ASSESSING GROUND-WATER RECHARGE IN THE ARID AND SEMI-ARID SOUTHWESTERN UNITED STATES

The quantification of ground-water recharge in arid settings is inherently difficult due to the generally low amount of recharge, its spatially and temporally spotty nature, and the absence of techniques for directly measuring fluxes entering the saturated zone from the unsaturated zone. Deep water tables in arid alluvial basins correspond to thick unsaturated zones that produce up to millennial time lags between changes in hydrologic conditions at the land surface and subsequent changes in recharge to underlying ground water. Recent advances in physical, chemical, isotopic, and modeling techniques have fostered new types of recharge assessments. Chemical and isotopic techniques include an increasing variety of environmental tracers that are useful and robust. Physically based techniques include the use of heat as a tracer and computationally intensive geophysical tools for characterizing hydrologic conditions in the unsaturated zone. Modeling-based techniques include high-resolution geospatially distributed water-budget calculations using geographic information systems (GIS). Application of these techniques to arid and semi-arid settings in the southwestern United States reveals distinct patterns of recharge corresponding to tectono-geomorphic setting, climatic and vegetative history, and land use. Analysis of recharge patterns shows that large expanses of alluvial basin floors are drying out under current climatic conditions, with little to no recharge to underlying ground water. Ground-water recharge occurs mainly beneath (1) upland catchments in which thin soils overlie permeable bedrock, (2) ephemeral channels in which flow may average only several hours per year, and (3) active agricultural areas.