Inverse modeling of a radial multistep outflow experiment for determining unsaturated hydraulic properties

Modeling flow and solute transport in the unsaturated zone on the basis of the Richards equation requires specifying values for unsaturated hydraulic conductivity and water potential as a function of saturation. The objective s of the paper are to evaluate the design of a transient, radial, multi-ste p outflow experiment, and to determine unsaturated hydraulic parameters usi ng inverse modeling. We conducted numerical simulations, sensitivity analys es, and synthetic data inversions to assess the suitability of the proposed experiment for concurrently estimating the parameters of interest. We cali brated different conceptual models against transient Row and pressure data from a multi-step, radial desaturation experiment to obtain estimates of ab solute permeability, as well as the parameters of the relative permeability and capillary pressure functions. We discuss the differences in the estima ted parameter values and illustrate the impact of the underlying model on t he estimates. We demonstrate that a small error in absolute permeability, i f determined in an independent experiment, leads to biased estimates of uns aturated hydraulic properties. Therefore, we perform a joint inversion of p ressure and flow rate data for the simultaneous determination of permeabili ty and retention parameters, and analyze the correlations between these par ameters. We conclude that the proposed combination of a radial desaturation experiment and inverse modeling is suitable for simultaneously determining the unsaturated hydraulic properties of a single soil sample, and that the inverse modeling technique provides the opportunity to analyze data from n onstandard experimental designs. (C) 1999 Elsevier Science Ltd. All rights reserved.