FOR 5288: Fast and invisible: Conquering Subsurface Stormflow
through an Interdisciplinary Multi-Site Approach

Abstract:

The transport of water-soluble organic matter (WSOM) during stormflow events is an important link between hillslope hydrology and biogeochemical cycling, as it determines the movement of organic carbon from soils to streams. Hydrological dynamics in hillslopes, particularly subsurface stormflow (SSF), exhibit substantial spatial and temporal variability, making quantification and generalization challenging. SSF can account for up to 90% of rainfall input to stream discharge during storm events, highlighting its importance in catchment hydrology. Despite its significance, current research frequently overlooks WSOM dynamics during SSF, which are not only key components of carbon cycling but may also serve as tracers for identifying potential critical source areas. This emphasizes the importance of studying hillslope hydrological dynamics and determining the factors that contribute to SSF spatial and temporal variability. Furthermore, the specific flow paths within hillslopes remain poorly understood, which complicates the identification of spatial sources and transport mechanisms for organic carbon. To fill these knowledge gaps, we conducted a field study in the Black Forest, Germany, using a trench system to collect lateral subsurface flows at two depths (0-100 cm and 100-200 cm) over several rain events. We analysed WSOM concentration and quality using absorbance and fluorescence properties to assess the variability in critical source areas. We also conducted isotopic analyses of oxygen (δ¹⁸O) and hydrogen (δ²H) of the same water samples to infer flow pathways with a conservative tracer. This approach provides valuable insights into the temporal dynamics and spatial heterogeneity of SSF. Our findings will contribute to our understanding of flow paths, transit times, and the characteristics of WSOM export, offering a deeper understanding of subsurface flow processes in catchments. Finally, the findings of this study can help to improve biogeochemical models and improve scaling of hillslope processes models, particularly in understanding their contribution to organic carbon transport via SSF.

Abstract:

Hydrological dynamics of hillslopes, particularly subsurface stormflow (SSF), are highly complex and variable in space and time. Frequently, available studies are often limited to single slopes or few storm events. As a result, the transfer of these findings to other slopes or catchments is associated with great uncertainties. Thus, for upscaling and model validation, a quantification of the hydrological dynamics of hillslopes and the factors influencing the spatial and temporal patterns of subsurface stormflow is urgently needed. Closely related to the hydrological dynamics of hillslopes is the export of organic carbon from the soils to the adjacent stream. However, the spatial sources of carbon are still largely unclear because the exact flow paths of SSF within the slope are not well known. In order to address this knowledge gap, we took a hydro-biogeochemical approach, that measures the water-soluble organic matter (WSOM; concentration, absorbance and fluorescence) along 480 locations on 100 hillslopes, in four contrasting catchments – varying from low to high mountain ranges (Sauerland, Ore Mountains, Black Forest, Alps). This enables us to derive empirical relations among different landforms (i.e., convergent, divergent and planar slope shapes, flow path lengths and valley shapes), bedrock and soil properties, and to quantify the spatial variability and stability of subsurface hydrological process patterns (e.g., flow directions, transit times, hydrochemical and biochemical composition). Distributed sampling of WSOM along the soil profile (6 WSOM samples per profile; both during wet and dry conditions) will help to assess the vertical and lateral subsurface flowpaths of water in the unsaturated and saturated zone, and the spatial discretization of source areas for SSF. We will use an array of state-of-the-art laboratory equipment and methods (TOC-Analyzer, Fluorescence Spectrometry) to analyze WSOM. First results will show depth profiles of WSOM in the four contrasting catchments from the low to high mountain ranges (Sauerland, Ore Mountains, Black Forest, Alps). By these depth profiles source areas of SSF can be detected.