Hydrologic Characteriza on of Desert Soils with Varying Degrees of Pedogenesis: 1. Field Experiments Evalua ng Plant-Relevant Soil Water Behavior

480 D use diff erent strategies to survive and fl ourish with erratically available water (Hamerlynck et al., 2002; Lombardini, 2006). Th e persistence of soil water within a particular depth range and its characteristic time scales of fl uctuation (Katul et al., 2007) help determine the resulting plant community. Th e depth to which infi ltration percolates and the degree to which water is retained at particular depths aff ect the relative advantage of shallow-rooted grasses compared with deep-rooted shrubs. Th e deeper unsaturated zone below the upper soil layers, including bedrock, can supply much of the water needed by many plants (Sternberg et al., 1996). Hamerlynck et al. (2002), investigating the contrasting water-use strategies of two codominant desert shrubs, found strong correlations between soil characteristics and the two species’ densities and health. Th ese factors are of heightened importance in arid climates, where much time may elapse between signifi cant infi ltrations. Directly, as well as indirectly through its eff ect on plants, soil water aff ects the habitat of animals, which in turn infl uence the patterns of soil water behavior through activities such as burrowing. Among the soil attributes aff ecting the ability of diff erent parcels of soil to transmit, concentrate, distribute, and store water are heterogeneity and anisotropy. In dry regions, the lateral fl ow and distribution of water is important at small spatial scales (Grayson et al., 2006). Shafer et al. (2007) and others have observed substantial systematic diff erences in soil hydraulic characteristics between intercanopy and undercanopy areas. Grayson et al. (1997) noted a dominance of vertical over horizontal fl ow except under the wettest conditions; areal variation in soil properties is likely to substantially infl uence the distribution of water during periods of high soil moisture following rain or fl ood. Both heterogeneity and anisotropy, although frequently neglected or oversimplifi ed in models of unsaturated-zone hydraulics, play an important role in structuring the ecosystem. Soil developmental processes are critical to the characteristic hydraulic behavior that infl uences a soil’s ability to support plant species and ecosystems. In desert soils, pedogenic development alters such attributes as clay and silt content, stoniness, soil structural type, degree of heterogeneity, and prevalence of macropores, which strongly infl uence the infi ltration, redistribution, and retention of water. At the land surface, biotic crusts and desert pavement vary in consistent ways with the age of the deposit and are important infl uences on runoff , infi ltration, and evaporation Hydrologic Characteriza on of Desert Soils with Varying Degrees of Pedogenesis: 1. Field Experiments Evalua ng Plant-Relevant Soil Water Behavior