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ARS Home » Research » Publications at this Location » Publication #73464
Title: ESTIMATING UNSATURATED SOIL HYDRAULIC PROPERTIES FROM TENSION DISC INFILTROMETER DATA BY NUMERICAL INVERSION

Author
item SIMUNEK, J - U.C. RIVERSIDE
item VAN GENUCHTEN, MARTINUS

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/12/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Reliable application of computer models to field-scale flow and transport problems demands a commensurate effort in quantifying a large number of model parameters. Knowledge of the unsaturated soil hydraulic properties is especially important when numerical models are used to simulate variably-saturated water flow and contaminant transport. Tension disc infiltrometers are increasingly being used for in measurement of the unsaturated soil hydraulic properties. Tension infiltrometers are especially useful for quantifying the effects of macropores and preferential flow paths on infiltration in the field. Thus far, tension infiltration data have been used primarily for evaluating the saturated hydraulic conductivity and the sorptivity parameter in Gardner's exponential model of the unsaturated hydraulic conductivity. In this paper we describe a parameter estimation procedure which combines nonlinear parameter optimization involving weighted least squares with a quasi three-dimensional numerical model which solves the variably-saturated flow equation. The unknown parameters in van Genuchten's model for the unsaturated soil properties are estimated from observed cumulative infiltration data during transient water flow by numerical inversion of the Richards' equation. Additional data, such as measured pressure heads or water contents can be optionally included in the parameter estimation procedure.

Technical Abstract: Tension disc infiltrometers are becoming increasingly popular devices for in-situ measurement of the unsaturated hydraulic properties of soil. Tension infiltration data are generally used to evaluate the parameters Ks and a in Gardner's exponential model of the unsaturated hydraulic conductivity. Either two measurements using different disc diameters, or measurements with a single disc but using multiple pressure heads, are then used. In this paper we describe a parameter estimation procedure which combines the Levenberg-Marquardt nonlinear parameter optimization method involving weighted least squares, with a quasi three-dimensional numerical model which solves the variably-saturated flow equation. By numerical inversion of Richard's equation the unknown parameters in van Genuchten's model of the unsaturated soil-hydraulic properties are estimated from observed cumulative infiltration data during transient water flow. Additional measurements of the pressure head or water content, can be optionally included into the parameter estimation procedure. We calculate objective functions based on available cumulative infiltration, pressure head, and water content measurements, and also on several combinations of these data. The behavior of the objective function in three-dimensional parameter space is evaluated by means of a series of two-dimensional response surfaces. The utility of the parameter estimation procedure is demonstrated using numerically generated data. The sensitivity of the procedure to different initial estimates of the model parameters is also discussed.