Physical significance of the parameters in the van Genuchten model

The van Genuchten model is widely used in fitting soil-water characteristic curves. However, it has numerous parameters with the complex physical significance, and especially the parameter α has not been well understood so far. This paper discusses the physical significance of the parameters α, m and n in the van Genuchten model on the basis of the results of both theoretical derivation and the measured values of the soil-water characteristic curves of the samples (silt sand, fine sand, medium sand, coarse sand), and focuses on the relationships between parameter α and air entry values or matrix potential at the inflection point (hi). The results illustrate that the relationship between the values of 1/α and hi is related to not only hi but also the parameters m and n, and the value of 1/α is approximately equal to hi for the medium sand and coarse sand, while it is greater than hi for the fine sand and silt sand. The air entry value is equal to value of 1/α for the coarse sand, and the complex relationships occur for the other media. In addition, there is a higher degree of accuracy when the water-soil characteristic curves are fitted as m and n are independent variables. It was necessary to give a constraint condition, i.e., m=1-1/n, when the unsaturated hydraulic conductivity is calculated.