Monitoring the generation and transformation of subsurface stormflow from hillslopes through the riparian zone

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Project D aims at better understanding the generation of subsurface stormflow (SSF) on hillslopes, the associated thresholds and controls, and to what extent the hillslope SSF physical and chemical signals are modified on the passage through the riparian zone. We intend to reduce the elusiveness of SSF as a streamflow generation process across topographical, climatic and land use gradients. The SSF signal, although most often generated on the hillslopes, is likely to become transformed on its path from the footslope through the riparian zone into the stream. This is problematic for SSF research as the most common experimental methods focus either on the hillslopes (observing shallow groundwater table fluctuations and/or subsurface flow in trenches) or on the SSF solute tracer signal identified on the hillslope and then measured in the stream. The transformation of the SSF signal can include time delays, changes in flow path direction and in the magnitude of flow. These physical effects mostly increase the residence time in the riparian zone and may thus lead to a chemical transformation of the SSF signal by mixing and/or biogeochemical reactions. By the time it reaches the stream, the original SSF signal might be heavily modified and hard to identify. Ultimately, the spatio-temporal variability of hillslope-stream connectivity will govern how and when hillslope SSF affects streamflow. To evaluate the relevance of hillslope SSF for streamflow quantity, timing and quality, Project D will address the following research questions:(1) What are the thresholds and dynamics of SSF generated on the hillslopes?(2) Are there SSF-specific tracers or combinations of tracers that could be used to identify hillslope SSF in streamflow? How site specific are these signatures?(3) How is the hillslope SSF physical and chemical signal transformed in the riparian zone and does this change seasonally or with catchment wetness state?(4) What are the major controls in time and space for SSF generation, as well as riparian zone transformation?We will tackle the challenges described above with an innovative multi-site experimental approach. This approach includes the development of a dual-use trench, allowing us to measure SSF outflow signals (quantity, timing, and natural tracers) at the upslope side of the trench, while at the same time serving as an injection site for artificial tracers at the downslope side of the trench. An array of piezometers installed downslope of the trench for water level observation and sampling will make it possible to trace the flowpaths from the footslope across the riparian zone to the stream. We will install three trenches in each of our four test catchments. In a next step, we can identify flow path directions, mixing with riparian zone groundwater and the extent of transformation of the chemical signal in the riparian zone. Based on this, we will be able to identify SSF-specific tracers across a range of landscape configurations.