Water repellency is an important property of many soils. It causes rainwater to penetrate into the soil as preferential flow paths, and solutes can reach the groundwater more rapidly than in the case of a homogeneous wetting. Water repellency depends on several factors which are principally related to the characteristics of the organic matter of the soil. A distinction between “potential” and “actual” water repellency and the assessment of the “critical soil water content” are introduced and highlighted in this paper. Persistence and degree of potential water repellency of dried samples were examined from 10 trenches in a dune sand with grass cover using the water drop penetration time and the alcohol percentage tests. The spatial variability of water repellency and, therefore, soil wetting was extremely high. The actual water repellency was measured on field-moist samples to obtain critical soil water contents. The soil is wettable above and water repellent below these values. The critical soil water content varies between 4.75 vol % at 5–10 cm and 1.75 vol % at 45–50 cm depth in this sandy soil.

How water moves in a water repellent sandy soil: 1. Potential and actual water repellency

The role of soil moisture in controlling water repellency: new evidence from forest soils in Portugal

Two common methods to assess soil hydrophobicity are the ‘Water Drop Penetration Time’ and ‘Molarity of an Ethanol Droplet’ techniques. For these, uncertainty exists regarding the representativeness of laboratory tests reflecting field conditions, their replicability, and the comparability of results between the two techniques. Using air-dried soils with a broad particle size and hydrophobicity range, this study shows that a high representativeness and replicability of results can be achieved. A close relationship between the two tests was found for highly, but not for moderately hydrophobic soils. Guidelines are suggested to increase representativeness, replicability and comparability of results in future studies. © 1998 John Wiley & Sons, Ltd.

On standardizing the ‘Water Drop Penetration Time’ and the ‘Molarity of an Ethanol Droplet’ techniques to classify soil hydrophobicity: A case study using medium textured soils

Water repellent soils exhibit hydrophobic properties when dry, resisting or retarding water infiltration into the soil matrix (Brandt, 1969a). Infiltration rates may be reduced by an order of magnitude, even in soils which visually appear to wet “normally” (Wallis et al., 1991). Repellent soils have been reported in many countries (DeBano and Letey, 1969) and may occupy large areas, such as the sandy soils of South and Western Australia (Bond, 1969a). However, severe repellency is relatively uncommon, and hence the condition is generally regarded as interesting but inconsequential. Indeed, the assumption of nonwater-repellent behavior is the norm in soil physics (Philip, 1969).

Soil Water Repellency

Water repellency in soils: a historical overview

The relation between apparent liquid-solid contact angle and surface tension was established for 3 water- repellent materials using both a series of aqueous ethanol solutions and organic liquids. The media included 2 artifically 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 phi = O. 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 (1) 90$ surface tension (gamma n), and (2) 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 characteristics of soils. These indices will be valuable in assessing erosion and runoff problems, and also the feasibility of surfactants to promote infiltration. (14 refs.)

C. L. Watson

Papers

Indices for Characterizing Soil-Water Repellency Based upon Contact Angle-Surface Tension Relationships