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a USDA-ARS Systems Research Lab, Bldg. 007, Rm 116, 10300 Baltimore Ave, Beltsville, MD 20705 USA
b USDA-ARS Great Plains Systems Research Unit, P.O. Box E, Ft. Collins, CO 80522 USA
c Dept of Botany, Duke University, Durham, NC 27708 USA
d USDA-ARS Grazing Lands Research Laboratory, P.O. Box 1199, El Reno, OK 73036-1199 USA
e Dept. of Environmental Sciences, Rutgers, The State University of NJ, 14 College Farm Rd., New Brunswick, NJ 08901 USA
f USDA-ARS Hydrology Laboratory, Bldg 007, Rm 112, 10300 Baltimore Ave., Beltsville, MD 20705 USA
dtimlin{at}asrr.arsusda.gov
Effective porosity, defined here as the difference between satiated total porosity and water-filled porosity at a matric potential of 33 kPa, has been shown to be a good predictor for saturated hydraulic conductivity (Ks) using a modified Kozeny-Carman equation. This equation is of the form of a coefficient (B) multiplied by effective porosity raised to a power (n). The purpose of this study was to improve the predictive capability of the modified Kozeny-Carman equation by including information from moisture release curves (soil water content vs. matric potential relation). We fitted the Brooks-Corey (B-C) equation parameters (pore size distribution index and air entry potential) to moisture release data from a large database (>500 samples). Values of Ks were also available from the same source. Inclusion of the pore size distribution index into the Kozeny-Carman equation improved the Ks estimation over using only effective porosity, but only slightly. The improvement came through a better estimation of large values of Ks. We also fit a relationship for the coefficient (B) of the Kozeny-Carman equation as a function of the two B-C parameters with a constant value of n = 2.5 for the exponent. Overall the use of Brooks-Corey parameters from moisture retention data improved estimates of Ks over using effective porosity (
e) alone. There is still considerable error in predicting individual Ks values, however. The best forms of the equation was when 
was included in the term for the coefficient for the modified Kozeny-Carman equation. The next best form was when was included in the exponent for e The two best models appeared to better preserve the mean, standard deviation and range of the original data.
Abbreviations: B-C, Brooks-Corey • RA, Rawls data • RMSE, root mean square error • SR, Southern region data
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