Near-Saturated Hydraulic Properties of Macroporous Soils

Simulations with dual-permeability models are sensitive to the description of the hydraulic properties in the near-saturated region. Yet mostly due to a lack of supporting data, very little is known about the suitability of different approaches or how the parameters describing hydraulic functions in these models can be estimated. In this study, a simple model of macropore hydraulic properties was derived from the generalized Kozeny-Carman equation, the pore-scale tortuosity factor proposed by Fatt and Dykstra, and the functional form for the soil water characteristic described by Ross and colleagues. The model was fitted to published data on near-saturation wetting characteristics and hydraulic conductivity for seven soils-five undisturbed soils in the field and two repacked under laboratory conditions. Considering the diffculties involved in measuring small changes in water content in the region close to saturation, the model described the field data quite well providing the tortuosity factor was set to zero. For a small data set consisting of near-saturated conductivity measurements on 12 Swedish soils, the pore size distribution parameter in the Ross et al. model could be related to clay content and macroporosity. It was concluded that this simple model shows promise, both as a description of macropore hydraulic properties in dual-permeability models and as a conceptual framework to support the development of pedotransfer functions for near-saturated and saturated hydraulic conductivity.