Abstract

Soil water evaporation plays a critical role in mass and energy exchanges across the land–atmosphere interface. Although much is known about this process, there is no agreement on the best modeling approaches to determine soil water evaporation due to the complexity of the numerical modeling scenarios and lack of experimental data available to validate such models. Existing studies show numerical and experimental discrepancies in the evaporation behavior and soil water distribution in soils at various scales, driving us to revisit the key process representation in subsurface soil. Therefore, the goal of this work is to test different mathematical formulations used to estimate evaporation from bare soils to critically evaluate the model formulations, assumptions and surface boundary conditions. This comparison required the development of three numerical models at the REV scale that vary in their complexity in characterizing water flow and evaporation, using the same modeling platform. The performance of the models was evaluated by comparing with experimental data generated from a soil tank/boundary layer wind tunnel experimental apparatus equipped with a sensor network to continuously monitor water–temperature–humidity variables. A series of experiments were performed in which the soil tank was packed with different soil types. Results demonstrate that the approaches vary in their ability to capture different stages of evaporation and no one approach can be deemed most appropriate for every scenario. When a proper top boundary condition and space discretization are defined, the Richards equation-based models (Richards model and Richards vapor model) can generally capture the evaporation behaviors across the entire range of soil saturations, comparing well with the experimental data. The simulation results of the non-equilibrium two-component two-phase model which considers vapor transport as an independent process generally agree well with the observations in terms of evaporation behavior and soil water dynamics. Certain differences in simulation results can be observed between equilibrium and non-equilibrium approaches. Comparisons of the models and the boundary layer formulations highlight the need to revisit key assumptions that influence evaporation behavior, highlighting the need to further understand water and vapor transport processes in soil to improve model accuracy.

Highlights

  • Soil water evaporation, associated with water movement and heat transfer, plays an important role in the water cycle and energy balance across land–atmosphere interface

  • Soil water evaporation oftentimes serves as the top boundary condition for water flow and is related to soil heat flux at the surface which serves as the top boundary condition of heat flow, interacting with the soil moisture and soil temperature and further influencing the carbon (Davidson and Janssens 2006; Koven et al 2013) and nitrogen cycles (Parton et al 2001)

  • The objective of this study is to evaluate the performance of three numerical modeling concepts along with multiple top boundary conditions in simulating water and heat transport in subsurface soil, and describing evaporation from soils

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Summary

Introduction

Soil water evaporation, associated with water movement and heat transfer, plays an important role in the water cycle and energy balance across land–atmosphere interface. The dynamic interactions of heat and mass transfer are oftentimes not considered in numerical models due to the complexity of field scenarios and the lack of field and laboratory data capable of testing such models. Models vary in their level of complexity and oftentimes rely on the use of fitting parameters or key simplifying assumptions without proper understanding of their implications. An alternative approach is to use controlled laboratory-scale experiments to generate data under transient yet controlled conditions These data can be used to systematically test key model assumptions and parameters. Use of the same modeling platform with different model assumptions tested against the experimental data allows for the investigation of the dominant mechanisms and can provide guidance for the improvement of simplified parameterizations and boundary condition development

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