3-PARAMETER LOGNORMAL-DISTRIBUTION MODEL FOR SOIL-WATER RETENTION

Many models for soil water retention have been proposed. However, most of these models are curve-fitting equations and do not emphasize the physical significance of their empirical parameters. A new retention m odel that exhibits increased flexibility was developed by applying thr ee-parameter lognormal distribution laws to the pore radius distributi on function f(r) and to the water capacity function, which was taken t o be the pore capillary pressure distribution function f(psi). This mo del contains three parameters that are closely related to the statisti cs of f(psi): the bubbling pressure psi(c), the mode psi0 of f(psi), a nd the standard deviation sigma of transformed f(psi). By comparison o f this model with three existing models (the van Genuchten model, the Brooks-Corey model, and the modified Tani model), it was shown that ps i(c), psi0, and sigma are all essential for a general retention model.