Supporting subsurface preferential flow in a small forested catchment from geophysical data and hydrological modelling

Subsurface flow at the hillslope scale is a critical process responsible for water redistribution and transport of nutrients to the stream. Despite its hydrological importance, understanding the mechanisms governing subsurface flow generation is still challenging. 

We investigated the case of a small forested catchment located in the Apennine mountains, Tuscany, central Italy, which experiences shallow lateral downslope water redistribution resulting in substantial differences in vadose zone water supply along the hillslope. We developed an integrated experimental and modelling approach in order to shed some light on the role of the subsurface structure on the generation of hillslope-scale subsurface flow in the study catchment.  

We used a combination of methods sensitive to different soil properties. Ground Penetrating Radar (GPR) surveys show a complex response reflecting the interplay of different factors such as the presence of rocks, banks and counterslope in the near-surface and thus highlighting the very heterogeneous soil that may control water flow patterns. Several Electromagnetic (EM) mappings were conducted and show top-down hillslope variations of soil electrical conductivity revealing that trees located at the footslope and that experience longer vegetative periods might benefit from larger soil moisture content compared to the smaller trees located on the hillslope top. Similar observations are made from the two parallel top-bottom hillslope Electrical Resistivity Tomography (ERT) transects. 

The geophysical results will be integrated into hydrogeological simulations using the CATHY model for different scenarios (e.g., initial soil moisture, preferential flow paths, drainable porosity, soil properties, bedrock topography or stratification of soils) to explore the main drivers for subsurface preferential flow.