Near surface runoff generation in a pre-Alpine headwater catchment

Near-surface flow pathways can be important contributors to runoff in headwater catchments with low conductivity soils. However, the high spatio-temporal variability and connectivity between surface flow and shallow subsurface flow makes it difficult to study these processes. As a result, they are still poorly understood, especially for well vegetated humid catchments. The TopFlow project, therefore, aims to enhance our understanding of the generation and connectivity of overland flow and shallow subsurface flow in a pre-Alpine headwater catchment with low permeability Gleysols. We installed 14 small (1 by 3 m) runoff plots at different topographic locations to cover the range in slope, vegetation, and wetness conditions across the catchment. At each plot, we measured overland flow (including biomat flow) and shallow subsurface flow from the rooting zone during two snow-free seasons. In addition, we collected groundwater, precipitation and soil moisture data. We also installed two larger plots (8 by >10 m), where we collected data during natural rainfall events and sprinkling experiments. Specifically, we conducted experiments to determine the surface flow path lengths and celerity of overland flow and shallow subsurface flow. Overland flow and shallow subsurface flow occurred frequently on most plots (on average for 40% of the 26 rainfall events for which data were collected) but the spatial and temporal variability in overland flow and shallow subsurface flow generation was high. The timing and relative importance of overland flow and subsurface flow varied as well. Runoff ratios increased with increasing soil moisture storage and precipitation, and were generally higher for sites with a higher Topographic Wetness Index. Runoff ratios were sometimes larger than 1, indicating the importance of connectivity between subsurface and surface flow. Flow path lengths and celerity also differed for the plots and can be explained by differences in soil characteristics and wetness conditions. Overall, these results highlight the importance of fast near surface flow pathways for runoff generation and its high spatial and temporal variability.