Spatial and temporal variation in near-surface runoff in a pre-Alpine headwater catchment

Near-surface flow pathways are important for the transfer of water and solutes from the hillslopes to the streams, particularly in head water catchments with low permeability soils. However, the high spatio-temporal variability in the occurrence of runoff makes it difficult to study these flow pathways and to upscale plots based measurements to the catchment scale. Furthermore, it is well known that the amount of overland flow at the bottom of a runoff plot depends on the size of the plot. To better understand the importance of near-surface flow pathways in pre-Alpine headwater catchments underlain by low permeability gleysols, we installed 14 small (1 × 3 m) trenched runoff plots in the Studibach catchment in the Alptal (Switzerland). They cover a range of topographic positions and vegetation covers. We measured the occurrence of overland flow (including biomat flow) and shallow subsurface flow through the topsoil (i.e., the main rooting zone), precipitation, and soil moisture during a snow-free season. In addition, we collected data in a second snow-free season from two large plots (>80 m²) and two nearby small plots. The results from the small plots showed that runoff ratios increase with increasing soil moisture storage and precipitation and are higher for areas with a greater topographic wetness index (TWI). To understand the effect of plot size on near-surface runoff, we compared the runoff characteristics (runoff ratio and runoff generation threshold) for the small and large plots for different events. Additionally, we determined the typical flow path lengths (and thus the effect of plot size) by applying blue dye and tracers to the surface of the plots during rainfall simulation experiments. These experiments showed that overland flow generally infiltrates within a short distance (<5 m, and often <1 m) but also exfiltrates again after flowing a short distance below the ground (<5 m). To better understand the importance of near-surface flow for runoff at the catchment scale, we compare the runoff thresholds and runoff ratios for the small and large plots to those for streamflow. We, furthermore, investigated the spatial pattern in near-surface flow generation across the catchment based on the relation between topography (TWI) and near-surface runoff generation. More specifically, we related the area where near-surface runoff is expected to occur and its connectivity to the stream, to the streamflow response for different sub-catchments and the catchment outlet.