A quasi-three-dimensional, numerical model of the occurrence and movement of groundwater in the central part of the Carrizo-Wilcox aquifer in Texas was developed to help us estimate groundwater availability and water levels in response to potential droughts and future pumping, including new well fields. Formations of the Paleocene-Eocene-age Wilcox Group, along with the overlying Carrizo Formation, make up a major aquifer system in Texas. This six-layer model is based on data on geological structure and depositional setting of the aquifer, hydrological properties, water-use survey estimates of historical groundwater withdrawals, and base flow of rivers and streams. New insights into how the downdip circulation of freshwater is affected by fault zones and a deep-basin geopressured zone are based on maps of total dissolved solids and equivalent water levels from the outcrop to depths of more than 10,000 ft. In addition, results of field studies using "environmental" tracers yielded regional estimates of recharge rates that broadly match estimates from previous models.

A steady-state model representing "predevelopment" (no pumping) conditions was calibrated against water levels measured before 1950 and historical low-flow measurements in streams. A transient version of the model was calibrated against water-level hydrographs and stream-flow data for the period from 1950 through 1990 and verified by comparison with water levels recorded between 1991 and 2000. Recharge rates, vertical hydraulic conductivity, specific storage, specific yield, and boundary-flux properties were calibrated using the model. Horizontal hydraulic conductivity is one of the better known attributes of the aquifer, given the number of pumping- and specific-capacity tests and the quality of regional mapping of the distribution and thickness of sandstones that make up the permeable architecture of the aquifer.

To demonstrate the use of the groundwater model as an evaluative and predictive tool, simulations were made of future water-level changes with assumed periods of normal and drought-of-record precipitation. Pumping rate is expected to continue to increase between 2000 and 2050, but at a rate slower than that of the past decade. Overall, total pumping from the Carrizo-Wilcox aquifer in the study area is expected to increase from 197,000 acre-feet per year in 2000 to 320,500 acre-feet per year in 2050. The simulated decline of water level related to groundwater pumping will occur mainly through a decrease in artesian storage. The model also suggests that the major rivers will continue to flow even with increased pumping and under drought conditions. The project is funded by the Texas Water Development Board as part of their Groundwater Availability Modeling (GAM) program.