SSF ANGLE - Comparing dynamics and pathways of subsurface stormflow among hillslopes

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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, so that 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. But the spatial sources of carbon are still largely unclear because the exact flow paths of SSF within the slope are not well known. We therefore propose a combined hydro-biogeochemical approach, that measures the hillslope groundwater dynamics including a characterisation of stable isotopes and the water-soluble organic matter (WSOM; concentration, absorbance and fluorescence) along 480 locations at 100 hillslopes in four contrasting catchments from the 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 groundwater dynamics to quantify the spatial variability and stability of subsurface hydrological process patterns (e.g., flow directions, transit times, hydrochemical and biochemical composition). A GIS based landscape analysis and a predictive modeling with the RoGeR model will be conducted prior to instrumentation that incorporates topographic, soil and land use information as well as spatial distribution of areas were SSF is expected be dominant, respectively, in order to sample representative hillslopes. Shallow groundwater dynamics due to SSF will be recorded in 480 wells across all equipped hillslopes. Distributed sampling of water stable isotopes and WSOM along the soil profile (12 isotope and 6 WSOM samples per profile; two times 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, Isotope Analyzer) to analyze stable isotopes and WSOM. The use of multivariate statistical techniques and machine learning tools will help to identify temporal and spatial patterns of subsurface hydrological process patterns. We will investigate hillslope-stream connectivity including the extent of contributing area with RoGeR predicting SSF fluxes, directions and dynamics which can later be used for predicting transport of soil organic matter from the hillslope to the stream.