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

Soil moisture couples ground, surface, and atmospheric water interactions via the processes of evapotranspiration, infiltration, and runoff generation (Grayson et al., 1997). Consequently, understanding the factors that influence the spatial distribution of soil moisture is vitally important to the accurate conceptualization and modeling of watershed processes. Typically, topographic indexing methods for the prediction of soil moisture have been studied in temperate or humid areas where the soil profile is often saturated and redistribution of soil moisture is driven by topography (Famiglietti et al., 1998; Grayson et al., 1997; Western et al., 1999). By contrast, in semi-arid environments, long periods of relatively dry conditions are punctuated by brief periods of saturation that result in lateral hillslope connectivity and runoff generation (McNamara et al., 2005). Given that lateral redistribution of soil moisture and subsequent runoff generation occur only briefly in semi-arid environments, the focus of hydrology in these watersheds should be on the mechanisms by which water inputs are retained in the watershed, rather than the mechanisms of lateral redistribution and runoff generation. The purpose of this study was to investigate the mechanisms by which a semi-arid watershed retains water, in the form of shallow soil moisture, at the hillslope scale. The following hypotheses were tested: 1) soil hydraulic properties that affect soil moisture retention vary with topography at the hillslope scale, and 2) soil moisture distribution trends at the hillslope scale are controlled by soil hydraulic properties. iv To test these hypotheses, a transect was laid out that traversed a set of opposing aspect (north and south facing) slopes in the Dry Creek Experimental Watershed. Soil moisture was monitored on 27 days during the spring dry down and summer, using time domain reflectometry at 35 sampling locations. Additionally, each sampling location was characterized for topographic attributes, soil physical properties, and soil hydraulic properties. The soil water retention curves of sampling locations were determined by removing soil cores and progressively drying them with an automated multistep outflow (AMSO) apparatus. The data obtained from the AMSO testing were then used in HYDRUS 1D to inversely estimate the van Genuchten parameters of the soil water retention curves of each sampling location. Correlations between sampling location attributes, and between soil moisture and sampling location attributes were determined. Results of laboratory analysis showed that north aspect sampling locations had 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 between 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. It is also concluded that these variations in soil hydraulic properties are the main drivers for the observed soil moisture patterns.