Groundwater Flow Model of the Presidio and Redford Bolsons Aquifers: Preliminary Results

We have developed a groundwater flow model for the Presidio and Redford Bolsons 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 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 4,000 feet thick; however, most wells are located in the upper 400 feet 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 1,952 square miles. 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 2,000 acre-feet per year is pumped in the United States portion of the aquifer, and another 9,000 acre-feet 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 Bravo, while river leakage to the aquifer is 4 percent. The greatest percentage of the budget is underflow entering from (95 percent) and exiting to (58 percent) outside the model area.