An evaluation of models of bare soil evaporation formulated with different land surface boundary conditions and assumptions

Abstract

Bare soil evaporation is a key process for water exchange between the land and the atmosphere and an important component of the water balance. However, there is no agreement on the best modeling methodology to determine evaporation under different atmospheric boundary conditions. Also, there is a lack of directly measured soil evaporation data for model validation to compare these methods to establish the validity of their mathematical formulations. Thus, a need exists to systematically compare evaporation estimates using existing methods to experimental observations. The goal of this work is to test different conceptual and mathematical formulations that are used to estimate evaporation from bare soils to critically investigate various formulations and surface boundary conditions. Such a comparison required the development of a numerical model that has the ability to incorporate these boundary conditions. For this model, we modified a previously developed theory that allows nonequilibrium liquid/gas phase change with gas phase vapor diffusion to better account for dry soil conditions. Precision data under well‐controlled transient heat and wind boundary conditions were generated, and results from numerical simulations were compared with experimental data. Results demonstrate that the approaches based on different boundary conditions varied in their ability to capture different stages of evaporation. All approaches have benefits and limitations, and no one approach can be deemed most appropriate for every scenario. Comparisons of different formulations of the surface boundary condition validate the need for further research on heat and vapor transport processes in soil for better modeling accuracy.

A non-equilibrium model for soil heating and moisture transport during extreme surface heating: the soil (heat–moisture–vapor) HMV-Model Version 1

“…(18) and (19), brought no discernible benefit to the model's performance. For less extreme conditions and for relatively coarsegrained sand Smits et al (2012) reached a similar conclusion. Nonetheless, K film r should not be discounted as unimportant, so further investigation into it's utility for modeling still seems warranted.…”

Improving the representation of hydrologic processes in Earth System Models

“…Moving forward, there is an opportunity to improve representation of storage and transmission of soil water in land models, while maintaining computational efficiency in mind, by (1) explicitly representing variably saturated flow, as is possible in the mixed form of Richards' equation [Celia et al, 1990;Maxwell and Miller, 2005;Kumar et al, 2009], to improve simulations of shallow groundwater dynamics; (2) explicitly represent airflow (vapor diffusion) through the soil to improve simulations of bare soil evaporation [Parker et al, 1987;Painter, 2011;Zeng et al, 2011;Smits et al, 2012]; and (3) explicitly represent macropore flow to simulate the nonuniform wetting of the soil matrix and the heterogeneity of flow paths at larger spatial scales Germann, 1981, 1982;SimunEk et al, 2003;Weiler, 2005;McDonnell et al, 2007;Maxwell and Kollet, 2008a;Nimmo, 2010;Yu et al, 2014]. Some integrated models have incorporated these processes-for example, the coupling of ParFlow and CLM represents threedimensional variably saturated flow and has now been applied at continental scales [Maxwell et al, 2015], CLM now parameterizes the diffusion of water vapor through a dry surface layer to improve simulations of bare soil evaporation [Swenson and Lawrence, 2014], and the LM3 model has a simple representation of macropores .…”

Column-scale unsaturated hydraulic conductivity estimates in coarse-textured homogeneous and layered soils derived under steady-state evaporation from a water table

“…In order to validate the applicability of Eq. (10), we employed data and hydraulic parameters from the Loveland sand in Anat (1965), the sand studied by Smits et al (2012) (Sand 1), the sand studied by Prevedello et al (2009) (Sand 2), and three coarsetextured soils from van Genuchten (1980) including Hygiene sandstone, Touchet silt loam, and Silt loam G.E.3 with VG parameters listed in Table 1.…”