Infiltration of water through thick unsaturated zones underlying intermittent streams in the western part of the Mojave Desert, southern California

Small amounts of recharge occur as infiltration of intermittent streamflow in washes in the western part of the Mojave Desert, near Victorville, California. These washes typically flow only a few days each year after large storms. To reach the water table, infiltrating water must pass through an unsaturated zone that is more than 130 meters (m) thick. Chloride, tritium, and stable isotope data from boreholes drilled in Oro Grande and Sheep Creek Washes indicate that infiltration of streamflow occurs to depths below the root zone, and presumably to the water table, along much of Oro Grande Wash and near the mountain front along Sheep Creek Wash. Infiltration does not occur to depths below the root zone along reaches of Sheep Creek Wash farther from the mountain front or at control sites away from the wash. Differences in infiltration at sites along each wash are the result of hydrologic variables such as proximity to the mountain front, quantity of streamflow and texture of the subsurface deposits. Differences in infiltration between the washes are the result of large-scale geomorphic processes. For example, Oro Grande wash is incised into the regional surface and infiltration has occurred at approximately the same location over recent geologic time. In contrast, Sheep Creek Wash overlies an active alluvial fan and the stream channel can wander across the fan surface.;An axial symmetric, 2-dimensional, unsaturated-flow model was developed using the computer program VS2D to test hypotheses about geologic and hydraulic controls on the movement of water through the unsaturated zone at a site along Oro Grande Wash. Heterogeneity within the unsaturated zone was estimated on the basis of transition probability/Markov chain 'realizations' of the subsurface geology developed using the computer program TPMOD. Hydraulic properties within the model grid were distributed on the basis of these realizations. Selected model scenarios were developed to test geologic and hydraulic controls on the movement of water. Model results show that infiltration from the wash is about 2 m/y and that thin, areally extensive clay layers in the subsurface having hydraulic conductivities of about 0.001 cm/h control downward movement and lateral spreading of water. Model results are consistent with rates of downward movement and lateral spreading away from the wash estimated on the basis of tritium data collected at the study site.