Two finite-element ground-water flow models were developed for the Edwards–Trinity aquifer system, west-central Texas, to gain a better understanding of the flow system; one ground-water flow model was developed at a large scale to simulate the regional system and contiguous, hydraulically connected units, and one model was constructed at a smaller more detailed scale to simulate the most active areas of the system. The study area is divided into four geographic subareas: the Trans-Pecos (9,750 square miles), the Edwards Plateau (23,750 square miles), the Hill Country (5,500 square miles), and the Balcones fault zone (3,000 square miles). The major aquifers within the study area are the Edwards–Trinity aquifer underlying the Trans-Pecos and Edwards Plateau, the Trinity aquifer underlying the Hill Country, and the Edwards aquifer in the Balcones fault zone. Hydraulically connected aquifers include the High Plains aquifer north of the Edwards Plateau, and the Cenozoic Pecos alluvium aquifer adjacent to both the Trans-Pecos and the Edwards Plateau along the Pecos River. Minor contiguous aquifers include the Dockum, Ellenburger– San Saba, Marble Falls, Hickory, and Lipan, which is adjacent to the Colorado River in Tom Green and Concho Counties, Texas.

The ground-water flow equations solved by the finite-element method are based on conservation of mass and energy. The equation for ground-water flow assumes laminar flow through a porous media. In places, the Edwards–Trinity aquifer system is a fractured karst system in which ground water flows through caverns and other features of secondary porosity development. The regional and subregional models were constructed to synthesize the known hydrologic boundaries and geologic structures into a heterogeneous continuum model of the karst ground-water flow system, rather than simulate the flow through specific fractures and caverns. A heterogeneous continuum or equivalent porous media approach uses an effective transmissivity and anisotropy for each element of the models. The models are calibrated both on water levels (representing potential energy) and estimates of recharge and discharge (for a realistic mass balance).

A two-dimensional one-layer large-scale model (55,600 square miles) was developed for the Edwards–Trinity aquifer system and contiguous, hydraulically connected units, in westcentral Texas. A quasi-three-dimensional, multilayer more detailed scale ground-water flow model (12,300 square miles) was applied to the major aquifers of the Edwards–Trinity aquifer system in the Hill Country and the Balcones fault zone, and in part of the Edwards Plateau.

The ground-water flow system in most of the study area within the Trans-Pecos and Edwards Plateau can be approximated with a one-layer regional model under steady-state conditions. Regionally, the Edwards–Trinity aquifer system in the Trans-Pecos and Edwards Plateau has been relatively static. Potentiometric maps from predevelopment and postdevelopment (winter 1974–75) indicate small differences in water levels. In local areas in the Trans-Pecos (in Pecos and Reeves Counties), ground-water withdrawals have exceeded recharge resulting in more than 300 feet of drawdown. Measurable differences between the 1974 and predevelopment potentiometric surfaces have been observed in small areas in the Trans-Pecos and in the northwestern part of the Edwards Plateau. The largest water-level declines in the Trans-Pecos have been observed in Pecos and Reeves Counties, and declines greater than 300 feet have been measured in Reeves County.

Comparison of pre- and postdevelopment water budgets for the regional model indicates that the increase in groundwater withdrawals has captured 20 percent of the water that would have naturally discharged to streams, and 30 percent of the natural discharge to springs after ground-water development. Induced recharge from streams to the ground-water system increased by 12 percent in the postdevelopment simulation compared to the predevelopment simulation.