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Water Vapor Diffusion in Relatively Dry Soil: III. Steady-State Experiments¹

ABSTRACT

Steady-state water vapor diffusion coefficients were measured for sorption and desorption on Adelanto loam and Pachappa loam at 27°C. The sorption diffusivities for both soil materials increase, and then decrease, with increasing water content. The desorption diffusivities for Adelanto loam are constant throughout the water content range studied, whereas the desorption diffusivities for the Pachappa loam exhibit a maximum similar to the sorption diffusivities. For both soil materials the transient and the steady-state diffusivity-water content curves have nearly the same shape and magnitude. Diffusivities calculated from adsorption isotherm data are similar to the measured diffusivities. Comparison of data from the three methods indicates that liquid diffusion becomes noticeable at water contents corresponding to relative pressures of 0.5 to 0.7 for Adelanto and 0.8 to 0.9 for Pachappa.

¹ Contribution from the Soil and Water Conservation Research Division, ARS, USDA.

² Research Physicist, U. S. Water Conservation Laboratory, Tempe, Arizona.

Received for publication December 23, 1963. Accepted for publication March 17, 1964.