Subsurface flow: From innovative experimental approaches to process modelling 
Convener: Peter Chifflard  | Co-conveners: Natalie Orlowski , Michael Rinderer , Taro Uchida , Ilja van Meerveld 
In catchment hydrology, subsurface flow is a well-recognized process but is still challenging to capture. Different terms exist to characterize subsurface flow, such as shallow subsurface runoff, interflow, subsurface stormflow, lateral flow or soil water flow. This reflects the different underlying concepts derived from various experimental and modeling studies in different environmental settings and for different spatial and temporal scales. Subsurface flow is responsible for the transport of nutrients and pollutants from the terrestrial to the aquatic ecosystems, which underlines its importance for the adjacent surface water bodies in terms of both water quantity and quality. This makes an accurate process understanding of subsurface flow essential.
Significant knowledge has been gained from experimental studies at the point and hillslope scales. These studies have identified controlling factors for subsurface flow (e.g., initial soil moisture, preferential flow paths, drainable porosity, precipitation inputs, soil properties, bedrock topography or stratification of soils). However, the importance at the catchment scale, and how these findings can be implemented in catchment scale rainfall-runoff models, remain poorly understood. This is mostly due to the nonlinearity of subsurface flow and due to a lack of knowledge in understanding where subsurface flow is generated within a catchment and when. Furthermore, simulation of subsurface runoff in catchment rainfall-runoff models is frequently based on calibration and validation for single rainfall-runoff events. However, such often isolated events, assuming 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 needed. New in-situ high-frequency measurements of tracers can help to bridge the gap between hillslope and point scale measurements and simulated catchment scale responses and thus improve the accuracy of these models.
This session aims to address the current state of the art for measurement, assessment and modeling of subsurface runoff processes. We welcome experimental and modeling studies on the following topics: (i) (Non-)Invasive methods for the investigation and monitoring of subsurface flow in space and time and its connection to the stream network; (ii) linking spatial patterns of subsurface flow with soil and lithological heterogeneity, including stratification of soils; (iii) assessment of the role of subsurface runoff for catchment response; and (iv) validation approaches to assess the accuracy of the simulated subsurface runoff using biogeochemical proxies (e.g. stable isotopes, dissolved silica, nitrate, dissolved organic carbon, trace elements etc).