Estimating Soil Water Retention Curve Including Hysteresis Using Inverse Modeling and Scott's and Mualem's Models in two Different Soils

Water movement in unsaturated soils is commonly affected by hysteresis which is often ignored in the mathematical description of water flow and solute transport. In this study، hysteresis in soil water retention curve was quantified at lysimeter scale in sandy loam and silty loam soils and results of the Scott’s and Mualem’s models were compared. Three undisturbed soil monoliths (80 cm diameter by 105 cm height) were collected for each soil and water movement was studied for various boundary conditions. Monoliths were equipped with different measuring devices to monitor soil water contents، pressure heads، and outlet water fluxes. Four unsaturated steady state water flow experiments were conducted on each monolith. Hydraulic properties were determined by inverse method using the WAVE model and a global multilevel coordinate search inversion algorithm. First soil hydraulic parameters in the soil water retention and hydraulic conductivity functions were estimated neglecting hysteresis and compared with the functions obtained using approaches considering hysteresis. Hysteresis was simulated using the simple empirical model proposed by Scott and using the universal conceptual model of Mualem. Results of measured and modeling data indicated that hysteresis was observed in both soils. The effect of hysteresis was different on various components of water flow (i. e.، pressure head، water content، and water flux) in two studied soils. Water retention curves، pressure head، and water content were significantly influenced by hysteresis in both soils. The Scott''s model considerably improved pressure head estimates in the silty loam 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.