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.

Assessments of water repellency of soils are commonly made on air‐dried or oven‐dried samples, without considering the soil water content. The objectives of this study were to examine the spatial and temporal variability of soil water content, actual water repellency over short distances, and the variations in critical soil water contents. Between 22 Apr. and 23 Nov. 1999, numerous samples were collected from a grass‐covered dune sand (typic Psammaquent), at six depths, eight times in transects and two times in soil blocks. The water drop penetration time (WDPT) test was used to measure the actual water repellency of the field‐moist samples and the potential water repellency after drying the samples at 25, 65, and 105°C. Highly spatial and temporal variability in water content and persistence of actual water repellency was found between the samples from all soil depths. At each depth we established an upper water content, below which samples were water repellent and a lower water content, above which samples were wettable. This water content range, called the transition zone, was different for each depth, and, for example, assessed at 0‐ to 2.5‐cm depth between soil water contents of 18 and 23% (vol./vol.), and at 16.5‐ to 19‐cm depth between 2 and 5% (vol./vol.). The potential water repellency of samples dried at 25 and 65°C was on some days less severe than the actual repellency of field‐moist samples on other days, thus underestimating the maximal persistence of water repellency that can occur in the field. Drying of the samples at 105°C significantly increased the potential water repellency.

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Knowledge of soil water repellency distribution, of factors affecting its occurrence and of its hydrological effects stems primarily from regions with a distinct dry season, whereas comparatively little is known about its occurrence in humid temperate regions such as typified by the UK. To address this research gap, we have examined: (i) water repellency persistence (determined by the water drop penetration time method, WDPT) and degree (determined by the critical surface tension method, CST) for soil samples (0-5, 10-15 and 20-25 cm depth) taken from 41 common soil and land-use types in the humid temperate climate of the UK; (ii) the supposed relationship of soil moisture, textural composition and organic matter content with sample repellency; and (iii) the bulk wetting behaviour of undisturbed surface core samples (0-5 cm depth) over a period of up to 1 week. Repellency was found in surface samples of all major soil textural types amongst most permanently vegetated sites, whereas tilled sites were virtually unaffected. Repellency levels reached those of the most severely affected areas elsewhere in the world, decreased in persistence and degree with depth and showed no consistent relationship with soil textural characteristics, organic matter or soil moisture contents, except that above a water content of c. 28% by volume, repellency was absent. Wetting rate assessments of 100 cm 3 intact soil cores using continuous water contact (-20 mm pressure head) over a period of up to 7 days showed that across the whole sample range and irrespective of texture, severe to extreme repellency persistence consistently reduced the maximum water content at any given time to well below that of wettable soils. For slightly to moderately repellent soils the results were more variable and thus hydrological effects of such repellency levels are more difficult to predict. The results imply that: (i) repellency is common for many land-use types with permanent vegetation cover in humid temperate climates irrespective of soil texture; (ii) supposedly influential parameters (texture, organic matter, specific water content) are poor general predictors of water repellency, whereas land use and the moisture content below which repellency can occur seem more reliable; and (iii) infiltration and water storage capacity of very repellent soils are considerably less than for comparable wettable soils.