It is well recognized that subsurface runoff is an important hydrological process transporting water, nutrients and pollutants from terrestrial to the aquatic ecosystems, but is still challenging to measure or model. Different terms exist to describe subsurface runoff, such as shallow subsurface runoff, interflow, subsurface stormflow, lateral flow or soil water flow. This reflects the different concepts derived from experimental and modeling studies in different environmental settings and at different spatial and temporal scales. Significant knowledge has been gained from these experimental studies at the point and hillslope scale, which have identified controlling factors for subsurface runoff (e.g., initial soil moisture, preferential flow paths, drainable porosity, precipitation inputs, soil properties, bedrock topography or soil layering). However, the importance of these controls on subsurface runoff at the catchment scale and how these findings can be implemented in catchment scale rainfall-runoff models remains poorly understood. Furthermore, most models are not directly calibrated on subsurface runoff and if it is done it is usually only for a few rainfall-runoff events that do not capture the whole range of initial conditions or thresholds for generating subsurface runoff. Thus, continuously measured proxies to assess the simulated subsurface runoff are needed. New, in-situ, high-frequency and high-temporal measurements of tracers can help to bridge the gap between point or hillslope scale processes and catchment data that are necessary for modelling these fundamental processes.

This session aims to address the current state of the art for measurement, characterizing and modeling subsurface runoff processes. We welcome all contributions on subsurface runoff processes and especially experimental and modeling studies on the following topics: (i) (Non-)Invasive methods to investigate and monitor subsurface runoff in space and time and its connection to the stream network; (ii) linking spatial patterns of subsurface runoff to soil and lithological heterogeneity, including stratification of soils; (iii) assessment of the importance of subsurface runoff for catchment scale runoff or chemical fluxes; and (iv) assessment of the accuracy of the subsurface runoff simulations using biogeochemical proxies (e.g. stable isotopes, dissolved silica, nitrate, dissolved organic carbon, artificial and/or environmental DNA).