Investigation of estimation of hydraulic parameters in heterogeneous soil

Data availability for in situ spatial variability assessment of hydraulic parameters is always limited in the vadose zone. In this work, laboratory and in situ experimental methods of parameter estimation were compared to investigate the best estimation method for heterogeneous soil. The Marquardt–Levenberg and non-linear least-squares optimisation algorithms were used for parameter estimation. The simulation error was minimised by selecting sensitive parameters during the numerical solution. The shape factor n was found to be the most sensitive parameter, followed by water content θ_s, saturated hydraulic conductivity (SHC) and the inverse of the air entry α. Compared with the in situ cumulative infiltration and simultaneous methods, the outflow method resulted in the best fit by minimising the error. During the comparison of outflow and cumulative infiltration methods, only θ_s showed a significant difference (p = 0.00). On the other hand, SHC showed a non-significant difference (p = 0.439) when the outflow and simultaneous methods were compared. During model predictions, the SHC measured by the simultaneous method showed reasonable estimates for surface horizon and weak correlations (0.79 and 0.77) with deep soil water content, which could be improved by adding more hydraulic parameters. The cumulative infiltration numerical solution resulted in the most reliable estimates of hydraulic parameters for in situ heterogeneous soil.