VAMOS II: Monitoring and modelling of non-equilibrium soil water dynamics and lateral subsurface flow in hillslope soils
The DFG-funded project is a joint research effort between the Helmholtz Centre for Environmental Research (UFZ, Prof. Hans-Jörg Vogel), the Leibniz Centre for Agricultural Landscape Research (ZALF, PD Dr. Horst H. Gerke) and the Technische Universität Dresden (TUD, Dr. Thomas Wöhling). The project deals with flow in unsaturated hillslope soils, specifically with the onset of lateral flow along impeding soil horizon boundaries and other heterogeneities. This cannot be described realistically even with spatially-distributed 3D numerical models because structural heterogeneities lead to non-equilibrium effects and hysteresis and consequently to lateral flow already at local water potentials close to zero, i.e. far before complete water saturation occurs as is commonly assumed.
In this trilateral project, we develop a conceptual framework to described non-equilibrium dynamics and hysteresis for 1D vertical flow in a physically consistent way. The analysis is 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 is accomplished by a dynamic lateral coupling of vertical 1D columns triggered by local water saturation (i.e. zero potential). This allows describing lateral flow at large scales with considerably reduced complexity. The project structured into three tasks:
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Unified concepts to model soil water hysteresis and hydraulic non-equilibrium (UFZ),
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Onset of lateral flow at hillslope scale (ZALF),
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Hillslope scale model development and evaluation of model structures (TUD).
Joint field and lab experiments are designed for model validation. The proposed model framework is expected to predict the onset and dynamics of lateral flow in unsaturated soils. Thus, it forms 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.
(A) Trajectories of water content and water potential in a lysimeter showing pronounced hydraulic non-equilibrium after heavy rain on relatively dry soil (Hannes et al. 2016). (B) Aerial image of the CarboZALF-D experimental field superimposed with a map of the apparent electrical conductivity (ECa) with the location of VAMOS lysimeters (yellow), augerings (black), soil profiles with suction-cup installations (red rectangles), and the distribution of ECa determined with EM38_DD by Dr.Koszinski (Figure by M. Sommer, ZALF, 2016).