Monitoring and modelling of non-equilibrium soil water dynamics and lateral subsurface flow in hillslope soils
Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
Final Report Abstract
The physically based Richards equation is today’s state-of-the-art in simulating water movement in soils. Some model assumptions are, however, often not valid for the application under natural conditions. The prime example is infiltration into initially dry soil caused by heavy rainfall events. Here, the so-called hydraulic non-equilibrium is observed. The infiltrating water moves locally fast through larger soil pores while the redistributed into finer pores by capillary forces is delayed. This phenomenon can lead to lateral flow in hillslope soils even before the entire pore space is saturated. The aims of the project were i) the development of a new conceptual approach to describe hydraulic non-equilibrium, ii) analyse experimentally how relevant these phenomena are for lateral flow in hillslope soils and iii) to extend the standard model to describe these phenomena by numerical simulations. The analysis was based on data sets from field and lysimeter experiments. While water movement is forced in the vertical direction in lysimeters, it is 3-dimensional in the field. Studying the discrepancy between lysimeter and field experiments should give insights into the onset of lateral and preferential flow. One important result is a new concept for describing hydraulic non-equilibrium by decoupling water content and water potential for the description of hydraulic conductivity. The Richards equation can still be used as the basis which enables the implementation of our new concept in existing software. We developed a robust numerical model that can simulate the phenomena of preferential flow, hydraulic non-equilibrium and hysteresis, which was previously only possible with different tools. The water content time-series from lysimeter and field experiments revealed that lateral flow occurs predominantly in spring and autumn along soil horizons with lower hydraulic conductivity. Lateral flow under variably-saturated conditions was also mimicked in novel lab experiments with intact soil monoliths. However, hydraulic non-equilibrium did not trigger lateral flows at horizon boundaries in a Hele-Shaw-Cell as originally assumed. The fate of a water tracer in sloping field soils could be observed only at suction cups in soil profiles downhill. Overall, the results of the project contribute to an improved theoretical and experimental basis for the description of water movement in soils.
Publications
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Photogrammetrische Erfassung von Strukturveränderungen der Bodenoberfläche zur Ermittlung des Bodenabtrags mittels SfM an einem Erosionsmesshang, Jahrestagung der Deutsche Bodenkundlichen Gesellschaft 2019 Bern, 22-30 August 2019.
Ehrhardt, A.; Gerke, H. H. & Deumlich, D.
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A robust solution to Richards' equation for complex soil hydraulic models using the Method Of Lines. Copernicus GmbH.
Mietrach, Robert; Wöhling, Thomas & Schütze, Niels
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Changes of soil surface micro-topography determined with Structure-from-Motion photogrammetry, Eurosoil 23-27 August 2021, Geneva (Virtual Congress)
Ehrhardt, A.; Gerke, H. H. & Deumlich, D.
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Modelling non-equillibrium unsaturated flow in soils during sudden pressure head changes by solving Richards' equation with a Method of lines approach. Copernicus GmbH.
Mietrach, Robert; Wöhling, Thomas & Schütze, Niels
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Wavelet Analysis of Soil Water State Variables for Identification of Lateral Subsurface Flow: Lysimeter versus Field Data, ISMC2021 – online conference, 18-22 May 2021, Poster presentation.
Ehrhardt, A.; Groh, J. & Gerke, H. H.
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Wavelet analysis of soil water state variables for identification of lateral subsurface flow: Lysimeter vs. field data. Vadose Zone Journal, 20(3).
Ehrhardt, Annelie; Groh, Jannis & Gerke, Horst H.
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Efficient 1.5-D hillslope model using 1D Richards’ and 1D Boussinesq equations coupled by the Method Of Lines.
Mietrach, Robert & Wöhling, Thomas
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Exploring the feasibility of using the soil temperature to identify preferential and lateral subsurface flows. Vadose Zone Journal, 22(1).
Ehrhardt, Annelie & Gerke, Horst H.
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Lateral and preferential flow identified using wavelet coherence analysis of water content time series in lysimeters and field soil, Jahrestagung der Deutsche Bodenkundlichen Gesellschaft 2022, 03-08 Sep 2022
Ehrhardt, A.; Groh, J. & Gerke, H. H.
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Linking soil structure and soil functions. Invited presentation, Krikham Conference, South Africa, August 2022
Vogel, H.-J.
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Richard's equation revisited; the challenge to reconstruct non-equilibrium field retention data with soil hydraulic models.
Wöhling, Thomas & Mietrach, Robert
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Soil Hydraulic Conductivity in the Stage of Non-Equilibrium. Invited presentation, SSSA International Annual Meeting, Baltimore, November 2022
Vogel, H.-J.
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Tracing lateral subsurface flow in layered soils by undisturbed monolith sampling, targeted laboratory experiments, and model‐based analysis. Vadose Zone Journal, 21(4).
Ehrhardt, Annelie; Berger, Kristian; Filipović, Vilim; Wöhling, Thomas; Vogel, Hans‐Jörg & Gerke, Horst H.
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Wavelet Analysis of Soil Water State Variables for Identification of Lateral Subsurface Flow: Lysimeter versus Field Data . Copernicus GmbH.
Ehrhardt, Annelie; Groh, Jannis & Gerke, Horst H.
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A Field Experiment for Tracing Lateral Subsurface Flow in a Post-Glacial Hummocky Arable Soil Landscape. Water, 15(6), 1248.
Ehrhardt, Annelie; Koszinski, Sylvia & Gerke, Horst H.
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Soil hydraulic conductivity in the state of nonequilibrium. Vadose Zone Journal, 22(2).
Vogel, Hans‐Jörg; Gerke, Horst H.; Mietrach, Robert; Zahl, René & Wöhling, Thomas
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Subsurface preferential flow occurrence and relevance in agricultural hillslopes: experimental evidence. Copernicus GmbH.
Filipović, Vilim; Ehrhardt, Annelie & Gerke, Horst H.
