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Evaluation of Closed-Form Analytical Models to Calculate Conductivity in a Fine Sand¹

ABSTRACT

The soil-water characteristic curve, —, was determined for an undisturbed core of fine sand over a range of negative pressure heads observed in the field, 0 to 150 cm, below about the 30 cm depth. These data were used to predict relative hydraulic conductivity in a computer code of closed-form analytical solutions developed by van Genuchten for the theoretical models of Burdine and Mualem. The equations to predict conductivity are based upon two parameters fit to observed — data by a non-linear least squares procedure. The two parameters are influenced by the choice of residual water content, _r, and unsaturated water content, _s. If _r is unknown, it is estimated in the program from observed data. A sensitivity analysis was performed to evaluate the reliability of the models. For this soil, calculated conductivities appear to be sensitive to _r. If — data are available over only a limited range, or if _s is too large, model-predicted _r-values may be near zero, whereas at large may actually be about 0.06 to 0.08 cm³/cm³. Predicted conductivities may differ by more than one order of magnitude at about = 150 cm, depending on the choice of _r.

¹ Contribution from the Department of Geoscience and Research Division, New Mexico Institute of Mining and Technology, Socorro, NM 87801. This research was funded in large part by the U.S. Bureau of Mines through the University of Nevada, Reno, under federal grant no. G11151132 UNIV NEV.

² Associate Professor of Hydrology, and Former Graduate Research Associate, respectively, New Mexico Institute of Mining & Technology, Socorro, NM 87801.

Received for publication January 3, 1984. Accepted for publication August 1, 1984.

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