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Measurement of Hydraulic Conductivity By Means of Steady, Spherically Symmetric Flows¹

ABSTRACT

The dependence of hydraulic conductivity upon pressure head can be determined from a series of steady, spherically symmetric flows: at any point the hydraulic conductivity is equal to the rate of change of the steady flux per unit solid angle with respect to the pressure head of that point, divided by its radial distance from the center of the water source. This method is very attractive for in-situ measurements in the field since it requires only one controlled boundary condition, but does not require one-dimensional flow. Experimental procedures are described and some results given. Hydraulic conductivity values derived from the pressure head in the source cavity itself were the same as those derived from tensiometer measurements near the source. A range of hydraulic conductivity values can be obtained faster with a series of different, steady pressure heads in the source cavity, rather than by measurements on a series of tensiometers located at several distances from the source.

Gravity tends to distort the spherical symmetry of the flow. An analytical solution for steady flow from a point source, for a soil with an exponential dependence of the hydraulic conductivity upon the pressure head, indicates that for most soils the effect of gravity near the cavity is small enough to obtain satisfactory hydraulic conductivity values. Gravity has no effect on the pressure head distribution directly below the cavity, suggesting that this is a preferred location for tensiometers. Actually, the analytical solution obtains the hydraulic conductivity in an exact manner for any position. The radius of the cavity should be kept as small as possible.

¹ Contribution from the North Central Region, Agricultural Research Service, USDA, in cooperation with the Wisconsin Agr. Exp. Sta., Madison, Wis.

² Soil Scientist. Present address-U. S. Salinity Lab., P. O. Box 672, Riverside, Calif. 92502.

Received for publication April 30, 1973. Accepted for publication October 23, 1973.