2-DOMAIN ESTIMATION OF HYDRAULIC-PROPERTIES IN MACROPORE SOILS

Estimation of hydraulic properties of soils having macropores is diffi cult, yet very important for describing soil-water flow dynamics. Conv entional approaches of defining macroporosity based on pore size may n ot be generally successful in quantitatively relating macroporosity to the dynamics of water flow. A definition of macroporosity based on wa ter flux at different degrees of water saturation can be expected to b e more useful. This study attempted to quantify the functional macropo rosity of field soil from in situ measurements of water content, theta (z,t), during drainage of an initially field-saturated soil. The soil was assumed to be a two-domain water flow system comprised of macropo res, which dominate the early drainage process, and the matrix pore sp ace, 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, is ohyperthermic Tropeptic Eutrustox) in Hawaii were used to test the app roach. The partitioned hydraulic conductivities obtained for the two d omains 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 macroporos ities obtained from drainage calculations for three soil depths were v ery similar to those obtained from water retention measurements on und isturbed soil cores from the same field site. The proposed approach th us appears to be a promising method for evaluating hydraulic propertie s for a well-drained soil profile containing macropores.