Environmental Engineering Reference
In-Depth Information
Figure 26.13
The effects of vegetation cover on soil erosion
(sediment yield and run-off) under low and high-intensity
rainfall in the Mediterranean environment.
ously argued that, in addition to protection from rain splash, the roots have a binding
effect, holding the soil against erosive forces. Whilst this will happen, there are additional
processes at work. Any soil grains detached by splash may become 'caught' by the leaves
of the plant, and will thus contribute to the mound. Plant litter also adds to the soil
organic matter around plants, improving infiltration by producing a better formed and
more stable soil structure. More infiltration leads to less overland flow and less erosion;
there also comes a time when the mound can divert any flowing water around it. A clear
positive feedback system thus exists between the plants and the mounds. Litter
production and lower erosion rates create better soil structures and soil moisture
conditions (more infiltration) as well as providing more plant nutrients (especially
nitrogen). They also provide more attractive, shady habitats for soil fauna (worms,
isopods), whose burrowing causes further mounding. The pattern of erosion at the micro-
scale is one of low-energy, minimum erosion beneath plants, and high-energy, maximum
erosion on bare ground between vegetation. This simple model of the interactions
between vegetation cover and erosion is complicated in the real world by other factors
such as rock type, aspect, the nature of grazing by animals and the extent of burnt areas.
Soils in Mediterranean areas often have a high stone content, whether due to stony
colluvial and alluvial deposits or to their shallowness over bedrock. Whilst the bare
surfaces of fine-grained soils can develop crusts with low infiltration rates, a layer of
stones can protect the surface of stony soils from direct raindrop impact. The process of
