Temperature Dependence of Water Retention Curves for Wettable and Water‐Repellent Soils

Abstract

The capillary pressure (ψ) in unsaturated porous media is known to be a function of temperature ( T ). Temperature affects the surface tension (σ) of the pore water, but possibly also the angle of contact (γ). Because information on the temperature dependence of γ in porous media is rare, we conducted experiments with three wettable soils and their hydrophobic counterparts. The objectives were (i) to determine the temperature dependence of the water retention curve (WRC) for wettable and water‐repellent soils, (ii) to assess temperature effects on the apparent contact angle γ A derived from those WRCs, and (iii) to evaluate two models (Philip‐de Vries and Grant‐Salehzadeh) that describe temperature effects on ψ. Columns packed with natural or hydrophobized soil materials were first water saturated, then drained at 5, 20, and 38°C, and rewetted again to saturation. Capillary pressure and water content, θ, at five depths in the columns were measured continuously. The observations were used to determine the change in γ A with T , as well as a parameter β 0 that describes the change in ψ with T It was found that the Philip‐de Vries model did not adequately describe the observed relation between ψ and T A mean value for β 0 of −457 K was measured, whereas the Philip‐de Vries model predicts a value of −766 K. Our results seem to confirm the Grant‐Salezahdeh model that predicts a temperature effect on γ A For the sand and the silt we studied, we found a decrease in γ A between 1.0 to 8.5°, when the temperature was increased from 5 to 38°C. Both β 0 and γ A were only weak functions of θ. Furthermore, it seemed that for the humic soil under study, surfactants, i.e., the dissolution of soil organic matter, may compound the contact angle effect of the soil solids.