Spatial and Seasonal Dependence of Vegetation Related Soil Hydrologic Properties in a Pinyon-Juniper Ecosystem

The influence of vegetation on infiltration capacity is one of several uncertainties associated with the ecohydrologic effects of woody plant encroachment into arid and semi arid land systems. Our objective was to determine how vegetation cover controls the spatial distribution of soil hydraulic properties in a pinyon juniper woodland. Hundreds of tension infiltration measurements were taken to measure fine scale spatial variability and seasonal changes in soil hydraulic properties within the P-J ecosystem, on the Colorado Plateau. Results from our studies show significant differences in hydrologic characteristics between canopy and intercanopy regions. Hydraulic conductivity under the canopy was found to be strongly influenced by water repellency. As a result the commonly accepted maxim that infiltration and hydraulic conductivity are greater under the vegetation may not be true in a P-J woodland. Biological soil crust appeared to not limit infiltration. Hydraulic conductivity was also found to be temporally variable, especially for bare soil. During the autumn, precipitation in the form of high intensity monsoonal thunderstorms appeared to have formed a physical crust on bare soil and consequently decreased infiltration. Conversely, the hydraulic conductivity of soil under the vegetation canopy and to a lesser extent soil covered with biological soil crust was relatively stable throughout the season