Numerical and experimental analysis of bare soil resistance in homogeneous and heterogeneous soils, A

Abstract

Resistance terms have been used in many studies to simplify the complex processes associated with water vapor transport across the land-atmosphere interface. Soil resistance, accounts for an additional resistance to evaporative losses associated with a drying soil surface. Early formulations were empirically derived from limited measured datasets, which confines their widespread applicability, but some newer formulations better relate to the physical processes at play. The Tang and Riley (2013) formulation avoids a direct empirical relation between soil resistance and the soil water content at some depth below the surface by considering liquid flow and gaseous diffusion determined from existing constitutive relationships. However, it has only been tested in a large scale, single phase model against global datasets. In this work, we utilize comprehensive data sets from wind tunnel evaporation experiments and a multiphase heat and mass transfer model to compare the accuracy of this resistance term with other prevalent soil resistance formulations and their sensitivity to model structure. Applying these measured and modeled data sets decreases reliance on simplifying assumptions and allows for more robust examination of the different formulations applied to homogeneous and heterogeneous soil surfaces. Results illustrate the significant variation in the behavior of different soil resistance terms over the course of the drying process. The mechanistic soil resistance performed well compared to more empirical approaches. Sensitivity of the soil resistance and the corresponding evaporation rate to the defined surface soil layer thickness used in the model structure and the diffusivity model varies across formulations and soil moisture conditions. Averaging soil state variables across a heterogeneous surface indicates limited capability to capture the overall evaporation rate behavior. These findings offer new insight into how well these soil resistance formulations relate to the physical processes associated with evaporation from the soil surface. The more physically-based resistance shows potential for more accurate estimation of evaporation from bare soil, but relies heavily on proper parameterization of soil transport properties associated with vapor transport to the surface.