When and where does near-surface runoff occur in a pre-Alpine headwater catchment?

Abstract. Although runoff processes have been described for many locations worldwide, there has been a lack of studies for poorly drained soils where most of the runoff may occur near the soil surface. Therefore, in this study, we aimed to improve the understanding of near-surface processes across a small headwater catchment with low permeable gleysols, which is typical for the Swiss pre-Alpine environment. We installed 14 small (1 m x 3 m) bounded runoff plots to collect overland flow (including biomat flow; OF) and shallow subsurface flow through the permeable topsoil, which we refer to as topsoil interflow (TIF). The runoff plots were located at different topographic locations and had a range of vegetation covers. For 27 rainfall events during the summer of 2022, we determined the occurrence and amount of OF and TIF. OF and TIF occurred for approximately half of the events, but the frequency of occurrence depended on the topographic wetness index (TWI) and vegetation cover. The runoff ratios (ratio between runoff produced divided by the total precipitation) increased with increasing precipitation and antecedent wetness conditions but did not correlate with the maximum rainfall intensity. Runoff ratios were highly variable and were generally higher for TIF than OF. Runoff ratios for OF were larger than one for some plots, indicating the occurrence lateral inflow to the plot from outside. Runoff ratio did not change after removing the upper boundary of the plot, suggesting that the actual flow-path lengths over the surface are short. Overall, this study highlights the importance of fast near-surface processes in pre-Alpine catchments underlain by low permeability gleysols, and that these processes occur across a range of catchment locations and land covers.