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

在排水良好的非饱和土壤中，水主要垂直移动。侧向流动开始于土壤接近水饱和和毛细力消失的位置。即使采用空间分布的三维数值模型，也不能真实地描述坡面上沿着阻碍土壤层边界和其他非均匀性的侧向流的发生。目前还没有一个基于过程的坡面非饱和带瞬态侧向流模型概念。发展这样一个概念的一个主要困难是，田间土壤中的水动力学表现出非平衡效应和滞后，由于结构的非均匀性。因此，侧向流已经在接近零的局部水势处被触发，即，如通常假设的那样，远早于完全水饱和发生。另一个困难是需要一个2D或3D表示的山坡和相应的高需求的数据和计算能力。在这个项目中，我们开发了一个概念框架来描述非平衡动力学和滞后一维垂直流在物理上一致的方式。分析将基于VAMOS蒸渗仪系统和TERENO-SoilCan蒸渗仪网络提供的独特数据集，这些数据集自2013年以来监测不同田间土壤（3D）和蒸渗仪（1D）的含水量和基质势。将通过由局部水饱和度（即零势）触发的垂直1D柱的动态横向耦合来实现山坡尺度的放大。该项目的结构是（1）统一概念来模拟土壤水分滞后和水力非平衡（Vogel），（2）山坡尺度的侧向流的开始（Gerke），（3）山坡尺度模型开发和模型结构的评估（Wöhling）。为了验证模型，我们将设计联合现场和实验室实验。所提出的模型框架预计将预测非饱和土壤中侧向流的发生和动力学。因此，预计将形成一个更现实的基础，以量化的临时变化的流动路径和旅行时间的山坡和集水区的规模，这是一个臭名昭著的问题，了解和预测溶质在饱和地下的运输。

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.