Two-Domain Estimation of Hydraulic Properties in Macropore Soils

Estimation of hydraulic properties of soils having macropores is difficult, yet very important for describing soil-water flow dynamics. Conventional approaches of defining macroporosity based on pore size may not be generally successful in quantitatively relating macroporosity to the dynamics of water flow. A definition of macroporosity based on water flux at different degrees of water saturation can be expected to be more useful. This study attempted to quantify the functional macroporosity of field soil from in situ measurements of water content, θ (z,t), during drainage of an initially field-saturated soil. The soil was assumed to be a two-domain water flow system comprised of macropores, which dominate the early drainage process, and the matrix pore space, which is responsible for drainage occurring after macropores are emptied. The unit hydraulic gradient approach of calculating hydraulic conductivity was extended and applied to the two-domain system. Field-measured data for a well-drained Wahiawa soil (clayey, kaolinitic, isohyperthermic Tropeptic Eutrustox) in Hawaii were used to test the approach. The partitioned hydraulic conductivities obtained for the two domains appeared qualitatively realistic, and when summed, resulted in a composite saturated conductivity which was close to that measured by the in situ instantaneous profile method. In addition, the macroporosities obtained from drainage calculations for three soil depths were very similar to those obtained from water retention measurements on undisturbed soil cores from the same field site. The proposed approach thus appears to be a promising method for evaluating hydraulic properties for a well-drained soil profile containing macropores. School of Ocean and Earth Science and Technology Contribution no. 3138.