Effect of Land use Change on Unsaturated Hydraulic Conductivity under Unsteady State Flow Condition and Evaluation of Some Global Databases

Zhang's method can be used to analyze the unsteady state flow of water in the soil. The aim of this study was determining a suitable method for estimating the amounts of saturated and unsaturated hydraulic conductivity in pasture, orchards, and other agricultural land uses and how it is affected by land use change. Originally, many orchards and cropped lands were pasture land. Field infiltration rate experiments were conducted using tension disc infiltrometer at tensions of 0, 3, 6, 10 and 18 cm of water with three replications for each land use. Hydraulic conductivity values for different land use were determined by Zhang method. To estimate hydraulic parameters of Van Genuchten soil hydraulic conductivity model in Zhang's method, Rawls et al. and Carsel and Parrish databases, extracted results of ROSETTA code and HYDRUS-2D software were used. HYDRUS-2D inverse modeling method, also, was used to estimate soil hydraulic properties. Relative Error (RE) and root mean square error (RMSE), as statistical parameters were used to evaluate the estimated hydraulic conductivity versus the measured one. Statistical parameters values showed that, in all the three land uses, Zhang model based on Carsel and Parish had the least amount of RE and RMSE and the highest accuracy in estimating the hydraulic conductivity values. In all land uses, other methods (except Carsel and Parrish) performed weakly at low tensions, but results improved at high tensions. During the land use changefrompasturetoorchardandfield crops,saturated hydraulic conductivity valuesincreaseddue toanincrease in soil organic matter in orchards and cropped lands.