A Dry Surface Layer Based Bare Soil Evaporation Model: Development and Evaluation

The bare-soil evaporation formulation, which is normally characterized by a soil resistance or soil evaporation efficiency (SEE) factor, is critical for properly modeling the hydrological and energy cycles. However, the empirically-based formulations are not effective in representing a wide range of atmospheric and moisture conditions, oftentimes biasing dry or wet conditions. In this study, a dry surface layer (DSL) model is theoretically developed and tested to relate the DSL thickness to topsoil water content by considering the evolution of soil moisture profiles within the topsoil. We then integrated the newly developed DSL model into a mechanically based soil resistance formulation that considers coupled liquid water flow and vapor diffusion. The DSL model and soil resistance formulation are validated against a comprehensive dataset under field conditions. The dataset includes precision evaporation fluxes, atmospheric measurements, and soil moisture measurements at various near-surface depths. DSL model results show good agreement with experimental data and improved predictions compared to existing DSL models currently available in literature. The newly developed DSL model also performs well for different soil textures and atmospheric conditions, which demonstrates wide applicability. The modeled SEE based on the DSL-based soil evaporation formulation agrees well with the experimental data for very dry soil water conditions, sans fitting parameters. Besides, the dependency of SEE on topsoil depth and atmospheric conditions, which is shown by the field measurements, is well captured by the DSL-based soil resistance model.