Monitoring and modelling of non-equilibrium soil water dynamics and lateral subsurface flow in hillslope soils

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In well-drained, unsaturated soils, water moves predominantly vertically. Lateral flow is initiated at locations where the soil approaches water saturation and capillary forces vanish. The onset of lateral flow along impeding soil horizon boundaries and other heterogeneities in hillslopes cannot be described realistically even with spatially-distributed 3D numerical models. A process-based model concept for transient lateral flow in the unsaturated zone of hillslope soils is still missing. One major difficulty to develop such a concept is that water dynamics in field soil exhibit non-equilibrium effects and hysteresis due to structural heterogeneities. Consequently, lateral flow is triggered already at local water potentials close to zero, i.e. far before complete water saturation occurs as is commonly assumed. Another difficulty is the need for a 2D or 3D representation of the hillslope and the corresponding high demand of both data and computing power.In this project, we develop a conceptual framework to described non-equilibrium dynamics and hysteresis for 1D vertical flow in a physically consistent way. The analysis will be based on unique data sets provided by the VAMOS lysimeter system and the TERENO-SoilCan lysimeter network, which monitor water contents and matric potentials in different field soils (3D) and lysimeters (1D) since 2013. Upscaling to the hillslope-scale will be accomplished by a dynamic lateral coupling of vertical 1D columns triggered by local water saturation (i.e. zero potential). This will allow to describe lateral flows at large scales with considerably reduced complexity.The project structure is (1) unified concepts to model soil water hysteresis and hydraulic non-equilibrium (Vogel), (2) onset of lateral flow at hillslope scale (Gerke), (3) hillslope scale model development and evaluation of model structures (Wöhling). For model validation we will design joint field and lab experiments.The proposed model framework is expected to predict the onset and dynamics of lateral flow in unsaturated soils. Thus, it is expected to form a more realistic basis to quantify the temporarily changing flow paths and travel times at the scale of hillslopes and catchments which is a notorious problem for understanding and predicting the transport of solutes in the variably saturated subsurface.