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Thermally Driven Liquid and Vapor Phase Transfer of Water and Energy in Soil¹

ABSTRACT

The basic rate equation developed from Onsager's theory and thermodynamics of irreversible processes was modified for liquid phase moisture diffusion. This was done by accounting for the thermal dependence of the phenomenological coefficients with Eyring's theory of rate processes.

The total simultaneous heat and water transfers through unsaturated soil samples were measured as a function of temperature gradient. By making use of the thermal dependence of vapor flow as reflected in the heat flux data, it was possible to separate the total water transfer into components of vapor phase flow and liquid phase flow. The rate equations were then tested and found to give reliable predictions of water and energy transfer through soil over the range of conditions applied. The interaction coefficients between heat and moisture transfer were also evaluated and found to be approximately the same, which was in agreement with the Onsager theory.

¹ Joint contribution Soil and Water Conservation Research Division, ARS, USDA, and Utah Agri. Exp. Sta., Logan. This work was supported in part by Western Regional Research Project W-68 and by contract with the U. S. Army Electronic Proving Ground, Fort Huachuca, Ariz. Approved by the Director, Utah Agr. Exp. Sta., as Journal Paper 184.

² Soil Scientist, Southwest Branch, Soil and Water Conservation Research Division, ARS, USDA, and Professor of Agronomy (Soil Physics), Utah State University, Logan, respectively.

Received for publication May 26, 1961. Accepted for publication October 2, 1961.