Natural recharge to the High Plains aquifer in Dawson County, Texas

The groundwater table has been generally rising in the southern part of the Texas High Plains aquifer since agricultural development started in the early twentieth century. This is in sharp contrast to the general decline in water levels in other parts of the aquifer. Water level rise in any aquifer system is an indication of recharge increase and/or discharge decrease. Groundwater in the study area has not been heavily pumped. Therefore, a decrease in pumpage would not have caused the observed rise of water levels. In this study, the effects of changes in agricultural practices, land use, and rainfall on natural recharge to the High Plains aquifer were investigated. Agricultural practices and land use have changed due to agricultural development in the area. In addition, the average annual rainfall increased between 1951 and 1996.; A calibrated groundwater flow model (MODFLOW) showed that the High Plains aquifer in northern Dawson County, Texas received 21 mm/yr of natural recharge prior to agricultural development. Water budget calculations and simulations with the calibrated groundwater flow model showed that net natural recharge to the aquifer system increased to a range of 32 mm/yr to 76 mm/yr following agricultural development (1951--1996). Because the study area lacks a well-developed surface drainage system, and contains numerous playa lakes, natural recharge is believed to be focused through the playa lakes. The calibrated model showed that approximately 35% of the runoff collected in playa lakes ultimately became natural recharge to the aquifer. Variations in natural recharge follow closely those of annual precipitation, and consequently, runoff. The fraction of runoff into playas that is recharge was nearly constant for all years considered following agricultural development. The recharge-to-runoff ratio ranged between 0.33 and 0.43.; The effect of agricultural development in the area on runoff was determined by running the calibrated model with pre-1950 land use and 1951--1996 monthly precipitation. Results of these simulations showed that average annual runoff increased from 33 mm/yr to 170 mm/yr as a result of agricultural development. Therefore, the increase in groundwater levels was caused by a combination of agricultural development and above-normal precipitation during 1951--1996.