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Determination of Soil Water Diffusivity by Sorptivity Measurements¹

ABSTRACT

A new method is proposed for determining the dependence of soil water diffusivity and conductivity on water content or pressure head in the tensiometer range. A weighted mean diffusivity is used to linearize the one-dimensional absorption problem. The resulting cumulative absorption is equated to that of the exact nonlinear solution to obtain an expression for the diffusivity in terms of sorptivity. To use this result, sorptivities must be measured for a series of step-function increases in the water content (pressure head) at the absorption interface. Such sorptivity measurements are quickly and easily made in situ.

The method was tested on a numerical example with nearly perfect results. Also, sorptivity measurements were made on laboratory soil columns and the derived hydraulic conductivities compared well with those measured directly under steady state conditions.

¹ Contribution from the Agricultural Research Service, USDA and the Wisconsin Agr. Exp. Sta., Madison.

² Soil Scientist, presently at the US Salinity Laboratory, P. O. Box 672, Riverside, CA. 92502.

Received for publication July 1, 1974. Accepted for publication September 5, 1974.