Factors Influencing Soil Moisture at the Hillslope Scale in a Semi-Arid Mountainous Environment.

Semi-arid, snowy regions are important because they represent some of the fastest growing population centers in the United States, and rapid land-use changes are occurring as development moves upland from urbanized valleys. A better understanding of the hydrology of these systems is becoming increasingly important for water resource planning and management. Previous studies have shown the correlation between soil moisture and static and dynamic variables, and have described the spatial variability of soil hydraulic properties at the hillslope scale, but have stopped short of correlating soil moisture directly to variations in soil hydraulic properties. We present preliminary results of a field study in which we investigate the correlation between soil moisture and soil hydraulic properties at the hillslope scale. The site includes a ~700 m transect in the Dry Creek Experimental Watershed (near Boise Idaho) with north- and south- facing slopes. A variety of topographic variables and soil moisture and tension will be measured at a spacing of 20 m along the transect during the dry down season in 2009. In addition, mini disc infiltrometer tests and laboratory multistep outflow tests on undisturbed cores will be used to determine unsaturated soil hydraulic properties along the transect. We hypothesize that the influence of the static and dynamic variables on soil moisture varies with time: topographic variables control soil moisture distribution during wet up, while soil hydraulic properties control soil moisture during the dry down period in the watershed. As climate change may result in the dry down period coming earlier each year, identifying the factors controlling soil moisture dynamics during and after the dry down period will be important in developing models that accurately predict changes in water supply patterns in semi-arid regions.