A soil water retention curve that can transition between Brooks-Corey and van Genuchten can be made hysteric in principle.

The van Genuchten-parameterization (vG) of the soil water retention curve (SWRC) has been very popular for over four decades, but it has some physical inconsistencies at the dry and the wet end. The recently introduced Rossi–Ippisch–Adaptation (RIA) of vG resolves these by introducing an air-entry value and eliminating the residual water content. Unlike any other parameterization of the SWRC, RIA can transition smoothly from a sigmoidal vG-type curve to a power-law Brooks-Corey curve. We elucidate how α determines the existence and location of an inflection point, which determines if the resulting curve is sigmoidal or not. We also present a criterion to determine the limit at which the RIA curve converges to the more parsimonious Brooks–Corey power-law form. As a preparation for future work, we explored if a hysteric version of RIA is feasible. Expressions for its main drying and wetting curves are provided, showing that hysteresis in RIA necessarily induces hysteresis in the shape parameters α and n. A practical closed-form relation is proposed to estimate a hysteretic parameter set for the main wetting curve when measurements are only available for the main drying curve.