Enhancement of Thermal Water Vapor Diffusion in Soil
1984
Cass, A. | Campbell, G. S. | Jones, T. L.
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 η of Philip and de Vries and the phenomenological enhancement factor β of Cary are calculated from the slope of the relationship between thermal conductivity and reciprocal relative pressure. The enhancement factor, η 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, η increased to 15. The inflection point water content for the η 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 β 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 β derived in this paper except at intermediate saturation and temperature values.
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