Analysis of a capillary hysteresis model based on a one‐variable distribution function

Parlange's (1976) model corresponds to a special case of Mualem's similarity hypothesis ƒ(ψ_w, ψ_d) = h(ψ_w)l(ψ_d) in which h(ψ) is set at unity for all ψ values. Interpreted in terms of the soil water domain theory this assumption implies that the relative pore volume of the domains is distributed uniformly in respect to the wetting radius (or to ψ_w). In this paper the proper mathematical equation is derived for calibrating the model from the experimental main drying curve. The applicability of Parlange's model for the soil water hysteresis is theoretically analyzed and extensively tested for different types of porous media. Theoretical hysteretic curves derived by direct implementation of Parlange's model are compared with experiments. These comparisons show that Parlange's model contradicts well‐known properties of the soil moisture characteristics. The good results reported by Parlange are not obtained when actual measured curves of the hysteresis loop are used. Whether the main branch of hysteresis for wetting or for drying is used in calibration, one obtains badly distorted shapes of hysteresis curves. Parlange's suggestion for calibrating the model on the basis of the main drying curve plus one additional point from the main wetting curve is considered too arbitrary to be reliable.