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

用一维有效连续尺度过程模型描述土壤水分向土壤-大气界面的蒸发运动在某些条件下是一种有效的方法，但在某些情况下可能导致错误的流量预测。这个子项目的目的是探索理查兹方程描述裸土蒸发的有效性限制，特别是在第二阶段蒸发中，土壤的非饱和水力传导率和土壤中的蒸汽扩散主导了蒸发过程。为了分析由于忽视热和水蒸气耦合流动过程而产生的局限性，并评估使用正确的液相流动水力导率函数的重要性，我们将通过使用有效过程模型的逆建模来评估各种蒸发情景的数据。场景包括瞬态条件下的实验，包括研究动态影响的流动中断。数据将由SP2综合正演模拟、实验室和现场条件下的小型渗滤仪实验以及联合渗滤仪实验（SP3）获得的虚拟现实提供。在反向评估中，我们将从完整的菲利普-德弗里斯模型和非平衡水流的扩展开始，并逐步将其简化为理查兹方程，以评估哪种模型复杂性对充分描述测量数据是必要的。我们特别感兴趣的是(i)确定液态水和水蒸气流量对地表附近总水流量的相对贡献，（ii）确定蒸发从第一阶段转移到第二阶段的时间，（iii）确定第二阶段蒸发过程中蒸发平面的位置，以及（iv）量化在实际情况中可以使用有效过程描述来正确预测大气中水通量的条件范围。

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.