Water Repellency Impacts on Soil Hydrology
Stefan H. Doerr1 and Coen J. Ritsema2
1University
of Wales Swansea, Department of Geography, School of Environment and Society, UK,
s.doerr@swan.ac.uk, Tel. +44 1792 295147, Fax. +44 1792 295955
2Alterra, PO Box 47, 6700 AA Wageningen, The Netherlands, Coen.ritsema@wur.nl
Keywords: water repellency, hydrophobicity, soil hydrology, water movement
The main hydrological impacts of soil water repellency are: (a) reduced infiltration; (b) increased
overland flow; (c) spatially localized infiltration and/or percolation, often with fingered flow
development; (d) modifications of the three-dimensional distribution and dynamics of soil
moisture; (e) enhanced streamflow responses to rainstorms; and (f) enhanced total streamflow.
Because of enhanced overland flow on, and increased erodibility of, water repellent soil,
slopewash, and sometimes the formation of rills and gullies, may be promoted.
The most frequently reported impacts of water repellency are those of reduced infiltration
into soil and thus increased overland flow. For example, the infiltration capacity of a water
repellent soil was found to be 25 times higher than for a similar wettable soil. Owing to the
reduced surface tension of the soil pore walls, a water repellent soil matrix will have a positive soil
water potential effectively leading to a capillary depression effect. Infiltration into a relatively dry
water repellent soil matrix will therefore only occur if a hydraulic head sufficient to exceed the
positive soil water potential is reached. The necessary ponding depth may decrease with time if
water repellency decays during water contact. These factors acting in combination usually result
in infiltration rates to increase during rainfall in contrast what is normal theory for wettable soils.
Thus rainwater will pond and, if rainfall is sufficient and surface detention is exceeded, Hortonian
(infiltration-excess) overland flow may occur. Where overland flow is considered the key effect of
water repellency, it is often not clear whether Hortonian or saturation overland flow, or a
combination of the two, is involved, particularly if the hydrophilic layer is thin, below the surface,
or discontinuous. Hortonian overland flow tends to be most pronounced where soils have an
uninterrupted water repellent layer. Otherwise, it may be intercepted by wettable patches. The
commonly observed temporal variability of water repellency also needs to be considered here.
Thus reductions in infiltration capacity and increases in overland flow can be expected to be most
pronounced following prolonged dry periods, when water repellency tends to become most
severe, however, it is often not known how long reduced infiltration associated with water
repellency persists during wet conditions.
Although not restricted to water repellent soils, water repellency can be particularly effective at
preventing or hindering downward water movement, directing it into structural or textural
preferential flow paths and creating an unstable irregular wetting front. Consequently soils may
not wet completely with the passage of a wetting front and water may be channelled via biopores,
cracks and pipes, thereby by-passing the soil matrix. Root channels and rodent burrows are
thought to represent particularly effective bypass route-ways through water repellent soil. Where
a water repellent layer is overlain by hydrophilic soil, infiltrating water tends to pond above the
former, which is then directed via lateral flow to preferential flow routeways, leading it through the
underlying water repellent layer. This phenomenon has, for example, been described for many
burnt soils and has been termed ‘distribution flow’. In contrast, instead of any fingered flow, a
uniform wetting front may develop in a water repellent layer if the overlying hydrophilic top-layer is
very thick. Preferential flow in general is thought to be reinforced by soil water hysteresis between
wetting and drying phases, a feature of most wettable soils but more pronounced in water
repellent ones.
As is evident from the above, water repellency influences the three-dimensional
distribution and dynamics of soil moisture. Effects will vary with the vertical position of the water
repellent zone, the frequency of preferential flow routeways through this zone and the severity
and persistence of its water repellency. For example, a strongly water repellent surface layer with
preferential flow routeways can lead to dry surface soil and higher soil moisture in the subsoil. It
may be that exudation of water repellent compounds is a strategy employed by some plants,
fungi or microorganisms to reduce evaporation, to reduce nutrient leaching in parts of the soil
profile, or to suppress the germination and growth of competing vegetation.
Water repellency-induced fingered flow can lead to considerable variations in water
content in an initially water repellent soil such that zones of very dry soil can be directly adjacent
to zones of wet soil. Such differences do not only result in the widely reported poor seed
germination and plant growth. Any type of preferential flow path formation can also lead to
accelerated leaching of surface-applied agrichemicals and an increased risk of surface and
groundwater contamination.
To date, some modelling approaches have considered water repellency at plot and small
field scales, but approaches for larger scales have yet to be fully developed. The spatial and
temporal variations in the occurrence and severity of water repellency at larger scales need to be
tackled in order to upscale existing modelling approaches. Modelling at larger scales would then
allow, for example, establishing to what degree water repellency actually contributes to, what are
perceived by many as ‘increasingly unusual’ hydrological events.
This contribution aims to provide a brief overview of the current knowledge of the impacts of
water repellency on soil hydrology and examines the prospects of addressing key research gaps.