In catchment hydrology, subsurface runoff is a well-recognized but still challenging process. The reasons are manifold. On the one hand, different terms such as shallow subsurface runoff, interflow, subsurface stormflow, lateral flow or soil water flow exist, which reflect the different underlying process concepts derived from various experimental and modeling studies in different environments at different spatial and temporal scales. On the other hand, subsurface runoff is often considered to be too slow and not relevant for the runoff peak or flood generation at the catchment scale. Nevertheless, subsurface runoff contributed significantly to the generation of recent floods especially in humid mountainous catchments. It is also responsible for the transport of nutrients and pollutants from the terrestrial into the aquatic ecosystems, which underlines its further importance for the adjacent surface water bodies. This makes an accurate process understanding essential.
Indeed, a lot of important research at the point and hillslope scale was carried out to identify controlling factors for subsurface runoff (e.g., initial soil moisture, preferential flow paths, drainable porosity, precipitation inputs, soil properties, bedrock topography or stratification of soils). But the importance at the catchment scale, and how these findings can be implemented in catchment rainfall-runoff models, are remain poorly understood. This is mostly due to the nonlinearity of subsurface runoff and to the missing knowledge about where subsurface runoff occurs within a catchment and when these areas are connected to the stream. Furthermore, the accuracy of the simulated subsurface runoff in catchment rainfall-runoff models is mainly calibrated and validated by single rainfall-runoff events where tracer hydrological studies and the determination of specific runoff components are available. It is obvious that these often isolated events with steady state conditions are not sufficient to capture the whole range of initial conditions and especially the thresholds for generating subsurface runoff. Thus, continuously measured proxies to assess the accuracy of the simulated subsurface runoff are required but still do not exist.
The proposed session will demonstrate current knowledge and progress about the generation of subsurface runoff and will raise to stimulate ongoing discussion, debate and learning, highlight areas for further work, and improve general knowledge. The proposed session welcomes studies on the following topics, but not limited to:
• (Non-)Invasive methods for investigation and monitoring subsurface runoff in space and time, from the most simple to the most sophisticated (e.g., TDR, GPR, artificial and natural tracer etc.).
• Tracking flow pathways of subsurface runoff and their connectivity with the stream in space and time.
• Linking spatial patterns of subsurface runoff with soil heterogeneity and lithological heterogeneity including stratification of soils.
• Assessing the role of subsurface runoff for catchment response by combining experimental studies at different spatial scales.
• Validation approaches to assess continuously the accuracy of the simulated subsurface runoff by using biogeochemical proxies (e.g. stable isotopes, dissolved silica, nitrate, dissolved organic carbon, trace elements etc).