Modeling multiyear observations of soil moisture recharge in the semiarid American Southwest

The multiyear, root zone soil moisture redistribution characteristics in a semiarid rangeland in southeastern Arizona were evaluated to determine the magnitude and variability of deep-profile, wintertime soil moisture recharge. Intermittent observations from 1990 to 1998 of average volumetric soil moisture under shrub and grass cover showed that significant recharge beyond 0.30 m principally occurs only in the wintertime when the vegetation is senescent and does not use the infiltrating water. Using the physically based, variably saturated flow model HYDRUS, wintertime observations were modeled to determine the recharge of soil moisture at different depth intervals in the vadose zone. Two approaches were carried out to estimate the soil model parameters. The first was to use basic soils data from detailed profile descriptions in conjunction with pedotransfer functions. The second parameter estimation strategy was to use an automatic parameter search algorithm to find the optimal soil parameters that minimize the error between the model-computed volumetric water content and observations. Automatic calibration of the model was performed using the shuffled complex evolution algorithm (SCE-UA), and it proved possible to satisfactorily describe the vadose zone observations using a simplified description of the soil profile with optimal model parameters. Simulations with the optimized model indicate that significant recharge of vadose zone does occur well beyond 0.30 m in winter but that such recharge is highly variable from year to year and appears correlated with El Nino episodes. This water could serve as a source of plant water for deeper-rooted plants that are active during the subsequent spring season, thereby exploiting a niche that the more abundant, shallower-rooted plants that are active during the summer rainy season do not. However, the year-to-year variability of the winter precipitation and consequent deep soil moisture recharge indicates that the deeper-rooted vegetation in this region must retain the ability to obtain moisture from the near surface in order to meet its water demands if necessary.