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Evaporation from bare sandy soils is the core component of the hydrologic cycle in arid environments, where vertical water movement dominates. Although extensive measurement and modeling studies have been conducted and reported in existing literature, the physics of dry soil and its function in evaporation is still a challenging topic with significant remaining issues. Thus, an overview of the previous findings will be very beneficial for identifying further research needs that aim to advance our understanding of the vapor flow resistance (VFR) effect on soil water evaporation as influenced by characteristics of the dry soil layer (DSL) and evaporation zone (EZ). In this regard, six measurement and four modeling studies were overviewed. The results of these overviewed studies, along with the others, affirm the conceptual dynamics of DSL and EZ during drying or wetting processes (but not both) within dry sandy soils. The VFR effect tends to linearly increase with DSL thickness (δ) when δ < 5 cm and is likely to increase as a logarithmic function of δ when δ ≥ 5 cm. The vaporization-condensation-movement (VCM) dynamics in a DSL depend on soil textures: sandy soils can form a thick (10 to 20 cm) DSL while sandy clay soils may or may not have a clear DSL; regardless, a DSL can function as a transient EZ, a vapor condensation zone, and/or a vapor transport medium. Based on the overview, further studies will need to generate long-term continuous field data, develop hydraulic functions for very dry soils, and establish an approach to quantify the dynamics and VFR effects of DSLs during wetting-drying cycles as well as take into account such effects when using conventional (e.g., Penman-Monteith) evaporation models.

Original Publication Citation

Wang, X. X. (2015). Vapor flow resistance of dry soil layer to soil water evaporation in arid environment: An overview. Water, 7(8), 4552-4574. doi: 10.3390/w7084552