Evaporation from soils under thermal boundary conditions: Experimental and modeling investigation to compare equilibrium‐ and nonequilibrium‐based approaches

Abstract

[1] In the shallow subsurface immediately below the land-atmosphere interface, it is widely recognized that the movement of water vapor is closely coupled to thermal processes. However, their mutual interactions are rarely considered in most soil water modeling efforts or in practical applications where it becomes necessary to understand the spatial and temporal distribution of soil moisture. The validation of numerical models that are designed to capture these processes is difficult due to the scarcity of field or laboratory data with accurately known hydraulic and thermal parameters of soils, limiting the testing and refinement of heat and water transfer theories. The goal of this paper is to perform controlled experiments under transient conditions of soil moisture and temperature and use this data to test existing theories and develop appropriate numerical models. Water vapor flow under varying temperature gradients was implemented on the basis of a concept that allows nonequilibrium liquid/gas phase change with gas phase vapor diffusion. To validate this new approach, we developed a long column apparatus equipped with a network of sensors and generated data under well-controlled thermal boundary conditions at the soil surface. The nonequilibrium approach yielded good agreement with the experimental results, validating the hypothesis that transport in the gas phase is better suited to be modeled with nonequilibrium liquid/gas phase change for highly transient field conditions where the thermal conditions at the land-atmosphere interface are constantly changing.

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

“…The difficult part is how to construct the proportionality coefficient. Nevertheless, there are at least a two ways to go about this: (a) largely empirically (e.g., Smits et al, 2011, and related approaches referenced therein); or (b) assume that S v = A wa J v (e.g., Skopp, 1985, or Novak, 2012, where A wa [m −1 ] is the volume-normalized soil water-air interfacial surface area and J v [kg m −2 s −1 ] is the flux to/from that interfacial surface. This second approach allows for a more physically based parameterization of the flux, viz., Novak (2012) proposed that the flux be driven be diffusion, so that R v = D v /r ep , where r ep [m] is the equivalent pore radius and D v is the diffusivity of water vapor in soil air.…”

“…This assumption is quite apropos for its original application, which was to describe the coupled heat and moisture transport in soils (and soil evaporation in particular) under environmental forcings associated with the daily and seasonal variations in radiation, temperature, precipitation, etc. (e.g., Milly, 1982;Novak, 2010;Smits et al, 2011). Under these conditions, assuming local equilibrium is reasonable because the time required to achieve equilibrium after a change of phase is "instantaneous" (short) relative to the timescale associated with normal environmental forcing.…”

“…Novak (2012) also recognized the inapplicability of the equilibrium assumption for very dry soils, but under more normal environmental forcing. On the other hand, even under normal (and much less extreme) soil moisture conditions both Smits et al (2011) and Ouedraogo et al (2013) suggest that non-equilibrium formulations of soil evaporation may actually improve model performance, which implies that the non-equilibrium assumption may really be a more appropriate description for soil evaporation and condensation than the equilibrium assumption. The present study is intended to provide the first test of the non-equilibrium hypothesis during extreme conditions.…”

Effects of Turfgrass Thatch on Water Infiltration, Surface Runoff, and Evaporation

“…A wet thatch could maintain a high air humidity at the soil surface, which reduces the differences of air humidity between the space immediately above and below soil surface (the drive of evaporation) [32]. Evaporative loss was thus lower when a thatch was placed at the soil surface, and the effects could be enhanced with an increased amount or thickness of thatch mass (Kentucky bluegrass vs. red fescue) (Figure 2).…”

Effect of Turbulence and Roughness on Coupled Porous-Medium/Free-Flow Exchange Processes

“…For example, the assumption of local thermodynamic equilibrium for phase transition and energy transfer has been questioned and analyzed, e.g., in Trautz et al (2015), Nuske et al (2014) and Smits et al (2011). , Shahraeeni and Or (2010) and Assouline et al (2010) investigated how the development of wet soil pore patches together with the environmental conditions affects evaporation behavior.…”