QUANTIFYING GROUNDWATER RECHARGE IN AN ARID ENVIRONMENT: PILOT VALLEY, UT-NV

Mountain block and mountain front recharge was quantified in Pilot Valley, a closed basin on the Utah-Nevada border. The Pilot Valley playa, a mud-salt pan remnant of Lake Bonneville, is bounded by the 10,000 ft Pilot Range to the west and the 7,000 ft Silver Island Mountains to the east. We have found that groundwater recharge to the playa comes by underflow from the lower and dryer Silver Island Mountains, not from the higher and wetter Pilot Range. Groundwater from the Pilot Range discharges in a series of springs and wet zones from the down-gradient end of the alluvial fan at the playa margin.

Several lines of shallow wells were placed at the playa-alluvial fan interface at the outlet of three sub-basins in the Silver Island Mountains, and a network of rain gauges was installed to estimate daily precipitation. Monthly water level measurements taken since September 2005 show the seasonal change in groundwater levels, including the flux of water during the spring and summer months. Rain hydrographs correspond closely to well hydrographs, with less than a month lag between large precipitation events and water level rise. Following a modified version of the water table fluctuation method, preliminary estimates on the Silver Island Mountains show that <1% of annual precipitation recharges the groundwater system, with a majority of the recharge occurring in the mountain front on the alluvial fans. Specific yield and transmissivity values at the playa-alluvial fan interface were derived from slug test data.

Water levels were also measured at monthly intervals in over 50 wells at the playa-alluvial fan interface at the base of the Pilot Range. The lineament of springs and wet zones at the playa margin were mapped, and a network of rain and temperature gauges was installed on the mountain front. A conceptual water budget was constructed as follows:

% Recharge = [Outputs (evapotranspiration from springs and playa margin) / Inputs (Precipitation)] * 100

Evaporation at the playa margin was estimated by the change in water levels in the wells, and evapotranspiration from the springs and wet zones was estimated based on plant type and seasonal temperatures. Preliminary estimates show very small recharge rates, similar to those on the Silver Island Range.