IMPACT OF LAND-USE CHANGE ON GROUNDWATER RECHARGE IN THE SOUTHWESTERN UNITED STATES

Recharge is a critical component of the water cycle for assessing sustainable water resources. Most recharge studies have focused on natural ecosystems; however, humans have exerted large-scale changes on terrestrial ecosystems in the last century, primarily through agricultural activities. The purpose of this study was to evaluate the effect of land use and land-use change on groundwater recharge using data from natural ecosystems, nonirrigated cultivated (dryland) ecosystems, and irrigated cultivated ecosystems in the Amargosa Desert, Nevada and the Southern High Plains, Texas. Matric-potential and environmental-tracer data from the vadose zone were used as archives of past changes in recharge. The results of the study indicate that recharge is highly correlated with land use: negligible recharge beneath natural ecosystems, moderate recharge beneath dryland ecosystems, and augmented but variable recharge beneath irrigated ecosystems. Low matric potentials, upward potential gradients, and accumulations of chloride and nitrate indicate little or no recharge beneath large areas of native vegetation. High matric potentials, low chloride and nitrate concentrations, and rising groundwater tables indicate induced recharge beneath areas of dryland agriculture. High matric potentials and low to moderate chloride and nitrate concentrations indicate substantially augmented recharge beneath areas of irrigated agriculture according to type and amount of irrigation. Conversion of native vegetation to irrigated agriculture is accompanied by increases in matric potential and downward displacement of accumulated chloride and nitrate. Induced recharge can flush accumulated chloride and nitrate to underlying aquifers, impacting water quality. The varied approaches of this study, which analyzed water-table fluctuations, matric-potential profiles, and environmental and applied tracers, complement each other in providing time-integrated and time-resolved information on the linkages between land use and groundwater recharge. Understanding these linkages is critical for optimal management of groundwater quantity and quality in the southwestern United States.