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

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 (φ 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 φ e distribution. These concepts are tested on experimental data for two widely different soils, a mollisol and an oxisol. The φ e is defined as total porosity minus soil water content at −33 kPa pressure head. The exponent of the K s ‐φ 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 φ 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 φ e distribution closely matched the experimental K s ‐derived distribution. The approach has a promise for large‐scale applications.