CHARACTERIZING RECHARGE IN BASINS OF THE DESERT SOUTHWEST

Recharge is a critical component necessary for the understanding of the regional hydrogeologic framework of basins in the desert southwest. Characterizing recharge processes is difficult because the mechanisms controlling recharge in arid and semi-arid basins vary spatially and temporally. As a result, measurements may miss the locations of recharge or may cover a time span where little or no recharge is occurring. Near-surface processes and properties control net infiltration, which is the amount, timing, and spatial distribution of water that eventually becomes recharge. Subsurface processes and properties control percolation, which is the lateral and vertical redistribution of water, and includes the travel time through the unsaturated zone. The major components controlling net infiltration are precipitation as rain or snow; infiltration and water storage capacity of the overlying soil; potential and actual evapotranspiration; and bedrock permeability. The major components controlling percolation 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 of geologic strata in the unsaturated zone. Although temporal fluctuations in net infiltration diminish with depth, resultant recharge is expected to vary on timescales of days to centuries. This makes decadal-scale climate cycles significant for the understanding of recharge. A simple conceptual and algebraic model was developed on the basis of the surface and subsurface processes to analyze the spatial and temporal components controlling net infiltration in more than 100 basins in the southwest on a monthly time step. Results indicate that net infiltration occurs in less than 5 percent of the area within a typical basin and only when the surface-water supply exceeds the storage capacity of the root zone plus the evapotranspiration over a fixed period of time. The simple model that accounts for the major mechanisms compares well with other detailed approaches used on the same basins. The model allows us to evaluate the role of decadal-scale climate cycles (El Niño/La Niña and the Pacific Decadal Oscillation) and will be used to look at the influence of future climate change on potential recharge in the desert southwest.