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Porosity and Soil-Water Diffusivity Relations¹

ABSTRACT

Water at –2mb pressure was absorbed by horizontal columns of three soil materials of differing texture each at three porosities, each porosity in duplicate. The soil materials were Adelanto loam, Pachappa loam, and Pine silty clay. From the water content distributions, soil-water diffusivities were calculated, assuming applicability of diffusion theory. For a total porosity change of about 0.05, Pachappa loam exhibited little change in soil-water diffusivity but Adelanto loam and Pine silty clay exhibited twofold to threefold changes in diffusivity at the same relative water content. Cumulative infiltration and distance of penetration of water into horizontal soil columns decreased 50% and 40%, respectively, for Adelanto and Pine and 15% and 12%, respectively, for Pachappa when the total porosity was decreased 0.05.

Water content distributions were calculated using exponential diffusivity-water content functions obtained by a least squares fit to the diffusivity data measured over a limited water content region. Predictions of water contents based on these functions were considerably in error.

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

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

Received for publication May 7, 1962. Accepted for publication September 28, 1962.