Thermohydromechanical Modeling of Evaporation from Unsaturated Soil

Abstract

The accelerating pace of development due to groundwater overwithdrawal throughout the world places increasing demands on the unsaturated zone. Estimating evaporation from unsaturated soil has been important for many geotechnical and geoenvironmental applications. As an example, the long-term performance of soil covers, which are widely used in mining and landfill applications to protect the environment, depends on evaporation from their surfaces. Evaporation, in turn, depends on moisture flow within an unsaturated soil mass, which is generally coupled with heat flow. Evaporation also depends on the void ratio and hydraulic and air conductivities of soil, which are, in turn, affected by stress and strain and the resulting soil settlement. As a result, a thermohydromechanical (THM) analysis for predicting evaporation is necessary. A two-dimensional (2D) finite-element program, θ-Stock, can perform THM analysis of unsaturated soil but works only within the soil, not in the atmosphere above the soil. In this study, a 2D model of soil–atmosphere interaction employing atmospheric coupling and using a modified Penman equation was implanted in θ-Stock to develop program EVAP1, which numerically estimates potential and actual evaporation from unsaturated soil using THM analysis. Because of the coupled character of the phenomena, the boundary conditions were determined by the soil state and the environmental data. The program EVAP1 was verified with published experimental and numerical studies on evaporation. The results of analyses using the verified program showed that neglecting THM behavior, and hence soil settlement, leads to an overestimation of evaporation from unsaturated soil surfaces.