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Theoretical Aspects of Flow Above the Water Table in Tile Drainage of Shallow Homogeneous Soils¹

ABSTRACT

Hypothetical conductivity-tension relationships were employed in a study to determine the effect of the thickness of the capillary fringe and the relative conductivity above the capillary fringe on drainage flow. Solutions were obtained with an electrical resistance network using equivalent conductivities.

A capillary fringe of 50 cm. increased the drainage coefficients in shallow soil by 24 to 57%, depending on the height of the water table. Smaller increases can be expected with decreasing thickness of the fringe and increasing depth of the impermeable layer. Agreement existed between the results of the network analyses and calculated drainage coefficients, including the effect of the capillary fringe with Hooghoudt's method.

The effect of the flow above the capillary fringe on the drainage coefficient appeared to be small so that the flux across the top of the capillary fringe can be assumed to be uniform. Steady-state drainage coefficients, therefore, can be used to calculate the rate of fall of the water table after cessation of rain when the region near the top of the capillary fringe becomes a source of almost uniform flow because of drainage of pore space.

For a certain water-table position midway between drains, the water table in the vicinity of the drains appeared to be higher with increasing thickness of the capillary fringe. Since hysteresis causes the fringe thickness to be greater for falling water tables than for rising water tables, lower water tables near the drains can be expected with the latter.

¹ Contribution from the Agr. Exp. Sta. of the Alabama Polytechnic Institute, Auburn.

² Associate Agricultural Engineer. The author acknowledges valuable assistance from J. M. Yeager, senior agricultural engineering student, in calculating equivalent conductivities and in carrying out network analyses.

Received for publication November 24, 1958. Accepted for publication January 13, 1959.