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SOIL HYDROPHOBICITY

Soils are generally considered to wet readily under rainfall or irrigation, but this does not apply to hydrophobic (water repellent) soils (Fig.1).

Hydrophobicity can reduce the affinity for soils to water such that infiltration or wetting may be delayed for periods ranging from as little as a few seconds to in excess of weeks. Soil hydrophobicity is thought to be caused primarily by a coating of long-chained hydrophobic organic molecules on individual soil particles. These substances may be released from a range of plants, decaying organic matter, soil fauna and micro-organisms either naturally or during burning (Fig.2)

Hydrophobicity tends to be spatially and temporally highly variable, which makes its effects difficult to observe and predict. It is often most prominent after prolonged dry spells and usually disappears after prolonged contact with water. Owing to the cultivation of certain frequently introduced plant species and the increase in wildfires in some regions, hydrophobicity has developed in previously unaffected areas. Amongst its effects are inhibited plant growth, increased overland flow and soil erosion (Fig.3), uneven spatial and vertical wetting patterns, reduced evaporation, and enhanced risk of groundwater pollution due to the generation of preferential flow pathways (Fig.4).

For an introduction to this topic written for the general public see also the article Fear of Water published recently in NERCís award-winning Planet Earth magazine.

Fig. 1: Water droplets resist infiltration into highly porous hydrophobic soil  (photo: Erik van den Elsen).

Fig. 2 Potential sources of hydrophobic substances

Fig. 3 Soil hydrophobicity can enhance overland flow particularly after wildfire as seen here in a burnt Eucalypt forest in Australia (photo: courtesy of Rob Ferguson)

Fig.4 Soil hydrophobicity can enhance preferential flow (as indicated by a tracer in this sandy soil), leading to accelerated contaminant transfer  (photo: courtesy of L.W.Dekker)