Parameter estimation for coupled unsaturated flow and transport

This paper deals with the estimation of soil hydraulic and transport parameters from transient unsaturated flow and tracer experiments using a combined simulations‐optimization approach. Hydraulic properties are defined by a modified form of van Genuchten's (1980) parametric model for two‐phase permeability‐saturation‐pressure relations, and transport properties are defined by an empirical parametric dispersion model. A nonlinear weighted least squares algorithm is used to estimate unknown model parameters by minimizing deviations between concentrations, water contents, and pressure heads obtained from hypothetical infiltration/redistribution/evaporation experiments, and those predicted by solving a numerical model of coupled unsaturated flow and transport. Simultaneous estimation of hydraulic and transport properties is found to yield smaller estimation errors for model parameters than sequential inversion of hydraulic properties from water content and pressure head data followed by inversion for transport properties from concentration data. Effects of random noise in data measurements, soil layering, and choice of improper parametric model on the parameter estimation process are discussed.