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The Measurement of Soil Moisture Diffusivity¹

ABSTRACT

The solution of the differential equation describing the flow of moisture in unsaturated porous media is difficult because of the dependence of the permeability on moisture content. In the case of horizontal flow this equation can be written in a form similar to the non-linear diffusion equation. The diffusivity in this equation is dependent upon the moisture content of the medium. Boltzmann has developed a treatment of the non-linear diffusion equation which allows one to calculate the diffusivity-moisture content function from a moisture content distribution curve. This treatment assumes that the moisture content is a function of a variable dependent on distance and the square root of the time.

Columns of sand at a constant moisture content throughout their length were prepared. Water was applied at one end of the columns and allowed to move into them for a measured period of time. The distribution of moisture content in the columns was then determined and calculations of the diffusivity-moisture content function were made. Data from one column gave moisture diffusivity values representing the entire moisture range of that column. The results indicate that there may be a maximum in the diffusivity-moisture content function at a moisture content less than saturation.

¹ Contribution from the Department of Agronomy, Illinois Agr. Exp. Sta., Urbana, Ill. Published with the approval of the Director. Part of a thesis submitted in partial fulfillment of the requirements for the Ph.D. degree at the University of Illinois by the senior author. Presented before Div. I, Soil Science Society of America, Aug. 17, 1955, Davis, Calif.

² Former graduate assistant and assistant professor of Soil Physics, respectively, University of Illinois, Urbana, Ill.

Received for publication November 11, 1955.