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The Soil Water Diffusivity Near Saturation¹

ABSTRACT

Many laboratory porous materials are considered to have a zone of tension-saturation in their water retentivity curve near saturation. This implies that the soil water diffusivity, D, is infinite in such a region. Here three new methods of measuring the diffusivity near saturation are presented. Experiments were carried out on a fine sand and all methods found D to be finite, yet large, having a value of approximately 1 to 2 x 10^–3 m² s^–1. Since the near-saturated conductivity of the sand is of the order 10^–5 m s^–1 this implies that the water retentivity curve near saturation has a finite but large slope of order 10² m. This was compatible with the measured retentivity data. The theory of one-dimensional absorption implies that the slope of the Boltzmann variable with respect to volumetric water content near saturation equals twice D divided by the sorptivity. The slope calculated this way was compatible with that found in absorption experiments.

¹ Contribution from the Plant Physiology Division, DSIR, Palmerston North, and Applied Mathematics Division, DSIR, Wellington, New Zealand.

² Scientists, DSIR.

Received for publication December 6, 1982. Accepted for publication March 17, 1983.