions using estimates of future groundwater demands based on demand numbers from the Regional Water Planning Groups.

The model predicts that the area near Cibolo Creek in northern Bexar, southern Kendall, and western Comal counties is the most susceptible to future water-level declines due to increased demand and potential droughts. If a drought similar to the drought-of-record occurs in the future, the model suggests that water levels may decrease as much as 100 ft in this area by 2010 and that a large part of the aquifer may be depleted in this area by 2030. The model suggests that water levels may decline nearly 100 ft in the Dripping Springs area by 2040. Hays, Blanco, Travis, southeastern Kerr, and eastern Bandera counties may experience moderate water-level declines (50 to 100 ft) in response to projected demands and potential drought as early as 2010. The model also suggests that major rivers may continue to flow seasonally even with increased pumping and under drought conditions.