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Hydraulic Conductivity Evaluation of the Soil Profile from Soil Water Retention Relations¹

ABSTRACT

Evaluation of the water content-suction relation applicable to field soil horizons that differ widely in texture was considered. Except for coarse-textured, organic matter-deficient horizons, it is concluded that soil water retention of sieved samples is significantly modified and does not represent the natural soil volume. Water content-suction data from measurements on carefully procured core samples of each horizon of a Typic Hapludult adequately represent their water retention characteristics. Using appropriate water content-suction data, the hydraulic conductivity-water content relation calculated by published procedures was compared with hydraulic conductivity measured on similar samples by a transient outflow procedure. The calculated hydraulic conductivity-water content relations for coarse grained systems or systems having a relatively narrow range of pore size and well-defined bubbling pressure was sufficiently accurate for many purposes. However, to obtain a useful evaluation of the unsaturated hydraulic conductivity of fine-textured horizons with a very wide range of pore size and a poorly defined bubbling pressure, the Marshall or Millington and Quirk methods had to be matched at a water content in the 0.1- to 0.3-bar range. Indiscriminate use of such methods of calculating hydraulic conductivity is inadvisable.

¹ Contribution from the Southern Branch, Soil & Water Conservation Research Division, ARS, USDA, in cooperation with the Univ. of Georgia Agr. Exp. Sta.

² Research Soil Scientist, USDA, Watkinsville, Ga. 30677.

Received for publication October 18, 1971. Accepted for publication March 1, 1972.