Field measurement of the hydraulic properties of soil

Abstract Disc permeameters are used here on two contrasting soils to determine absorption and transmission characteristics in the potential range from saturation down to ψ 0 = −100 mm. One soil was a recently-ploughed loam, the other a cracking-prone heavy clay. Despite the dominance of gravity in the former case, and of capillarity in the latter, a rapid onset of geometrically-induced steady flows q ∞ , was observed from disc permeameters in both cases. For the loam a twin-disc analysis of q ∞ from discs of different radii, had to be used to determine the sorptivity S 0 . Whereas for the clay the ample square-root-of-time behaviour gave S 0 as well. The conductivity K 0 was then derived from q ∞ via Wooding's equation. Another value of K 0 derived for the ploughed loam from a standard determination with a large, buffered ring, did not agree with that from the disc permeameter. This failure arose because of the vertical drop-off in K that rendered impossible the attainment of one-dimensional flow from the inner ring. The loam possessed the hydraulic properties expected of such a medium-textured soil that had recently been disturbed by ploughing. The clay however had a 40-fold jump in K right at saturation reflecting the presence of cracks.

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