Insights into Subsurface Stormflow Dynamics using multitracer approaches

Subsurface stromflow (SSF) is an important runoff generation mechanism in hillslope catchments. However, since the process occurs below ground, it is difficult to observe and measure. So far, the dynamics and thresholds of SSF occurrence remain elusive. To gain insights into the mechanisms and to determine SSF quantities and their dynamics, we installed three SSF trenches in a first-order catchment in the Black Forest, Germany. We selected hillslopes with different landuse and topography and excavated slope-perpendicular trenches to bedrock (approx. 15 m wide, 2.5 m deep). The trenches are split by soil depth to collect subsurface flow from a top and from a bottom layer. The flow is channeled to tipping buckets for measuring discharge, and autosamplers for semi-continuous water sampling. The water samples are then used to measure multi tracers like stable water isotopes, dissolved organic carbon as well as major anions and cations. In November and December 2023, the catchment experienced three extreme, multi-day rainfall events. The generated subsurface discharge showed distinct differences in volume among the trenches (up to 100% of upslope-area-corrected flow volume). Most flow (70-90%) occurred in the lower trench sections. Top layer flow was only activated during peak discharge in the bottom layer. Using the multitracer approach, we can gain first insights into the dynamics of the different natural tracers and relate them to the observed subsurface flow variations, possible flow pathways and transit times. Ultimately, we aim to compare these findings to data from three other trenched research catchments to gain a more general understanding of the underlying subsurface stormflow generation mechanisms.