Inverse modeling towards parameter estimation of the nonlinear soil hydraulic functions using developed multistep outflow procedure

Abstract

Modeling and monitoring vadose zone processes have a key role to effectively identify and solve important emerging issues in hydrology and related disciplines. However, major gaps and challenges remain concerning the further conceptualization of the zone complexities and parameterization of relevant models. Consequently, the main focus of this investigation was to establish a modern developed version of the multistep outflow experiments for the vadose zone characterization and modeling. Besides, soil hydraulic properties (K-θ-h) were determined by the Mualem/Van Genuchten model based on the inverse solution of the Richards equation for the four loamy textured soils. The adjustable parameters were estimated with the least-square approach through the implementation of the generalized reduced gradient (GRG) algorithm. Then, the inverse manner was evaluated by comparing optimized hydraulic functions with those measured independently using an equilibrium condition. The results revealed the optimized attributes complied well with those determined using equilibrium analysis and steady-state situations. The reliable estimation of the parameters to inversely optimize the K-θ-h functions was possible only by measuring the cumulative outflow as a function of time. The simultaneous optimization of the unsaturated hydraulic functions carried out from a single transient experiment. This protocol appears well suited for vadose zone simulation. Furthermore, along with the tensiometer elimination and no independent measurements of Ks and θs, the developed version is generally more cost-effective, faster and more convenient than the conventional method. Finally, we conclude the combination of proposed procedure with GRG algorithm can provide a reasonable procedure to inversely estimate the hydraulic parameters.