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.