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a Institute of Soil Science, Univ. of Hannover, Herrenhaeuser Str. 2, 30419, Hannover, Germany
b Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
c U.S. Army Cold Regions Research and Engineering Lab., 72 Lyme Rd., Hanover, NH 03755
* Corresponding author (rhorton{at}iastate.edu)
The capillary pressure (
) in unsaturated porous media is known to be a function of temperature (T). Temperature affects the surface tension (
) of the pore water, but possibly also the angle of contact (
). Because information on the temperature dependence of 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 A derived from those WRCs, and (iii) to evaluate two models (Philip-de Vries and Grant-Salehzadeh) that describe temperature effects on . 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, 
, at five depths in the columns were measured continuously. The observations were used to determine the change in A with T, as well as a parameter ß0 that describes the change in with T. It was found that the Philip-de Vries model did not adequately describe the observed relation between and T. A mean value for ß0 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 A. For the sand and the silt we studied, we found a decrease in A between 1.0 to 8.5°, when the temperature was increased from 5 to 38°C. Both ß0 and A were only weak functions of . 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.
Abbreviations: SD, quartz sand • SDphob, hydrophopic quartz sand • SL, wettable humic soil • SLphob, hydrophobic humic soil • ST, wettable silt • STphob, hydrophobic silt • T, temperature • TDR, time domain reflectometry • WRC, water retention curve • , soil water content • , surface tension • lg, liquid-air surface tension • , capillary pressure • , angle of contact • A, apparent contact angle • sd, sessile drop contact angle
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