A Hydrogeophysics Approach to Investigate Upland-Stream Connectivity on a Rangeland Hillslope in the Upper Crow Creek Watershed in Southeastern Wyoming

Abstract:

Nearly all streamflow at one time passes through or over a hillslope prior to reaching the stream network. Knowledge of how this water is attenuated and delayed over a variety of hillslope conditions is critical for understanding the mechanisms controlling the spatial and temporal distribution of streamflow response. In semi-arid rangeland environments, these mechanisms remain unclear. Non- invasive geophysical methods coupled with traditional hydrological tools can provide the necessary insights into the dynamics of how upland hillslopes and streams are episodically connected. We collected approximately 1.4 km of ground penetrating radar (GPR) data to characterize the subsurface structure of a rangeland hillslope in the Upper Crow Creek Watershed and identify subsurface fractures. A field study integrating variable intensity rainfall simulation with time-lapse electrical resistivity tomography (ERT) was then conducted on four runoff plots positioned on the hillslope, to quantify the real-time partitioning of rainfall into surface and subsurface response. Runoff varied significantly across the plots due to spatial variability in vegetation cover, with a maximum and minimum peak runoff rate of 82 mm hr^-1 and 7 mm hr^-1 respectively. Time-lapse ERT data suggest the potential for infiltrated water to move preferentially through fractured regions in the subsurface identified by the GPR. Conductivity probes and pressure transducers positioned in the adjacent stream showed changes in electrical conductivity and depth following rainfall applications, suggesting hillslope connectivity with fairly rapid subsurface flow. Information obtained from this study will allow us to improve our understanding of the coupling of upland hillslopes and adjacent stream networks in complex rangeland environments.