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The Temperature Dependence of Isothermal Moisture vs. Potential Characteristics of Soils¹

ABSTRACT

A method has been developed for rapid, transient measurement of hysteretic soil-moisture characteristics as a function of temperature. While a varying soil-water pressure was imposed on a thin sample by means of flexible membranes held in firm contact with the soil, water content was measured by gamma-ray attenuation, and matric potential was measured with tensiometers. The applied pressure was cycled through a program designed to obtain hysteretic () main and scanning curves. Isothermal characteristics were measured for 181-µm glass beads, Plainfield (Typic Udipsamments) sand, and an undisturbed core of Plano (Typic Argiudolls) silt loam at several temperatures in the 4° to 50°C range. At each temperature the measurements included main drying and wetting curves covering the range from 0.30 to 0.05 m³ water/m³ for glass beads, 0.30 to 0.17 for sand, and 0.45 to 0.37 for silt loam. A model has been developed to quantify the temperature dependence as a function of . Combined with an isothermal hysteresis model of Mualem, this model requires only three characteristic functions to represent all hysteretic () curves for a given medium at all temperatures. Model calculations for the sand and silt loam data indicate that except near saturation, the temperature effect is greater than can be accounted for by the temperature dependence of the surface tension of pure water. The results rule out several possible explanations but they support the hypothesis that the concentration and effectiveness of dissolved surfactants increases with temperature.

¹ Contribution from Water Resources Div., U.S. Geological Survey, Menlo Park, CA, and Dep. of Physics and Dep. of Soil Science, Univ. of Wisconsin-Madison. Done in part while the senior author was a graduate student in Physics at the Univ. of Wisconsin-Madison. Research supported in part by the Wisconsin Water Resources Center, the Wisconsin Alumni Research Foundation, and the U.S. Dep. of Agric. (Hatch formula funds project 142-UW-2607).

² Physicist, U.S. Geological Survey, Water Resources Div., 345 Middlefield Rd., Menlo Park, CA 94025; Professor, Dep. of Physics and Dep. of Soil Science, Univ. of Wisconsin-Madison, Madison, WI 53706.

Received for publication August 5, 1985.

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