Identification of effective process formulations for evaporation of water from bare soil

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Describing the evaporative movement of soil water towards the soil-atmosphere interface with one-dimensional effective continuum-scale process models can be a valid approach under some conditions, but may lead to erroneous flow predictions under certain circumstances. The aim of this subproject is to explore validity limits of the Richards equation for describing bare soil evaporation in particular during stage-two evaporation, where the soil's unsaturated hydraulic conductivity and vapor diffusion in soil dominate the evaporation process. To analyze limitations arising from neglect of coupled heat and water vapor flow processes and to assess the importance of using a correct hydraulic conductivity function for liquid phase flow, we will evaluate data from a variety of evaporation scenarios by inverse modeling with effective process models. Scenarios cover experiments under transient conditions and include flow interruptions to study dynamic effects. Data will be provided by virtual realities obtained by comprehensive forward modeling by SP2, small lysimeter experiments under laboratory and field conditions, and a joint lysimeter experiment (SP3). In the inverse evaluations, we will start with the full Philip-de Vries model and extensions for non-equilibrium water flow and simplify it stepwise towards the Richards equation in order to assess which model complexity is necessary to adequately describe the measurement data. Our particular interest lies in (i) identifying the relative contributions of liquid and vapor water flow to the total water flow near the surface, (ii) determining the time at which evaporation shifts from stage-one to stage-two, (iii) determining the position of the vaporization plane during stage-two evaporation, and (iv) quantifying the range of conditions where effective process descriptions can be used in practical situations to correctly predict the water fluxes to the atmosphere.