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 have been made to establish a critical soil moisture threshold, demarcating water-repellent and non-repellent conditions. The reported thresholds vary widely and the exact relationship between hydrophobicity and soil moisture remains far from being understood. Using field and laboratory measurements, this study explores the effect of soil moisture on water repellency for Portuguese sandy loam and loamy sand forest soils. The results indicate that for these soils, repellency is absent when soil moisture exceeds 28%, but show that after wetting, repellency is not necessarily re-established when soils 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 moisture as indicated in the literature. It is suggested that, after wetting, re-establishment of repellency may also require a fresh input of water-repellent substances. The mechanisms of wetting and drying in water-repellent soils are discussed and associated hydrological implications are explored.