Accurate assessment of water flow and contaminant transport in unsaturated porous media at the field scale is often hindered by difficulties associated with obtaining reliable estimates of soil hydraulic properties. The unsteady drainage-flux method is one of the commonly used methods to measure in situ unsaturated hydraulic properties of soils. However, the properties obtained by this method using instantaneous profile data analysis may not be the best estimation of actual values of hydraulic properties. We present an improved analysis of the data from drainage experiments using inverse modeling, which uses nonlinear regression methods to estimate hydraulic parameters. Parameter identifiability is evaluated through sensitivity and uniqueness analyses. We used the combination of the inverse modeling program, UCODE, with the flow simulator, STOMP, for inverse modeling. Applying the inverse method to a field drainage experiment in sandy soil showed that all the van Genuchten (1980) hydraulic parameters could be estimated uniquely when both water content (θ) and pressure head (h) data were used. The parameter estimates by inverse technique using both θ and h data simulated the flow better than the parameter values obtained by the conventional instantaneous-profile analysis method. After the spatial and temporal sensitivities were analyzed, a more rational experimental design was recommended.

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