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 recharg e. Intermittent observations from 1990 to 1998 of average volumetric soil m oisture under shrub and grass cover showed that significant recharge beyond 0.30 m principally occurs only in the wintertime when the vegetation is se nescent and does not use the infiltrating water. Using the physically based , variably saturated flow model HYDRUS, wintertime observations were modele d to determine the recharge of soil moisture at different depth intervals i n the vadose zone. Two approaches were carried out to estimate the soil mod el parameters. The first was to use basic soils data from detailed profile descriptions in conjunction with pedotransfer functions. The second paramet er estimation strategy was to use an automatic parameter search algorithm t o find the optimal soil parameters that minimize the error between the mode l-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 optim al model parameters. Simulations with the optimized model indicate that sig nificant recharge of vadose zone does occur well beyond 0.30 m in winter bu t that such recharge is highly variable from year to year and appears corre lated with El Nino episodes. This water could serve as a source of plant wa ter for deeper-rooted plants that are active during the subsequent spring s eason, 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 so il moisture recharge indicates that the deeper-rooted vegetation in this re gion must retain the ability to obtain moisture from the near surface in or der to meet its water demands if necessary.