Combined effects of rainfall-runoff events and antecedent soil moisture on runoff generation processes in an upland forested headwater area

Abstract

In this study, we investigate the combined effect of different rainfall-runoff event types and antecedent soil moisture (ASM) on runoff processes in the headwater elementary discharge area of a small forested upland catchment. The study focuses on (i) the relationship between soil moisture thresholds and runoff gener- ation; (ii) the combined effect of ASM and tree vicinity and (iii) the relationship between different rainfall-runoff event types and different types of runoff (base- flow and stormflow). The results suggest that ASM has a strong impact on local runoff generation processes. Soil water content (35%-36%) threshold exceedance was related to stormflow runoff generation caused by the activation of quick pref- erential flow paths in the soil during storm events, especially in the upper and the deepest soil layers. At the same time, unexpected non-linear increases in baseflow runoff ratios were documented during dry, precipitation-free, periods and when the 31%-34% soil moisture threshold was exceeded, presumably due to the hydrological connection of farther slope areas during these conditions. Multiple stormflow periods, which exhibited the lowest runoff coefficient, were the most significant events in terms of water retention and soil water recharge due to increased vertical hydrological connectivity enabling more rapid transport to dee- per soil layers. However, this rainfall type occurred least often over the study period. The important role of forest stands (individual trees) in creating spatial patterns of soil moisture and preferential infiltration paths to deeper soil layers was also confirmed. These results contribute towards a better conceptualisation of hydrological behaviour in elementary headwater discharge areas and highlight the potential dangers associated with expected increases in extreme weather events.