Effects of Gravity on Evaporation for Soil‐Limited Conditions

This manuscript systematically investigates the effects of gravity during soil‐limited (Stage 2) evaporation from bare soil. Analytical and numerical solutions to the one‐dimensional Richards soil moisture flow equation are developed based on simplified hydraulic functions that depend exponentially on soil water matric potential. Semi‐infinite domains with either freely draining or hydrostatic bottom boundary conditions and finite domains with no‐flow bottom boundary conditions are considered. Cases with gravity are directly compared to those without. The simplified solutions are verified by comparison to numerical solutions with fully realistic soil hydraulic functions for two soils at the extreme ends of the range of possible soil textures. Cumulative evaporation versus time and soil volumetric water content versus depth at selected times are determined for each case from the simplified solutions. These results are reported in terms of non‐dimensional variables that are universal, spanning all soil textures and other conditions. For the semi‐infinite domain cases cumulative evaporation with gravity is reduced by several orders of magnitude for very coarse soils at times of practical interest while gravity has a negligible effect for the finest textures at those times. For the finite domain cases gravity is less important because cumulative evaporation is ultimately limited by the fixed amount of water in the profile. Plain Language Summary This manuscript examines how gravity influences evaporation of water from initially wet soils during the second stage of drying when the evaporation rate is limited by the internal resistance to water flowing up to the soil surface that exists within soil pores. The paper uses analytical and numerical solutions of the standard flow equation and produces results that should be valid for all soil textures. Key Points Gravity has large effects on soil‐limited evaporation for coarse textured soils Gravity has negligible effect on soil‐limited evaporation for very fine textured soils The Richards equation with exponential hydraulic functions yields good estimates of cumulative evaporation for various boundary conditions