T. O. Keefer

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.

Runoff simulation sensitivity to remotely sensed initial soil water content

A variety of aircraft remotely sensed and conventional ground-based measurements of volumetric soil water content (SW) were made over two subwatersheds (4.4 and 631 ha) of the U.S. Department of Agricultures Agricultural Research Service Walnut Gulch experimental watershed during the 1990 monsoon season. Spatially distributed soil water contents estimated remotely from the NASA push broom microwave radiometer (PBMR), an Institute of Radioengineering and Electronics (IRE) multifrequency radiometer, and three ground-based point methods were used to define prestorm initial SW for a distributed rainfall-runoff model (KINEROS; Woolhiser et al., 1990) at a small catchment scale (4.4 ha). At a medium catchment scale (631 ha or 6.31 km2) spatially distributed PBMR SW data were aggregated via stream order reduction. The impacts of the various spatial averages of SW on runoff simulations are discussed and are compared to runoff simulations using SW estimates derived from a simple daily water balance model. It was found that at the small catchment scale the SW data obtained from any of the measurement methods could be used to obtain reasonable runoff predictions. At the medium catchment scale, a basin-wide remotely sensed average of initial water content was sufficient for runoff simulations. This has important implications for the possible use of satellite-based microwave soil moisture data to define prestorm SW because the low spatial resolutions of such sensors may not seriously impact runoff simulations under the conditions examined. However, at both the small and medium basin scale, adequate resources must be devoted to proper definition of the input rainfall to achieve reasonable runoff simulations.

Southern oscillation effects on daily precipitation in the southwestern United States

The effect of the Southern Oscillation on daily precipitation in the southwestern United States is examined by using the Southern Oscillation Index (SOI) to perturb parameters of a stochastic daily precipitation model. Daily precipitation is modeled with a Markov chain-mixed exponential model and seasonal variability of model parameters is described by Fourier series. The hypothesized linkage between the SOI and the model parameters is of the form Gi(N, t) = Gi(t) + biS(N, t − τi) where Gi(N, t) is the perturbed parameter i for day t of year N, Gi(t) is the annually periodic parameter i for day t, bi is a coefficient, S is the SOI, and τi is a lag in days. Daily precipitation data for 27 stations in California, Nevada, Arizona, and New Mexico were analyzed. Perturbations of the logits of the dry-dry transition probabilities resulted in statistically significant improvements in the log likelihood function for 23 stations and perturbations of the mean daily rainfall resulted in significant increases for 18 stations. The most common lag identified was 90 days, suggesting the possibility of conditional simulations of daily precipitation. Seasonal effects were detected, confirming the results of previous analysis with groups of stations.

Long‐term meteorological and soil hydrology database, Walnut Gulch Experimental Watershed, Arizona, United States

A 17 year (1990-2006) meteorological and soil hydrology database has been developed for the Walnut Gulch Experimental Watershed in southeastern Arizona. Data have been acquired at three automated weather stations, 5 soil profile trench sites, and 19 locations dispersed across the watershed colocated with recording rain gauges. Meteorological elements measured at the weather stations include air temperature, relative humidity, wind speed, wind direction, barometric pressure, solar radiation, photosynthetically active radiation, and net radiation. Soil hydrology properties measured at the weather stations, trench sites, and rain gauges include soil moisture, soil temperature, soil heat flux, and soil surface temperature. Data are available at http://www.tucson.ars.ag.gov/dap.

Quantifying Extreme Rainfall Events and Their Hydrologic Response in Southeastern Arizona

AbstractHydrologists are concerned with high-intensity rainfall and peak runoff rates for stormwater infrastructure designs, post-event assessments, and mitigation of environmental impacts. In the southwestern United States the need for accurate information about these rates is increasingly important as population growth and associated development are projected to exceed national averages. Design storm totals for various durations and return period frequencies are routinely derived from the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 and are commonly used as input to hydrologic models to estimate peak runoff rates and runoff volumes. For the southwestern United States during the North American Monsoon, NOAA relies on sparse rain gauge networks to measure rainfall from limited area convective storms primarily at daily time steps and estimates of subdaily event intensities are derived by temporal downscaling from a few point locations. The USDA, Agricultural Research Service, Southwest W...