Soil moisture patterns and streamflow source areas in semi-arid catchment

Hydrologists are challenged with partitioning processes controlling hillslope hydrology and stream responses over various scales. Several topographic and wetness indices have been proposed for use in distributed hydrologic models. This study both investigated hillslope flowpaths and tested available terrain and wetness based indices for modeling soil moisture patterns and hillslope hydrologic processes in semi-arid headwater catchments. Soil moisture patterns were characterized from soil moisture content measurements from the near-surface and simulated (SHAW model) soil moisture contents at depth. Weekly soil moisture maps for the 2005 water year show a saturated subsurface source area near the channel head early in the fall wet-up period and apparently maintained base flow for the ephemeral stream. Statistical analyses of the controls on soil moisture concurred with the development and later reduction of the saturated subsurface source area and respective subsequent initiation and cessation of streamflow at the site. Seasonal soil moisture patterns were statistically compared with site physical and biological characteristics to assess the temporal and spatial variability of controls on soil moisture. Results suggest that controls on soil moisture patterns and hillslope processes at the site vary with hydrologic regime and are different from hydrologic controls commonly observed under humid, steady state conditions. Modified topographic wetness indices were created to characterize the observed hydrologic controls. Observed soil moisture patterns were more positively correlated with modified indices than indices commonly reported in literature. Additionally, the performance of modified and commonly used indices exhibited seasonality indicating that a multi-index approach to distributed modeling may be necessary to accurately simulate seasonal variation in soil moisture patterns in semi-arid climates.