Multi-Scale Variability of Soil and Hydraulic Properties and Impacts to Arid Water Balance.

The Mojave Global Change Facility (MGCF), located in the Mojave Desert in Nevada, extends for several hundred meters across a north-facing alluvial fan complex. The specific objectives are to evaluate the spatial structure (correlation and variability) present in soil texture, bulk density, and soil hydraulic properties; determine the impact of soil texture on depth of wetting at the meso-scale; and, determine whether a relationship exists between plant response and texturally induced differences in the water balance. Soil properties were obtained at two scales. First, soil was collected at two depths (0 to 5 cm and 5 to 10 cm) at each of 96 test plots, analyzed for texture and bulk density, and used to obtain soil hydraulic (van Genuchten) properties using the pedotransfer function method. Second, hydraulic properties were obtained at ~20cm increments from undercanopy to interspace microsites using a tension infiltrometer. Data were examined for spatial structure using semi-variograms. We then simulated soil water balance at both scales using a Richards-equation based computer model, field collected (eddy covariance) evapotranspiration estimates for a one-year period, and shrub coverage at the site. The results show a distinct fining of texture at both depths in the northeast quadrant of the site (p<0.05). The textural patterns led increasing saturated water content and increasing air entry pressure from the southwest to the northeast of the facility. Subshrub-scale variability in hydraulic properties was also significant, showing that soil structure does exert an important influence on soil properties, and the overall water balance at small scale.