SSSAJ Journal of Natural Resources and Life Sciences Education
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Published in Soil Sci Soc Am J 27:507-511 (1963)
© 1963 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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A Test of the Validity of the Diffusion Equation for Unsaturated Flow of Soil Water1

Stephen L. Rawlins and Walter H. Gardner2

ABSTRACT

A review of the derivation of the diffusion equation from the potential equation for unsaturated flow of liquids in porous materials reveals that the only additional assumption required is that diffusivity is a unique function of water content. The validity of this assumption is tested by following absorption of water into a horizontal column of soil with a gamma ray densitometer. To ensure that required boundary conditions were met, the soil was evacuated to eliminate entrapped air or air pressure build-up ahead of the wetting-front; and water was applied quickly, evenly, and at constant negative head through a plane sintered glass bead plate, pressed against the bare plane end of the column to initiate flow. Diffusivity vs. water content curves constructed from these data reveal that diffusivity is not a function of water content alone.

NOTES

1 Scientific Paper 2307, Department of Agronomy, Wash. Agr. Exp. Sta., Washington State Univ., Pullman, Wash. Work was conducted under Project 1463, a contributing project to Regional Research Project W-68, "Water Movement in Soil." Presented before Div. I, Soil Science Society of America, St. Louis, Mo. Nov. 27, 1961.

2 Formerly Acting Instructor in Soils, and Professor of Soils, Dept. of Agron., Washington State Univ., respectively. The senior author is now Asst. Soil Physicist, Dept. of Soils and Climatology, The Connecticut Agr. Exp. Sta., New Haven.

Received for publication November 1, 1962. Accepted for publication January 16, 1963.






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