Extension of the similarity hypothesis used for modeling the soil water characteristics

Two independent domains models are derived by using an extended similarity hypothesis according to which the pore water distribution function is interpreted to be ƒ(r, ρ) = h(r)h(ρ). When this hypothesis is applied to the Néel diagram and the Mualem diagram, it yields two different models which permit prediction of the hysteretical relationship from only one branch of the main hysteresis loop. The models yield an extremely simple formula for the relationship between the boundary drying and wetting curves. Moreover, a universal nondimensional hysteresis is derived for all soils. Comparison with experiments shows that the model which uses the Néel diagram fails to reproduce a reasonable shape of the hysteresis curves. However, much better results are achieved by the second model, which uses the Mualem diagram. In some cases the predicted curves are in very good agreement with the observed ones. For soils in which blockage against air entry plays a significant role the model is less efficient as a predictive tool. It yields poor results in the region of high water content, while the predicted curves are very close to the measured ones in the range of low water content.