Groundwater Modeling of the Southern High Plains Aquifer - Effects of Pre- and Post-Development Recharge on Water Availability

The Southern High Plains (SHP) aquifer underlies approximately 30,000 square miles of the High Plains of western Texas and eastern New Mexico. The availability of groundwater is critical to the economy of this region, as 95 percent of groundwater pumped is for irrigated agriculture. As part of the water planning process adopted by the State of Texas, a new groundwater flow model was developed for the SHP aquifer. The purpose of the model is to determine the adequacy of existing groundwater supplies to meet projected 50-year demands.

The model incorporates a number of recently developed conceptual models of recharge and discharge on the SHP, identified through model calibration and confirmed by recently completed field studies. Based on a hydraulic conductivity field developed through the correlation of hydraulic conductivity to depositional systems, pre-development regional recharge rates are generally less than those used in previous studies by a factor of about 2 – 10, primarily because of lower average hydraulic conductivity used in the model. Greater recharge rates are identified where soils are less permeable and likely caused more runoff to playas relative to regions that consist of more permeable soils. Springs and seeps along major draws served as important points of discharge that were a significant control on the groundwater flow system under predevelopment conditions.

Under post-development conditions, recharge has increased substantially due to changes in land use. Key model inputs that have changed with time and may change in the future include the interdependency between aquifer recharge and land use, and the amount, timing and physical mechanisms of return flow from irrigated fields. Locally, other sources of enhanced recharge are important, such as infiltration of storm water runoff within urban areas. Studies are ongoing regarding key hydrologic processes that have changed with time which affect groundwater availability estimates.