Enhanced Drainage and Altered Streamflow Sensitivity of Soil Water Storage through Topographically-Driven Connectivity between Stream and Aquifer

The catchment-scale pattern and processes of soil water storage on sloping topography is key to understanding the functions of soil moisture in atmospheric boundary layer processes. While the impact of an adjacent flow regime (e.g., overland flow, groundwater flow) on soil water storage variations is well understood, the role of the fluvial system in the evolution of soil water storage has not been thoroughly explored in Earth System Models (ESMs). In this study, we use a novel Bidirectional Exchange Scheme in Surface and Subsurface (BE3S) and demonstrate one approach to evaluating streamflow sensitivity of soil water storage under different surface water-groundwater connectivity and streamflow conditions. We study the development of topographically-driven soil moisture distribution in a series of comparative numerical experiments with different boundary conditions (BC) between the phreatic aquifer and stream. The numerical experiments are performed with both the fully-coupled interfacial BC (two-way exchange) and zero-flux BC (no flow/exchange) between the phreatic aquifer and stream. Our results show that hill to valley convergence facilitates efficient vertical percolation in uplands and impedes drainage in valleys while accounting for the influence of subsurface heterogeneity on the surface water-groundwater connectivity. The effect of hills-to-valleys lateral drainage on vertical percolation are seen in the differences in streamflow sensitivity of soil water storage that yields lower values of sensitivity under the fully-coupled BC. The contrast in streamflow sensitivity of soil water storage between catchments is mainly induced by the differences in catchments' drainage ability that results from a combination of topography, groundwater depth, and subsurface heterogeneity.