Event-based dynamics of the chemical composition of subsurface stormflow across seasons

Subsurface stormflow (SSF) is a streamflow generation process that is difficult to observe. It has therefore been challenging to evaluate the relevance and magnitude of SSF contributions to streamflow quantity and quality. However, some earlier studies have shown that it can deliver substantial amounts of water to the stream at the time scale of an event.  One possible approach for detecting SSF in streamflow has been to sample SSF on hillslopes, characterize it by analyzing various tracers and search for this SSF fingerprint in stream water samples. In this study, we ask the following questions: Does subsurface stormflow generated on hillslopes and moving downslope towards the stream have a typical chemical fingerprint or signature by which we could recognize it in the stream? And does this signature vary over time? Here, we present data from a headwater catchment near Freiburg, Germany, where we installed three trenches to measure SSF flow rates and to obtain SSF samples for chemical analysis. We collected SSF samples from the three trenches over multiple events during spring 2023, fall 2023 and spring 2024 and analyzed them for dissolved organic carbon and major ions. We compared chemical SSF signatures through the events, across seasons and between the three trenches. Preliminary analyses indicate that the SSF signature changed during events, with SSF signatures at the beginning and at the end of events being remarkably similar to each other. Results also hint at a seasonal stability of SSF signatures. In our presentation, we are going to present a detailed analysis of the dynamics of the chemical SSF signature. This dataset provides a unique opportunity to evaluate the chemical composition of subsurface stormflow in sub-daily resolution at three different hillslopes and to improve our capability to recognize contributions of SSF to streamflow.