Water and heat transport in shallow subsurface soil and across the soil-air interface: simulation, experiments and parameterization

Abstract

Knowledge and understanding of heat and water transport in vadose zone and across the soil-air interface is important to predict the hydrological process, pesticide vaporization and transport, and greenhouse gas emission. However, properly representing the mass & energy exchange process in modeling efforts has been a challenge due to the complexity of the numerical modeling scenarios and the scarcity of data capable of testing such models. Models differ in representations of physical processes and spatial heterogeneity, formulations of the processes, and parameterizations in those formulations, contributing to uncertainties in modeling predictions. Increasing confidence in model predictions requires advancing the understanding of the fundamental physical processes and improving the model representations of the mass & energy exchange processes. This study aims to advance our understanding and modeling of mass & energy exchange processes at the soil-air interface to improve the model predictions of surface fluxes and subsurface soil conditions. To achieve this goal, fine-scale experiments and numerical studies are used to investigate some of the physical processes/parameterizations that are often overlooked/oversimplified in numerical models of mass & energy exchanges at the soil-air interfaces. The first work evaluates the performance of modeling concepts along with soil evaporation formulation (i.e., top boundary condition) in simulating water and heat transport in subsurface soil, and describing evaporation from soils. Results indicate that neglecting the vapor transport process can lead to smaller soil evaporation predictions compared with the experimental results. Results also show that the choice of evaporation formulation has a significant effect on the model predictions. A numerical model that considers the vapor transport process and non-equilibrium phase change generally agrees well with the observations. This model is modified and used in the second work, which focuses on the effect of soil vertical heterogeneity on soil moisture and evaporation dynamics. Results show that the top soil layer can significantly affect the evolution of soil moisture profiles and evaporation dynamics hence illustrates the importance of characterizing top soil properties. A dry surface layer (DSL) develops fast when a coarse-textured layer is on the top which significantly suppresses evaporation. The above understandings are integrated into the third work, in which a DSL-based bare soil evaporation formulation is developed and validated against a comprehensive field dataset. The integrated DSL-based soil resistance formulation improves the application of the original parameterization to the dry soil moisture region, expanding its potential region of application, especially in arid and semi-arid regions. Considering the model’s usability for varying soil moisture conditions, soil textures, and atmospheric boundary conditions, the integrated DSL-based soil resistance formulation can improve the bare-soil evaporation prediction when used in large-scale models or remote sensing methods. The work provides a critical step toward improving our understanding and modeling of multiphase fluid transport mechanisms in subsurface soil and mass, energy exchange at the soil-air interface. Results include improved understanding of the effect of coupling vapor transport process and accounting for soil vertical heterogeneity in numerical models on surface fluxes and subsurface soil conditions. With the new understanding, this work results in the development of a DSL-based soil resistance formulation to mechanistically represent mass exchange processes at the soil-air interface. New knowledge and modeling approaches will result in improved predictions for environmental, agricultural, and engineering fields.