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Velocity-Gradient Relationships for Steady-State Unsaturated Flow of Water in Nonswelling Artificial Soils¹

ABSTRACT

The relationship between flow velocity and hydraulic gradient was studied for steady-state unsaturated flow of water in horizontal columns of two nonswelling porous media, one a mixture of sand, ground silica, and kaolinite, and the other a mixture of sand and ground silica only. The flowing water contained 0.1% phenol to inhibit bacteria. Soil-moisture tension was measured with three tensiometers positioned within the soil column. Gradients at different velocities could be determined at a single location. Tension changes were always from low to high, to eliminate hysteresis effects. This required the use of a graphical analysis to calculate corresponding values of velocity and gradient at a given constant tension.

In general, essential proportionality between flow velocity and tension gradient at given tension was found for both materials. Very slight greater-than-proportional tendencies were present in the sand-silica-kaolinite mixture, but were not consistently related to tension. In the sand-silica mixture a less-than-proportional flow response was observed at low and intermediate tensions, but became proportional as the tension increased. Minimal pore-volume saturations achieved were 0.74 for the sand-silica-kaolinite, and 0.65 for the sand-silica.

¹ Contribution from the Soil & Water Conservation Research Division ARS, USDA, in cooperation with the Department of Agronomy, Purdue University, and the South Dakota Agr. Exp. Sta. Journal Paper no. 789 of the South Dakota Agr. Exp. Sta., and no. 3215 of the Purdue University Agr. Exp. Sta. Presented before Div. S-1, Soil Science Society of America, Nov. 9, 1967, at Washington, D.C.

² Soil Scientist, USDA, Brookings, South Dakota, and Professor of Soils, Purdue University, Lafayette, Indiana.

Received for publication December 13, 1967. Accepted for publication February 19, 1968.