Celerity, velocity and flow path lengths of near-surface flow pathways: insights from tracer experiments during artificial rainfall

In catchments with low permeability soils, near-surface flow pathways can quickly transport water and solutes from the hillslopes to the stream network. Gaining insight into these pathways is essential for predicting changes in stream chemistry and improving flood forecasting. Despite their importance, near-surface flow pathways have rarely been assessed for well vegetated catchments in temperate climate. To better understand the importance of these flow pathways in terms of their ability to transfer water to the stream (celerity), transport solutes (particle velocity), and their flow path lengths, we conducted artificial rainfall simulation experiments on two large (>80 m²) trenched runoff plots in a small headwater catchment underlain by gleysols in the Swiss pre-Alps. One plot is located in a natural clearing in an open mixed forest and the other in a wet pasture. Together they represent the dominant land cover types in the region.

We applied streamwater to the surface of the plots using sprinklers and tracers after overland flow and lateral flow through the topsoil had reached steady state. Deuterium-enriched water was applied to the surface via the sprinklers, while Uranine and NaCl were applied as a line tracer at multiple distances from the trench. NaBr was injected into the topsoil at ~20 cm depth. Samples of overland flow and topsoil interflow were collected for several hours after tracer application, while the sprinklers continued to apply water to the surface. To determine the lengths of the overland flow pathways, we applied brilliant blue dye on the surface at different distances from the trench. The celerity of overland flow and topsoil interflow was determined by temporarily adding more water to the surface of the plots at different distances from the runoff collectors.

The breakthrough curves for both plots highlighted the rapid transport of water and solutes, as well as the high interaction between overland flow and topsoil interflow. The average of the maximum particle velocity (calculated for the different tracers) was 51 ± 14 m h^-1 for overland flow and 30 ± 9 m h^-1 for topsoil interflow for the plot in the natural clearing. The particle velocity was lower for the plot in the pasture: 24 ± 1 m h^-1 for overland flow and 17 ± 6 m h^-1 for topsoil interflow. The celerity was 2-3 times higher than the particle velocity for overland flow and similar to the velocity for topsoil interflow. The blue dye experiments highlighted that overland flow pathways are relative short for most locations (< 5m) and confirmed the considerable interaction between overland flow and topsoil interflow. In summary, these results highlight the high connectivity between overland flow and topsoil interflow and the critical role of macropores and soil pipes in rapidly transporting water and solutes from the hillslopes to the streams.