The Influence of Soil Hydraulic Properties On Soil Moisture in a Semi-Arid Mountainous Environment.

Semi-arid, snowy regions are important because they represent some of the fastest growing population centers in the country. These lands are experiencing rapid land-use changes as urban development moves upland from the urbanized valleys. Understanding the mechanisms by which watersheds in these areas process water inputs is key to accurate hydrologic modeling for estimation of ground water recharge, streamflow, and other quantities. Here we investigate the mechanisms by which a semi-arid watershed retains water, in the form of shallow soil moisture, at the hillslope scale. Soil moisture was monitored on 27 days during the spring dry-down and summer, using time domain reflectometry at 35 sampling locations along a 650-m transect covering north- and south-facing slopes in the Dry Creek Experimental Watershed (DCEW) near Boise, Idaho. Each sampling location was characterized for topographic attributes, soil physical properties, and soil hydraulic properties. The soil water retention curves of samples from these locations were determined using multistep outflow testing and parameter estimation. Correlations between sampling location attributes, and between soil moisture and sampling location attributes were determined. North-aspect sampling locations have higher levels of organic carbon, lower percentages of sand-sized particles, and higher percentages of silt- and clay-sized particles. These differences in organic carbon and texture are correlated to variations in soil water retention at the sampling locations. Observed soil moistures were well correlated to soil physical and hydraulic properties across a wide range of soil moisture conditions. The hydraulic properties of DCEW soils show substantial variation with topography, and in particular, aspect. Our results suggest that the soil hydraulic properties are the primary drivers of the observed soil moisture patterns.