Partitioning Surface and Subsurface Flow in a Semi-arid Rangeland Watershe

This thesis couples variable intensity rainfall simulation with surface geophysical measurements to study the partitioning of surface and subsurface flow processes at the ecological site scale. Ecological sites, hillslope scale soil-vegetation complexes, are the basic landscape unit for the management of rangeland ecosystems. Moreover, they provide a useful framework to study complex rangeland ecohydrologic processes. My research used observational hydrogeophysics field experiments to link ecological site characteristics to hydrologic function. I conducted over 20 experiments integrating rainfall simulation with high-resolution time-lapse electrical resistivity tomography on hillslope plots at five different ecological sites within the Upper Crow Creek Watershed in southeastern Wyoming. First order constraints on subsurface structure at these sites were made by jointly interpreting electrical resistivity tomography, seismic refraction surveys and ground penetrating radar datasets. My results show that ecological sites have a characteristic hydrologic response that can be quantified. ANOVA analysis found statistically significant differences in how ecological sites partition rainfall into runoff response. Sites ranged from infiltrating 100% of the applied rainfall to converting over 40% of the rainfall into surface runoff. Multiple linear regressions indicated that a large amount of the variability in surface hydrologic response could be explained by parsimonious models consisting only of ground cover variables. From the time-lapse resistivity datasets I was able to track the movement of the wetting front in the subsurface. ANCOVA results found significant differences in the rate of wetting front migration, which can be attributed to different ecological site characteristics. Joint interpretation of the geophysics results provided information about subsurface features that influenced hydrologic response at these sites. This research directly supports the concept of mapping the variability in rangeland hydrology at the ecological site scale and represents the first step in the process of coupling these two observational platforms to quantify the partitioning of rainfall into surface and subsurface flow components.