Hysteresis of the Water Retention Curve: Wetting Branch Simulation Based on the Drying Curve

Hysteresis of the soil water retention curve (SWRC), manifested as a difference between equilibrium curves of soil wetting and drying (hysteresis loop), is a phenomenon specific to soil hydrology, which is of practical importance for calculating irrigation norms. We have attempted to derive a model of the wetting curve from the drying curve. Modeling was based on parameters proposed by van Genuchten 6] and certain soil physical characteristics. SWRC hysteresis characteristics were obtained experimentally using capillarimetric measurements for wetting and drying curves at soil water pressures ranging from 0 to –800 cmH₂O. Two models, М-1 and М-2, based on the hypothesis of dissimilarity of parameters α for wetting (α_w) and drying (α_d) and the constancy of other parameters of the van Genuchten SWRC approximation for both curves of the hysteresis loop, have been developed for wetting curve assessment based on the drying curve. The M-1 error (RMSE = 0.05 cm^–1) was less than that of M-2 (RMSE = 0.06 cm^–1), which used clay content and soil density as predictors, as well as that of the well-known model proposed by Kool and Parker 11]. This approach to derive the wetting curve from the drying curve for a presumed correlation between the values of one parameter and equal values of the other parameters, can be used to predict an estimate of the SWRC hysteresis for a specific soil.