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Comparison of Three Field Methods to Characterize Apparent Macropore Conductivity

USDA-ARS-BA-NRI Systems Research Lab., Bldg. 007, Rm. 008, 10300 Baltimore Ave., Beltsville, MD 20705

L. R. Ahuja

Great Plains Systems Research Lab., 301 S. Howes, P.O. Box E, Ft. Collins, CO 80522

M. D. Ankeny

D.B. Stevens and Assoc., 6020 Academy Rd. NE, Suite 100, Albuquerque, NM

*Corresponding author.

ABSTRACT

The objective of this study was to investigate some field-oriented methods designed to characterize the flow rates of both the soil matrix and macropores near saturation. An apparent macropore conductivity was obtained by the difference between saturated and unsaturated hydraulic conductivities at –0.6 kPa of soil water pressure. Saturated conductivities were measured in double-ring infiltrometers with tensiometers. Soil matrix conductivities were calculated from measurements of soil water pressures and drainage rate during redistribution and were also measured with a thin sand-cement crust. Another set of data for saturated and unsaturated conductivities was obtained from unconfined ponded measurements in 76.2-mm-diam. rings and a tension infiltrometer. Unsaturated conductivities and calculated apparent macropore conductivities from the crust and redistribution methods were comparable. Unsaturated conductivities and apparent macropore conductivities from the tension infiltrometer and unconfined ponded measurements were greater than those from the infiltration-redistribution method. We concluded that apparent macropore conductivity for the profile can be characterized by using ponded infiltration rates and matrix conductivities near saturation determined from redistribution data. This method accounts for the continuity of the macropore flow system in subsurface layers and involves a larger area for macropore infiltration measurement.

Joint contribution from the USDA-ARS National Agricultural Water Quality Lab., Durant, OK, and the National Soil Tilth Lab., Ames, IA.

Received for publication March 18, 1992.

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