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Macroporosity to Characterize Spatial Variability of Hydraulic Conductivity and Effects of Land Management¹

ABSTRACT

To develop simplified methods of hydraulic characterization of field soils and effects of management, frequency distribution of macroporosity (or effective porosity) in a soil is investigated as a measure of its saturated hydraulic conductivity distribution. The effective porosity (_e) of a soil is related to its saturated hydraulic conductivity (K_s) by a generalized Kozeny-Carman equation. The exponent of this relationship is assumed to vary within a narrow range (value of 4 or 5). The equation is then combined with scaling theory to derive the frequency distribution of K_s scaling factors from the _e distribution. These concepts are tested on experimental data for two widely different soils, a mollisol and an oxisol. The _e is defined as total porosity minus soil water content at –33 kPa pressure head. The exponent of the K_s-_e relationship is found to be nearly 4 for the soil-core data of both soils, while for a smaller set of in-situ field data for oxisol, which was within a narrow range of _e, the value of the exponent was smaller. There was a considerable scatter in the relationships. However, with the exponent set equal to 4 or 5 the distribution of K_s scaling factors derived from _e distribution closely matched the experimental K_s-derived distribution. The approach has a promise for large-scale applications.

¹ Contribution from Water Quality and Watershed Research Laboratory, USDA-ARS, P. O. Box 1430, Durant, OK 74702.

² Soil Physicist and Geologist, Durant, Professor of Soil Science, Univ. of Hawaii, Honolulu, and Professor of Soil and Water Science, Univ. of California, Davis, respectively.