Minasny and McBratney, 2007), and the use of tables * 1 Supported by the National Natural Science Foundation of China

Abstract

ABSTRACT The van Genuchten (vG) function is often used to describe the soil water retention curve (SWRC) of unsaturated soils and fractured rock. The objective of this study was to develop a method to determine the vG model parameter m from the fractal dimension. We compared two approaches previously proposed by van Genuchten and Lenhard et al. for estimating m from the pore size distribution index of the Brooks and Corey (BC) model. In both approaches we used a relationship between the pore size distribution index of the BC model and the fractal dimension of the SWRC. A dataset containing 75 samples from the UNSODA unsaturated soil hydraulic database was used to evaluate the two approaches. The statistical parameters showed that the approach by Lenhard et al. provided better estimates of the parameter m. Another dataset containing 72 samples from the literature was used to validate Lenhard's approach in which the SWRC fractal dimension was estimated from the clay content. The estimated SWRC of the second dataset was compared with those obtained with the Rosetta model using sand, silt, and clay contents. Root mean square error values of the proposed fractal approach and Rosetta were 0.081 and 0.136, respectively, indicating that the proposed fractal approach performed better than the Rosetta model. Many soil and water management and environmental protection practices require knowledge of the evolution of water and solutes in the subsurface. During the past several decades, a large number of computer models have been developed to simulate water flow and contaminant transport in saturated and unsaturated soils and fractured rock. Their application is often restricted by a lack of hydraulic property information involving the soil water retention curve (SWRC) and the unsaturated hydraulic conductivity. Furthermore, due to inherent temporal and spatial variability of the hydraulic properties in the field, large numbers of samples are generally required to properly characterize the spatial distribution of the hydraulic properties. Accurate characterization and estimation of the SWRC has been a major focus of research for more than 60 years. Many empirical model