Estimating hysteresis in the soil water retention curve from monolith experiments

Water movement in unsaturated soils is commonly affected by hysteresis, a phenomenon often ignored as to keep the mathematical description of water flow and solute transport simple. In this study, the importance of hysteresis was quantified at lysimeter scale on two Belgian soils, a loamy and a sandy loam soil. Undisturbed soil monoliths (79 cm diameter by 105 cm height) were collected in each soil and the flow studied for various boundary conditions. Each monolith was equipped with different measuring devices to monitor online soil water content, pressure head, and outlet water fluxes. Four unsaturated steady state flow experiments were conducted on each monolith. Hydraulic properties were obtained by inverting the governing flow equation of Richards, using a global multilevel coordinate search inversion algorithm. Measured soil water contents, pressure heads, and outlet water fluxes were used in the objective function. First soil hydraulic parameters in the retention and hydraulic conductivity functions were estimated neglecting hysteresis and compared with the functions obtained using approaches considering hysteresis in their analysis. Hysteresis was incorporated in the retention curve using the simple empirical model proposed by Scott et al. (1983) and using the universal conceptual model of Mualem (1977). Hysteresis in both soils was characterized by a relative large difference between the main drying and wetting soil water retention curves. However, results showed that the effect of hysteresis was different on various components of water flow in the two studied soils. The Scott's model considerably improved pressure head estimates in the loamy soil and Mualem's model somewhat improved the soil water content estimates in the sandy loam soil. Outlet water fluxes in both soils were less sensitive to hysteresis. In general, the simple hysteretic model of Scott was more successful in studying hysteresis in both studied soils. (C) 2012 Elsevier B.V. All rights reserved.