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

Water repellency (hydrophobicity) is known to be temporally variable. Most studies indicate that soils are most repellent when dry and least repellent or non-repellent (hydrophilic) when moist. In several studies, attempts ha ve been made to establish a critical soil moisture threshold, demarcating w ater-repellent and non-repellent conditions. The reported thresholds vary w idely and the exact relationship between hydrophobicity and soil moisture r emains far from being understood. Using field and laboratory measurements, this study explores the effect of soil moisture on water repellency for Por tuguese sandy loam and loamy sand forest soils. The results indicate that f or these soils, repellency is absent when soil moisture exceeds 28%, but sh ow that after wetting, repellency is not necessarily re-established when so ils become dry again. It is thought that short-term and seasonal changes in soil water repellency are not simply a function of variations in soil mois ture as indicated in the literature. It is suggested that, after wetting, r e-establishment of repellency may also require a fresh input of water-repel lent substances. The mechanisms of wetting and drying in water-repellent so ils are discussed and associated hydrological implications are explored. (C ) 2000 Elsevier Science B.V. All rights reserved.