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

Water repellency (hydrophobicity) of soils is a property with major repercu ssions for plant growth, surface and subsurface hydrology, and for soil ero sion. Important advances have been made since the late 1980s in identifying the range of environments affected by water repellency, its characteristic s and its hydro-geomorphological impacts. This review summarises earlier wo rk, but focusses particularly on these recent advances and identifies remai ning research gaps. The associations of water repellency with (a) soils other than coarse-textu red ones, (b) an expanding list of plant species, and (c) a widening range of climates other than seasonally dry types have been recurrent themes emph asised in recent literature. Nevertheless, knowledge about the extent of wa ter repellency amongst world soils is still comparatively sparse. Its origi n 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 incomplet e. The transient nature of water repellency has been found to be mainly ass ociated with fluctuations in soil moisture, but the precise processes and r equired 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 hydr ophobic layers have been made since the late 1980s in identifying and separ ating the various effects of such layers on surface and subsurface water fl ow. It has become evident that these effects in turn are influenced by vari ables such as the frequency and effectiveness of flow pathways through hydr ophobic layers as well as their position and transitory behaviour. Recent l iterature has continued to highlight the role of water repellency in promot ing soil erosion and it is now recognised that it can promote rainsplash de tachment and soil loss not only by water, but also by wind. Major research gaps, however, remain in (a) isolating the erosional impact of water repell ency from other factors, and (b) identifying the exact role of, and the int eractions between the different variables controlling development and effec tiveness of flow pathways through hydrophobic soil. Improved understanding of the effects of soil water repellency will enable its overall role in sur face and subsurface hydrological and erosional processes to become more cle arly defined. (C) 2000 Elsevier Science B.V. All rights reserved.