TECHNIQUES FOR ESTIMATING RECHARGE IN ARID BASIN AQUIFERS: A CASE STUDY FROM TRANS-PECOS, TX

A vital aspect in determining the sustainable yield of groundwater resources is to have reasonable estimates for groundwater recharge. In arid regions like Trans-Pecos, West Texas, methods of estimating recharge (e.g. the 1% rule, Maxey-Eakin method, etc.) overlook the contribution of direct recharge on the basin floors so that this component of recharge is considered negligible in the total water budget. However, this recharge may have significant effects on the overall water budget estimate for the basins and on basin water quality. Observed trends in groundwater nitrate concentrations and the presence of young (<70 year old) water indicate that recharge does occur through the basin floors and can be significant. We use a different type of model (INFIL 3.0.1) to estimate the volume and spatial distribution of recharge within the Trans-Pecos basin aquifers. The INFIL model improves upon the current estimates in that it considers site-specific data (e.g. lithology, vegetation, and runoff) rather than using estimates based primarily on elevation, mean annual precipitation, and general evapotranspiration. Using the INFIL model we estimate that in two basins, Red Light Draw and Eagle Flats, up to 15% of potential recharge is occurring as widespread recharge on the basin floors. Preliminary results indicate that similar amounts of recharge occur through the basin floors of Wild Horse and Michigan Flats and Lobo and Ryan Flats. These improved estimates of the volume and spatial distribution of recharge are integral to understanding the hydrology and long-term sustainability of arid basin aquifers.