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Enhancement of Thermal Water Vapor Diffusion in Soil¹

ABSTRACT

The enhancement of water vapor diffusion in soil resulting from a temperature gradient was determined using a transient state thermal conductivity measurement. The method of Parikh et al. (1979) was adapted for this study to allow measurement of the thermal conductivity as a function of temperature, water content, and pressure. The data allowed separation of thermal conductivity from thermally induced latent heat transport. Both the mechanistic enhancement factor of Philip and de Vries and the phenomenological enhancement factor ß of Cary are calculated from the slope of the relationship between thermal conductivity and reciprocal relative pressure. The enhancement factor, approaches 1 for dry soil and increases rapidly to values around 10 for a sand and a silt loam at water contents above half saturation and temperatures above 22°C. At high water content and 3.5°C, increased to 15. The inflection point water content for the vs. saturation curve was about twice as high in silt loam as in sand. Measurements conform well to the pattern predicted by a theoretical examination of enhancement factor behavior. Experimental results were also compared with two recent quantitative models of ß from Jury and Letey (1979) and Cary (1979). These model predictions did not agree with measured values, nor did they conform to the theoretical limits of ß derived in this paper except at intermediate saturation and temperature values.

¹ Contribution from the Dep. of Agronomy and Soils, Washington State Univ., Pullman, WA 99164, and Battelle, Pacific Northwest Lab., Richland, WA 99352. This work is supported by the Dep. of Energy's Low-Level Waste Management Program under Contract no. DE-AC06-76RL0 1830.

² Research Associate, Professor of Soils, Washington State Univ., and Research Scientist, Pacific Northwest Lab., respectively. The work was conducted while the senior author was on leave from the Univ. of Natal, South Africa.

Received for publication March 8, 1983. Accepted for publication September 10, 1983.

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