Determination of water content in drying soils: incorporating transition from liquid phase to vapour phase

Abstract

In arid and semi-arid environments, soil profiles often exhibit a liquid–vapour displacement known as evaporation front characterised by a critical matric potential (ψ_me) or water content (θ_e) located somewhere inside the unsaturated zone above a watertable (WT). The objective of this study was to determine the θ_e including the range of water content (θ) in the transition zone from liquid to vapour both theoretically and experimentally for different soil textures under saline and non-saline WTs. Characteristic shapes of water content and salt concentration profiles were the criteria to obtain θ_e experimentally, and the θ–diffusivity relationship was used to compute the θ_e and θ range in the transition zone. Measured θ_e values of 0.05 and 0.12 m³/m³ under non-saline WT and 0.07 and 0.15 m³/m³ under saline WT were in agreement with the computed values of 0.05 and 0.10 m³/m³ for sandy loam and clay loam soils, respectively. The model calculates roughly the same θ_e for saline and non-saline conditions. Besides experimental soils, θ_e and range of θ in the transition zone were calculated for silty loam and coarse sand. The lighter the soil texture, the smaller is θ_e and the steeper the transition zone. The results were further compared with those calculated by different authors.