Analysis of rate-limiting processes in soil evaporation with implications for soil resistance models

Abstract Numerical integrations of coupled equations of moisture, vapor and heat diffusion in soil are analyzed to explore the relative roles of vapor and liquid fluxes in rate-limiting the transfer of water to the soil–atmosphere interface. Approximate analytical integrations of a simpler isothermal system are then introduced to explore the interactions of vapor and liquid transport. Although vapor diffusion dominates total moisture flux near the soil surface, both models indicate that liquid transport deeper in the soil limits evaporation at daily time scales for all but very dry soils. The rate-limiting role of the liquid flow is demonstrated by the insensitivity of the integrated-coupled equations to the molecular diffusivity of water vapor (Da). The mechanism underlying the insensitivity is that the depth of the drying front ( L ∗ ) shrinks in order to compensate for reductions in Da in such a way that the capillary rise to the drying front can still be transported to the surface. The feedback mechanism that causes L ∗ to shrink is the mass imbalance that would occur for reduced vapor transport out of the drying front and essentially unchanged liquid transport into it. Implications for the utility and interpretation of soil resistance terms (defined as proportional to the ratio of L ∗ to Da) employed in SVAT models are discussed.

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