Heat and mass transfer in porous media under the influence of near-surface boundary layer atmospheric flow

Abstract

Bare-soil evaporation is one of the governing processes responsible for controlling heat and water exchanges between the land and lower part of the atmospheric boundary layer with direct implications to meteorology and climatology, waste isolation and storage, vadose zone remediation, and water management. Despite its obvious importance to a wide range of scientific and industry disciplines, this process remains poorly understood. This is due in part to evaporation being a complex multiphase phenomenon that must be described and understood in terms of a variety of processes that occur simultaneously at different scales; bare-soil evaporation involves the strong coupling of phase change kinematics, internal transport mechanisms, soil hydraulic and thermal properties, and atmospheric demand (. Many assumptions and simplifications are made during the description and simulation of bare-soil evaporation in order to reduce complexity as well as to address knowledge gaps resulting from a lack of high spatial and temporal datasets capable of testing and refining existing heat and mass transfer theory in coupled systems involving flow in porous media and free-fluid. There are therefore a large number of different aspects of bare-soil evaporation that need to be carefully and rigorously studied. The purpose of this research was to investigate several of the most poorly understood or least characterized areas of this phenomenon using a multifaceted approach that included precision experimentation and detailed numerical modeling. Specific investigations included: (1) testing the applicability of the combined heat-pulse and sensible heat balance method for determining evaporation rates in situ, (2) evaluating non-equilibrium phase change under different boundary and initial conditions, and (3) exploring the effects of heterogeneous porous surfaces on conditions in the shallow subsurface and near-surface boundary layer. Findings from these three studies led to the refinement of heat and mass transfer theory in continuum scale numerical models and the realization that further improvement will require upscaling to a larger experimental scale in order to be able to observe the feedback mechanisms between the land and atmosphere. This prompted an additional two studies that focused on using a climate-controlled closed-circuit wind tunnel interfaced with an intermediate scale (7.3 m) long soil tank to investigate the feedbacks between key atmospheric and soil state variables in addition to the effect of scale on observed evaporative behavior in terms of fetch.