Estimates of Effective Recharge in the Amargosa Desert Using a Simple Elevation-Dependent Chloride Mass-Balance Method and a Minimum Effective Recharge Elevation

Abstract:

Recharge estimates in desert environments are complicated by several factors. Deserts have low and infrequent rainfall that frequently occur in the form of sudden thunderstorms. For the most part, precipitation on desert valleys evaporates without much, if any, infiltration due to high evaporation rates, and precipitation on mountain tops quickly turns into runoff that has little time to infiltrate where it originated. Furthermore, water flow in washes is usually sizable and rapid but short lived and ephemeral, thus difficult to gage. Here we present recharge calculations for the Amargosa Desert, NV, using a simple elevation‑dependent chloride mass-balance method modified by estimating a Minimum Effective Recharge Elevation (MERE). For each defined watershed, an average groundwater chloride concentration is used and the chloride deposition rate is assessed in the subject watersheds over all land elevations above the estimated MERE by assuming two constant spatial and temporal concentrations of chloride in precipitation and that precipitation is a function of elevation. The first chloride concentration used corresponds to contemporary values of the region and the second concentrations corresponds to an upper bound as an attempt to correct for either greater past chloride loading or a higher past precipitation with chloride concentration remaining constant. The MERE for the Amargosa desert was appraised as 1,200 m AMSL, based upon the elevation of the orifice of Cane Spring (1,237 m AMSL), one of the lowest‑elevation permanent spring on the Nevada National Security Site. This minimum elevation is the cutoff altitude for our calculations at which precipitation is considered to make a significant contribution to recharge. Elevation-dependent precipitation was evaluated relating elevation data from the online mapping software ACME Labs Mapper 2.1 and precipitation data from the PRISM Climate Group’s PRISM model. The Parameter-elevation Regressions on Independent Slopes Model (PRISM) developed by the Oregon State University is a knowledge-based system that uses point measurements of precipitation, temperature, and other climatic factors to produce continuous, digital grid estimates of climatic parameters. Finally, we compare our calculated estimates to those from the State Engineer.