DETERMINING REGIONAL GROUND-WATER RECHARGE IN THE GREAT BASIN

Ground-water recharge in arid basins of the Great Basin varies spatially and temporally. This variability is caused by the varying distribution of net infiltration in the shallow subsurface and the lateral and vertical redistribution of water within the unsaturated zone. The major components affecting net infiltration in order of importance are the quantity and duration of precipitation (rain and snow accumulation and melt); infiltration rate and water storage capacity of the shallow subsurface material; evapotranspiration; and the bulk permeability of the subsurface material below the root zone. The major components influencing the lateral and vertical redistribution of water in the unsaturated zone are the amount of net infiltration; the thickness of the unsaturated zone; and the effective flow path porosity, which is controlled by faults, fractures, and the hydrologic properties and layering of sediments and bedrock in the unsaturated zone. Although temporal fluctuations in net infiltration diminish with depth, resultant recharge is expected to fluctuate on timescales of days to centuries making decadal-scale climate cycles significant to understanding recharge.

Simple water-balance calculations were used to analyze the spatial and temporal components that influence net infiltration, and subsequently ground-water recharge, in the Great Basin, and to evaluate the dominant components in each subbasin. The calculations are done on a monthly time step and account for rainfall, snow accumulation, snowmelt, soil water storage, potential evapotranspiration, runoff, and infiltration below the root zone. Calculations indicate that, in a typical basin, net infiltration occurs in less than 5 percent of the area and only when surface-water supply exceeds the storage capacity and potential evapotranspiration in the shallow subsurface over a fixed period of time. This method compares well with the recharge results determined by more physically detailed numerical models, and with other approaches used on the same subbasins. These calculations use 50 years of climate data that allows for the evaluation of decadal-scale climate cycles (El Niño/La Niña and the Pacific Decadal Oscillation) which will help determine the influence of future climate change on water availability for basin recharge.