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Water Uptake By Plants: II. The Root Contact Model¹

ABSTRACT

In the standard analysis of root water uptake, it is assumed there is a constant root membrane resistance in series with a soil resistance which is dependent upon the soil moisture diffusivity. The relation of root extraction rate to soil water content and to soil water potential predicted by this standard model was compared to the results of divided root experiments. The extraction rates predicted by the theory were as much as eight times larger than the measured values. A reasonable fit between theory and experiment could only be obtained by assuming in the theoretical calculations that the rooting density was 100 times smaller than that measured in the experiments. An alternative model emphasizing the possibility of a root-soil contact effect was developed. It was assumed that as the soil dried, the surface area of the roots in contact with the soil decreased, which caused an increase in the root membrane resistance. By assuming that the effective root membrane permeability was simply proportional to the relative saturation of the soil, a much better fit to the data was obtained than by using the conventional theory.

¹ Published by permission of Director of Research, Wisconsin Agric. Exp. Stn., Univ. of Wisconsin, Madison, WI 53706. Research supported in part by a grant from the Mobil Foundation.

² Former Research Assistant in Physics, Univ. of Wisconsin, now Physicist, US Geological Survey, Water Resources Division, 345 Middle-field Road, Menlo Park, CA 94025; Professor, Dep. of Physics and Soil Sci.; Professor, Dep. of Soil Sci.; both University of Wisconsin, Madison 53706.

Received for publication December 6, 1977. Accepted for publication July 8, 1977.

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