Field Calibration and Validation of Solute Transport Models for the Unsaturated Zone

Abstract

AbstractData from two solute transport field experiments, one obtained from soil solution samplers and the other from soil coring, are used to illustrate the parameter estimation problem for two models describing area‐averaged vertical transport of mobile solutes. The two models, the lognormal transfer function model (TFM) and the convection dispersion equation (CDE), were selected because of their simplicity and because they represent different hypotheses for the governing mechanisms determining longitudinal solute spreading on the field scale. The two parameters of each model are estimated using three calibration procedures: sum of squares optimization, method of moments, and maximum likelihood. The three calibration procedures yield different estimates for the parameters using a given data set, and different trends in parameter variation at different depths or times in a given experiment. Calculated uncertainty in the parameter estimates was high enough that no conclusion could be drawn about which model better described the data from the solution sampler experiment, but the TFM provided a better description of the coring experimental data after calibration than the CDE. It was concluded that future experiments on field scale solute transport through the unsaturated zone will have to monitor movement below 5 m in order to yield a data set suitable for distinguishing between different models.