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Thermal Conductivity of Soils as a Function of Temperature and Water Content¹

ABSTRACT

Apparent thermal conductivities of loamy sand, loam, and silty clay loam soils were measured with a cylindrical heat probe at several water contents and temperatures. Values of were also calculated with the de Vries model. Results show that the model may be used satisfactorily to calculate . However, improvements may be needed to account for the enhancement of vapor transfer at high temperatures (45° C) in medium to fine textured soils. The heat probe method of measuring the thermal conductivity requires a correction factor to allow for errors due to the entrapment of air at the surface of the heat probe, when used at water contents ranging from 0 to about 30% of saturation.

The contribution of vapor distillation to thermal conductivity was analyzed by comparing at 25 and 45° C. The ratios ₄₅/₂₅ were nearly equal to unity when < 5% of total pore space was filled with water. The ratios increased, due to vapor distillation, as water filled the pores and reached maximum values of 2.17, 2.71, and 2.96 when 22, 27, and 35% of total pore space was filled with water, corresponding to soil water potentials of –0.8, –9.0, and –8.0 bars for loamy sand, loam, and silty clay loam, respectively. As the water content increased further, the ratios decreased and approached unity when > 50% of total space was filled with water.

The apparent thermal conductivity was independent of water content at very low water contents. The water molecules are in layers only a few molecules thick. The water content below which the apparent thermal conductivity is not affected by water content is a function of the soil temperature and the clay content. In our experiments these water contents were 0.03 and 0.05 cm³/cm³ at 45° C and 0.13 and 0.18 cm³/cm³ at 25° C for the loam and silty clay loam respectively.

¹ Technical Paper no. 5000, Oregon Agric. Exp. Stn., Corvallis, OR 97331. Contribution from the Dept. of Soil Science, Oregon State University. Research supported in part by the Office of Water Research and Technology, USDI.

² Formerly Research Assistant, Dept. of Soil Science, OSU, now Associate Professor, Dept. of Irrigation, Pahlavi Univ., Shiraz, Iran and Professor, Dept. of Soil Science, Oregon State University, respectively.

Received for publication November 13, 1978. Accepted for publication January 24, 1979.

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