The riparian zone as a gatekeeper for subsurface stormflow

Subsurface stormflow (SSF) can be a major streamflow generation process in small catchments. It is known that SSF generated on the hillslopes of the catchment may change both in its chemical and quantitative composition on the way to the stream. This occurs primarily due to processes in the riparian zone. The riparian zone plays the role of a "reactor" where mixing, storage, and biogeochemical transformation of the hillslope SSF composition occurs.  However, we still lack a comprehensive understanding of this “gatekeeper function” of the riparian zone, controlling the timing and spatial patterns of connectivity and the chemistry of the water being transferred from the hillslopes into the stream.
In our study we aim to investigate the SSF signal transformation in the riparian zone. We installed three “dual-use trenches” per catchment in four different catchments located in Germany and Austria. With this novel dual-use trench approach we are able to measure hillslope SSF as well as inject tracer into the riparian zone. We measure response dynamics, timing, flow volumes and chemistry at the upslope side of the trench. We will identify tracers or tracer combinations that characterize SSF and can be used to identify hillslope SSF in riparian zone groundwater and stream flow. The inter-comparison of the four different catchments allows us to evaluate the influence of landscape and climate characteristics. We then use tracer injections at the downslope side of the dual-use trench in combination with an array of shallow groundwater observation wells downslope of the trench to investigate the physical and chemical transformation of hillslope SSF in the riparian zone. This array of wells extends both upstream and downstream of the trench, enabling us to trace the transformation of the uninterrupted physical and chemical signal of SSF on the adjacent hillslopes on its passage to the stream and to evaluate the influence of parafluvial flow.
Here, we present first data on tracer concentrations in hillslope SSF and riparian zone groundwater from our test catchments. Ultimately, we aim to develop a conceptual matrix, by which it will be possible to estimate the degree of SSF transformation in the riparian zone, depending on watershed characteristics (topography, soil depth and soil hydraulic properties) and hydrological conditions (antecedent wetness of the watershed and seasonal dynamics).