Quantifying the spatio-temporal variability of shallow subsurface flow across three hillslopes in a semi-arid, sub alpine watershed

Abstract:

Distinguishing the local and non-local watershed characteristics that influence the spatial and temporal response of shallow subsurface flow within hillslopes is requisite for quantifying streamflow quantity, timing, and quality. We evaluated local and non-local (upslope) topographic influences on shallow water table duration, magnitude, spatial extent (32 shallow groundwater recording wells) and compared these to incremental changes in stream discharge across 3 hillslopes in the Lubrecht Experimental Forest, MT. Our results corroborate prior findings of the role upslope accumulated area (UAA) and local slope (as combined in the topographic wetness index) for shallow groundwater response, but emphasize the importance of considering the two variables independently. Increasing UAA resulted in greater durations of saturation across each well position (R²= 0.78; p < .05). Local slope was a significant predictor of mean water table height (R²= -0.86; p < .05). We used our empirical findings to inform a distributed hydrologic model (EcH₂O) and quantify the spatio-temporal variability of hydrologically connected contributing area. The spatial extent and timing of hydrologically connected contributing area was synchronized with changes in stream reach discharge adjacent to each hillslope. These relationships suggest that the organization of hillslope topography is a necessary context for predicting runoff source contributions to streams in space and time.