Subsurface stormflow source area identification using depth profiles of the water-soluble organic matter

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.