How does hydrologic connectivity on hillslopes mantled by soils with shrink-swell properties respond to changing precipitation patterns?

In soils with vertic (shrink-swell) properties, the locations and density of macropores (pores > 75 μm) are dynamic—changing in response to moisture conditions. Given that macropores are responsible for roughly 70% of water flow in soils, this transient network of macropores governs both the pathways and residence time of water along a hillslope which feeds back to control groundwater recharge and delivery of solutes to streams. Here, we use a multifaceted field-based approach over wet and dry seasons to quantify how soils with shrink-swell properties govern hillslope hydrologic connectivity (HHC) — the potential for continuous subsurface flow along a hillslope. To quantify hillslope connectivity and flow paths, we installed soil monitoring arrays (i.e., soil moisture, matric potential, and soil water chemistry) at several depths in four locations (e.g., summit, backslope, footslope and toeslope positions) on a hill mantled by shrink-swell soils. For two years we continuously measured soil moisture and matric potential and collected soil water chemistry data weekly when water was present. In addition, we conducted electrical resistivity tomography (ERT) surveys at all four hillslope positions under three different seasonal soil moisture states to characterize the spatial variability in soil moisture patterns and their relationship to HHC. Preliminary analysis shows a greater degree of vertical connectivity through the soils during summer months when high evapotranspiration rate can dry soil and form macropores, with vertical connectivity observed to depths of ~1 m under given rainfall events, compared to winter months when wetter conditions persist and soils swell. The degree to which vertical recharge persists during summer months was also observed to vary with precipitation patterns. Greater vertical recharge was observed to occur in the summer marked by intense but infrequent rainfall events, while soils swelled shut between 10-50 cm, reducing vertical recharge, when rainfall was less intense but more frequent. Soil resistivity variations measured by ERT support these patterns. Together, these data suggest groundwater recharge patterns and throughflow in hillslopes mantled by shrink-swell soils are highly dependent on precipitation patterns during summer months.