The relation between apparent liquid‐solid contact angle and surface tension was established for three water repellent materials using both a series of aqueous ethanol solutions and organic liquids. The media included two artificially treated sands and one naturally occurring soil. With one exception there was an approximate linear relationship between the cosine of the contact angle and the surface tension. The relationship for the forest soil tended to be curvilinear when using ethanol solutions of high surface tension. The repellency of the materials was best differentiated by using their intercepts with cos θ = 0. A simple drop test using the same materials and ethanol solutions placed the materials in the same order of repellency. The drop test was further developed to allow classification of porous media on the basis of (i) 90° surface tension (γn), and (ii) stability to water interaction. The tests can be readily performed both in the laboratory and the field. Two indices were then proposed to describe water‐repellent characteristies of soils. These indices will be valuable in assessing erosion and runoff problems, and also the feasibility of surfactants to promote infiltration.

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The equations λ = λi[(γ/η) cos H]½ and D = Di[(γ/η) cos H] have been presented to describe the effect of surface tension, γ, viscosity, η, and solid‐liquid contact angle, H, on penetrability, λ, and diffusivity D, where λi and Di are the intrinsic penetrability and diffusivity, respectively. The equations were tested by studying horizontal liquid‐infiltration at 2 mbars suction of nine organic liquids and 0.01N CaSO4 in glass beads, Pachappa clay loam, and a water‐repellent soil. The equations were found to describe infiltration quite well for all liquids studied in glass beads and Pachappa soil. The equations apply to organic liquids but not to 0.01N CaSO4 in a water‐repellent soil.