Advances in Water Resources, 22,(5), 431-444, 1999
Inverse Modeling of a Radial Multistep Outflow Experiment for Determining Unsaturated Hydraulic Properties
Stefan Finsterle and Boris Faybishenko
Lawrence Berkeley National Laboratory, Earth Sciences Division
University of California, Berkeley, CA 94720
Abstract.
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 objectives of the paper are to evaluate the design
of a transient, radial, multistep outflow experiment, and to determine unsaturated hydraulic
parameters using inverse modeling. We conducted numerical simulations, sensitivity
analyses, and synthetic data inversions to assess the suitability of the proposed experiment
for concurrently estimating the parameters of interest. We calibrated different conceptual
models against transient flow and pressure data from a multistep, radial desaturation
experiment to obtain estimates of absolute permeability as well as the parameters of the
relative permeability and capillary pressure functions. We discuss the differences in the
estimated parameter values and illustrate the impact of the underlying model on the
estimates. We demonstrate that a small error in absolute permeability, if determined in an
independent experiment, leads to biased estimates of unsaturated hydraulic properties.
Therefore, we perform a joint inversion of pressure and flow rate data for the simultaneous
determination of permeability and retention parameters, and analyze the correlations
between these parameters. 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 nonstandard experimental designs.
