A Basin-Scale Model of Pahrump Valley, Nevada for Water Resources Management

Pahrump Valley, located approximately 70 kilometers west of Las Vegas, is the most heavy allocated groundwater basin in Nevada. The inhabitants of the basin, including the town of Pahrump (population ~38,000), rely on the underlying basin fill and carbonate aquifers as the sole source of water for irrigation, commercial, public and domestic uses. Current well inventories include approximately 10,000 domestic and 300 irrigation wells. The sustainable basin yield for Pahrump Valley – the amount of groundwater that can be extracted annually without depleting the aquifer – has been estimated at 19,000 ac-ft/yr. Pumpage inventories indicate that the sustainable basin yield has been continuously exceeded since the early 1950s, with annual pumpage rates as high as 48,000 ac-ft/yr.

A three-dimensional, basin-scale groundwater model has been constructed to provide a framework to assess the water resources of Pahrump Valley and to evaluate alternative water management scenarios. The model incorporates the geological framework model developed by the USGS for the Death Valley Regional Flow System model. Recharge to the valley floor aquifers is computed according to empirical methods that relate mean annual precipitation within high-elevation mountain block watersheds to net infiltration. Evapotranspiration in the valley floor is determined through the delineation of phreatophytes from high resolution vegetation maps and representative plant water use coefficients. Transient model calibration is performed using the inverse parameter estimation program PEST given extensive pumpage inventories from 1913 to 2003 and water levels from 89 observation wells. Model results including calibrated hydraulic parameters, an updated sustainable basin yield estimate and impacts of future pumpage will be presented.