Soil water repellency: its causes, characteristics and hydro-geomorphological significance

Water repellency (hydrophobicity) of soils is a property with major repercussions for plant growth, surface and subsurface hydrology, and for soil erosion. Important advances have been made since the late 1980s in identifying the range of environments affected by water repellency, its characteristics and its hydro-geomorphological impacts. This review summarises earlier work, but focusses particularly on these recent advances and identifies remaining research gaps. The associations of water repellency with (a) soils other than coarse-textured ones, (b) an expanding list of plant species, and (c) a widening range of climates other than seasonally dry types have been recurrent themes emphasised in recent literature. Nevertheless, knowledge about the extent of water repellency amongst world soils is still comparatively sparse. Its origin by the accumulation of long-chained organic compounds on or between soil particles is now widely accepted, but understanding of their exact chemical composition and means of attachment to particle surfaces remains incomplete. The transient nature of water repellency has been found to be mainly associated with fluctuations in soil moisture, but the precise processes and required conditions for the changes from hydrophobic to hydrophilic and vice versa are so far only poorly understood. Significant advances relating to the hydro-geomorphological impacts of hydrophobic layers have been made since the late 1980s in identifying and separating the various effects of such layers on surface and subsurface water flow. It has become evident that these effects in turn are influenced by variables such as the frequency and effectiveness of flow pathways through hydrophobic layers as well as their position and transitory behaviour. Recent literature has continued to highlight the role of water repellency in promoting soil erosion and it is now recognised that it can promote rainsplash detachment and soil loss not only by water, but also by wind. Major research gaps, however, remain in (a) isolating the erosional impact of water repellency from other factors, and (b) identifying the exact role of, and the interactions between the different variables controlling development and effectiveness of flow pathways through hydrophobic soil. Improved understanding of the effects of soil water repellency will enable its overall role in surface and subsurface hydrological and erosional processes to become more clearly defined. (C) 2000 Elsevier Science B.V. All rights reserved.