We have developed a preliminary groundwater flow model for the Presidio and Redford bolson aquifers as part of the Texas Water Development Board’s Groundwater Availability Modeling Program. The aquifers, located along the Rio Grande, supply drinking water for Presidio, Texas, and Ojinaga, Mexico, as well as livestock and irrigation water. The model will be used as a tool for regional water planning and groundwater management in that area. The current model consists of four layers, three representing bolson deposits and younger alluvium, and one layer representing older units beneath the bolson. At the basin center, the bolson is up to 4000 ft (1200 m) thick; however, most wells are located in the upper 400 ft (120 m) of the bolson. The model grid straddles the Rio Grande and consists of 340 rows by 200 columns of quarter mile square cells with an active area of 1952 mi2 (5056 km2). In our conceptual model of the groundwater system, recharge enters through surrounding mountains and through stream channels, but not directly through the impervious valley floor. Groundwater moves from higher elevations through the bolson and into the Rio Grande Alluvium, where evapotranspiration exceeds recharge. Approximately 2000 acre-ft (2,500,000 m3) per year is pumped in the United States portion of the aquifer, and another 9000 acre-ft (11,000,000 m3) per year are permitted in Mexico.

Historical water levels in the study area change little through time. Therefore, our model will be calibrated to steady-state conditions. Preliminary calibration results suggest that recharge to stream beds and surrounding mountains is about one percent of the total flow budget and evapotranspiration accounts for about 6 percent. Approximately 36 percent of groundwater discharges to the Rio Grande and Rio Conchos, while river leakage to the aquifer is four percent. The greatest percentage of the budget is under flow entering from (95 percent) and exiting to (58 percent) outside the model area.