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Soil Moisture Transport Due to Thermal Gradients: Practical Aspects¹

ABSTRACT

The present status of knowledge concerning thermally induced transport of moisture in soil is reviewed. This includes the various theories on mechanisms of transport and experimental data showing the magnitude of moisture flow in various porous materials. Wherever possible these data were chosen to show the relative importance of thermal versus head-type flow and some general trends are noted.

Simple equations are developed to describe the thermally induced moisture flow near the soil surface which arises from transient thermal gradients produced by the diurnal temperature cycle. Calculations of the moisture flux over 2 ten-hour time intervals for a typical field situation indicate that thermal water transport should be considered whenever moisture, salt, or heat fluxes are being studied in the soil's surface layers.

The upward flow of soil water against a moisture content gradient in the winter is also considered. A sample calculation of the amount of thermally driven moisture was made using data available in the literature. The result suggested that the thermal moisture flow was too small to account for the net movement of soil moisture into the frost zone.

¹ Contribution from the Soil and Water Conservation Research Division, ARS, USDA, in cooperation with the Dep. of Water Science & Engineering, University of California, Davis. Presented at the annual meeting of the Soil Science Society of America, Columbus, Ohio, Nov. 1, 1965, as part of the Symposium on "Practical Applications of Recent Advances in Knowledge of Flow of Water in Soils."

² Soil Scientist, USDA, Snake River Conservation Research Center, Kimberly (Twin Falls), Idaho.

Received for publication November 16, 1965. Accepted for publication April 6, 1966.