Influence of rainfall event characteristics and antecedent conditions on subsurface stormflow response of two forested hillslopes

Abstract. Subsurface stormflow (SSF) is a critical runoff-producing mechanism in many upland and mountainous environments, yet the complex relationships between antecedent conditions, rainfall characteristics and SSF response are still not fully understood. Worldwide, the small number of SSF collection systems (trenches), as well as the generally small number of investigated SSF events limit our ability to generalize the findings and explore the influence of a broader range of storm sizes, intensities, antecedent wetness conditions and different hydrogeologic settings. In this study, we present a comprehensive analysis of SSF event characteristics in combination with rainfall event characteristics (depth and intensity), and antecedent conditions. The analysis is based on data collected over a 2-year period at two forested hillslope sites. Our results show that SSF volume is primarily controlled by total rainfall (Ptot) and antecedent wetness, with volumes being up to three orders of magnitude larger under wet initial conditions. The peak SSF flow rates of smaller events were correlated with Ptot and antecedent conditions, but for larger events (Ptot>ca. 20 mm), rainfall intensity and rainfall amount preceding peak rainfall intensity were more influential predictors than antecedent conditions. The steepness of the rising limb of the SSF hydrograph was correlated with Ptot and rainfall intensity. The antecedent soil moisture index (ASI) together with Ptot showed a high correlation with most SSF characteristics. The seasonal analysis revealed that, statistically, the largest SSF volumes occurred in winter and spring, while the highest peak flows were observed in spring and summer. Our results highlight the complex interactions among SSF responses, rainfall characteristics, and antecedent wetness conditions, underscoring the value of long-term monitoring across different seasons.