Focused ground-water recharge in the Amargosa Desert basin: Chapter E in Ground-water recharge in the arid and semiarid southwestern United States (Professional Paper 1703)

The Amargosa River is an approximately 300-kilometer-long regional drainage connecting the northern highlands on the Nevada Test Site in Nye County, Nev., to the floor of Death Valley in Inyo County, Calif. Streamflow analysis indicates that the Amargosa Desert portion of the river is dry more than 98 percent of the time. Infiltration losses during ephemeral flows of the Amargosa River and Fortymile Wash provide the main sources of ground-water recharge on the desert-basin floor. The primary use of ground water is for irrigated agriculture. The current study examined ground-water recharge from ephemeral flows in the Amargosa River by using streamflow data and environmental tracers. The USGS streamflow-gaging station at Beatty, Nev., provided highfrequency data on base flow and storm runoff entering the basin during water years 1998–2001. Discharge into the basin during the four-year period totaled 3.03 million cubic meters, three quarters of which was base flow. Streambed temperature anomalies indicated the distribution of ephemeral flows and infiltration losses within the basin. Major storms that produced regional flow during the four-year period occurred in February 1998, during a strong El Niño that more than doubled annual precipitation, and in July 1999. The study also quantified recharge beneath undisturbed native vegetation and irrigation return flow beneath irrigated fields. Vertical profiles of water potential and environmental tracers in the unsaturated zone provided estimates of recharge beneath the river channel (0.04–0.09 meter per year) and irrigated fields (0.1–0.5 meter per year). Chloride mass-balance estimates indicate that 12–15 percent of channel infiltration becomes ground-water recharge, together with 9–22 percent of infiltrated irrigation. Profiles of potential and chloride beneath the dominant desert-shrub vegetation suggest that ground-water recharge has been negligible throughout most of the basin since at least the early Holocene. Surface-based electrical-resistivity imaging provided areal extension of borehole information from sampled profiles. These images indicate narrowly focused recharge beneath the Amargosa River channel, flanked by large tracts of rechargefree basin floor. 1 Present address Berkeley, California (amystew@gmail.com). Introduction and Setting The Amargosa Desert Basin contains a large quantity of high quality ground water (Walker and Eakin, 1963; Claassen, 1985; Kilroy, 1991). Beneficial uses of ground water include sustenance of wildlife at discharge points within Ash Meadows National Wildlife Refuge and support of irrigated agriculture within the Amargosa Farms area (fig. 1). Recharge of ground water is a relatively uncharacterized component of the hydrologic budget. The basin is part of the larger Death Valley regional ground-water flow system. While numerous studies have examined the Death Valley system at a regional scale (Winograd and Thordarson, 1975; Harrill and others, 1988; D’Agnese and others, 1999; Tiedeman and others, 2004), the sparsely populated Amargosa Desert Basin—especially its upper reaches—has received relatively little study. Considerable uncertainties remain in subsurface residence times and fluxes under current climatic and land-use conditions. Appraisal of ground-water resources and management of competing demands from developing land-use patterns requires an improved understanding of ground-water recharge in this desert environment. Population, Land Use, and Water Withdrawals The Amargosa Desert Basin is sparsely populated and relatively undeveloped. The average population density is less than one person per square kilometer. Primary land uses are farming and recreation. The town of Beatty straddles the Amargosa River just upstream of its entry into the basin (fig. 1). The estimated population of Beatty in 2000 was 1,085 (U.S. Census Bureau, 2003). The Amargosa Farms area spreads across the outlet of the basin. The census division that includes the farms area, Amargosa Valley, had an estimated population in 2000 of 1,142. In view of rapid population growth in nearby areas, the Amargosa Desert Basin has substantial potential for growth. Population estimates at the county level extend back to 1870. The early population of the county jumped roughly six-fold after prospectors found gold near Beatty in 1904, and rose again in the 1950s and 1960s during expansion of irrigated agriculture and defense activities at the Nevada Test Site (fig. 2). Even so, total county population remained below 10,000 before 1981. 108 Focused Ground-Water Recharge in the Amargosa Desert Basin 0 5 10 15 Kilometers 20 116°45' 116°30' 116°15' 116°0' 36°30' 36°45' 37°0' 37°15'