Temperature dependence of water retention curves for wettable and water-repellent soils

The capillary pressure (psi) in unsaturated porous media is known to be a function of temperature (T). Temperature affects the surface tension (sigma) of the pore water, but possibly also the angle of contact (gamma). Because information on the temperature dependence of gamma 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 gamma(A) derived from those WRCs, and (iii) to evaluate two models (Philip-de Vries and Grant-Salehzadeh) that describe temperature effects on psi. Columns packed with natural or hydrophobized soil materials were first water saturated, then drained at 5, 20, and 38 degrees C, and rewetted again to saturation. Capillary pressure and water content, theta, at five depths in the columns were measured continuously. The observations were used to determine the change in gamma(A) with T, as well as a parameter beta0 that describes the change in psi with T. It was found that the Philip-de Vries model did not adequately describe the observed relation between psi and T. A mean value for beta0 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 gamma(A). For the sand and the silt we studied, we found a decrease in gamma(A) between 1.0 to 8.5 degrees, when the temperature was increased from 5 to 38 degrees C. Both beta0 and gamma(A) were only weak functions of theta. 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.