REGIONAL GROUNDWATER FLOW MODELING OF THE SOUTHERN HIGH PLAINS AQUIFER - CONCEPTUAL MODELS APPLIED AND INSIGHTS GAINED

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 water is critical to the economy of this region, as 95 percent of groundwater pumped is used for irrigated agriculture. The first comprehensive groundwater models of the SHP aquifer were published during the mid-1980s. Additional regional modeling efforts since that time have consisted primarily of updates to previous models.

As part of a state-wide water resources planning process adopted by the State of Texas, a new groundwater model is being developed for the SHP aquifer. The model consists of predevelopment and transient calibration periods. The predevelopment model calibration is complete and the transient calibration is in progress.

Calibration of the predevelopment model provided new insights into the hydrogeology of the SHP aquifer, and led to the application of new conceptual models of recharge and discharge not identified in previous modeling studies. Regional recharge rates estimated through model calibration are less than those used in previous studies, primarily because of the lower average hydraulic conductivity used in the model determined using numerous well logs and specific capacity tests. In addition, higher recharge rates are applied in the northern portion of the study area, where soils are less permeable and are therefore likely to cause more runoff to playas, through which most of the recharge to the SHP aquifer occurs. This conceptual model of recharge is consistent with that proposed by previous authors (e.g., Wood and Sanford [1995]) who have evaluated recharge on the SHP, but is different from that used in previous regional models.

Springs and seeps along the draws, and to a lesser extent salt lakes within the SHP, appear to have served as major discharge controls on the regional groundwater flow system under predevelopment conditions. Whereas previous modeling studies emphasized groundwater discharge along the eastern caprock escarpment aquifer boundary, in the current model interior seeps and springs account for nearly half of the total groundwater discharge from the aquifer.