Tracing near-surface runoff in a pre-Alpine headwater catchment

Abstract. Near-surface flow pathways (i.e., overland flow and topsoil interflow) play a crucial role in runoff generation and solute transport in steep humid catchments with low-permeability gleysols but remain understudied. We conducted sprinkling experiments on two large (>80 m2) trenched runoff plots in the Swiss pre-Alps. One plot was located in a natural clearing in an open forest and the other in a grassland. After reaching steady state conditions, we applied uranine and NaCl to the surface as line tracers, injected NaBr into the subsurface (at ∼ 20 cm depth), and added deuterium-enriched water via the sprinklers to assess the particle velocities for near-surface flow pathways and the interaction between overland flow and topsoil interflow. We compared these velocities with the celerity, which was determined by temporarily adding more water to the plots at different distances (2, 4 and 6 m) from the runoff collectors. To trace overland flow and determine its flow path lengths, we also applied brilliant blue dye at different locations on the surface of the plots. The breakthrough curves highlight the rapid transport of water and solutes. The average (over all tracer applications) of the maximum velocities for overland flow and topsoil interflow were 51 and 30 m h−1 for the plot in the clearing, and 24 and 17 m h−1 for the plot in the grassland, respectively. The tracer breakthrough curves highlight the interaction between overland flow and topsoil interflow as the NaBr that was injected in the subsurface mainly exited the plot in the clearing via overland flow. Similar to the results for the velocity, the celerity for overland flow was higher than for topsoil interflow at both locations. The celerity of overland flow was 2–3 times higher than the velocity for both locations. This was also the case for topsoil interflow in the grassland plot. For topsoil interflow in the clearing the celerity and velocity were relatively similar, which we attribute to the importance of flow through large macropores. The overland flow pathways were relatively short for most locations (<5 m) and confirmed the considerable interaction between overland flow and topsoil interflow as the dye often resurfaced a few meters below the initial infiltration points. Together, these results highlight the interaction between overland flow and topsoil interflow and the important role of macropores and soil pipes (particularly in forested areas) for the rapid transport of water and solutes from steep, vegetated hillslopes to the streams.