Slope-aspect dependent hillslope water storage capacity, threshold responses and runoff generation

Abstract:

Evidence has shown that the subsurface modulates the water cycle by connecting key hydrologic stores and fluxes. However, we do not fully understand how differences in subsurface regolith depth and porosity-permeability gradients alter vertical and lateral hydrologic flowpaths, which mediate streamflow magnitudes and timing. Additionally, aspect-dependent solar radiation inputs can produce striking differences in vegetation and subsurface weathering patterns across adjacent hillslopes. These differences provide a natural laboratory to investigate the biophysical controls on the retention and release of water in hillslopes. To investigate how co-evolved vegetation and weathering patterns drive hydrologic responses of north and south facing hillslopes, we conducted seismic refraction surveys of subsurface structure and monitored soil moisture and groundwater along two hillslopes at the Blue Oak Ranch Reserve (coastal central CA). Our seismic refraction surveys, corroborated with soil pits, indicate similarities in mobile regolith depth across north and south facing slopes. Our results show that while shallow water storage was consistently elevated on north facing slopes, streamflow was only generated following rapid increases in water storage on south facing slopes. This suggests that hillslopes with differing aspects may support specific watershed functions. This work highlights how natural experiments, such as slope aspect contrasts, can improve our scientific understanding and predictive power of how the critical zone structure may drive runoff generation.