EFFECTS OF TIME-VARYING SOIL PROPERTIES AND CANOPY STRUCTURE ON PALEORECHARGE IN THE MOJAVE DESERT

In arid regions, high evapotranspiration and low precipitation rates make soil moisture a very important variable to the sustainable development of desert ecosystems, as well as subsurface contaminant transport. We combined a long-term (18,000 year) climatological record and time-varying soil hydraulic properties to evaluate the role that soil evolution might play in paleorecharge in arid settings. The HYDRUS-1D model, which solves Richards Equation for variably saturated flow, was used to simulate paleorecharge. Results were compared to soil water potential and chloride profiles from surrogate field data collected at the Nevada Test Site. Different saturated hydraulic conductivities and α values were used for five different time periods (0.05, 0.5, 4, 10, and 100 thousand years ago (ka)) in the simulations. The values were taken from research conducted at the Mojave National Preserve, CA. Because seasonal vegetation plays a significant role in the desert hydrologic cycle, daily estimates of evapotranspiration (ET) from monthly averages were calculated for a Quaternary Period, Mojave Desert-type canopy that included evergreens, drought deciduous, annuals and grasses. Ground cover percentage and leaf area index (LAI) were used as bases for the partitioning of ET into soil evaporation and plant transpiration, throughout the active seasons for each growth form. Different root distributions were also used to simulate the different root water uptake of each plant type. Observed data from 1998-2000 at the Amargosa Desert Research Site (ADRS), NV were used to calibrate the model. The results show that incorporating vegetation parameters and time-varying soil hydraulic properties could further improve upon predictions of deep flux during various past periods.