Coupled free-flow/pore-network modeling of nonequilibrium nonisothermal evaporation in soils at the land surface: Impact of water film flow and mass and heat transfer across air-water interfaces

Evaporation is a key physical process governing mass and energy fluxes at the land surface. Soil evaporation is controlled by a wide range of factors including atmospheric temperature, humidity, and air flow, and soil physical processes. In particular, water film flow along soil grain surfaces and the nonequilibrium mass and heat transfer across the pore-scale air-water interfaces are expected to strongly influence the evaporative processes. However, these pore-scale mechanisms remain poorly understood and are not rigorously represented in macroscopic mathematical models for evaporation. We extend a prior coupled free-flow/pore-network modeling framework to examine these nonequilibrium and nonisothermal processes at the pore-scale. The modeling framework couples single-phase air flow, vapor transport, and heat transfer in the atmosphere with two-phase air-water flow, vapor transport, and heat transfer in the soil. The air flow, vapor transport, and heat transfer in the atmosphere are modeled by the Navier-Stokes formalism, while the air-water flow, vapor transport, and heat transfer in the soil are represented by a dynamic compositional pore-network model. The prior modeling framework neglected water films and assumed thermodynamic equilibrium between air and water at the pore-scale. In the present work, we extend the modeling framework to include water film flow and nonequilibrium mass and heat transfer across the air-water interfaces in the soil. Employing this new coupled free-flow/pore-network modeling framework, we conduct a series of numerical simulations to quantify the impact of water film flow and nonequilibrium air-water interfacial mass and heat transfer on the evaporation in soils for a wide range of atmospheric conditions and soil types. Our simulations suggest that water film flow increases the water supply from deeper soils to the land surface leading to greater evaporation rates. However, the nonequilibrium mass and heat transfer limitation across the air-water interfaces reduces evaporation in certain atmospheric flow regimes.