Environmental Engineering Reference
In-Depth Information
DESERTIFICATION AND SOIL EROSION
The processes which lead to the erosion and degradation of the land surfaces of arid,
semi-arid and sub-humid areas are collectively known as desertification . The term refers
to adverse impacts on all ecosystem components (soil, water, vegetation, wildlife) and the
process is considered to be due to human impacts on land use. Although normally
associated with desert fringes (e.g. the Sahel on the southern margins of the Sahara),
Mediterranean regions are highly susceptible to desertification processes because of the
properties of their climate, soils and vegetation. Against the backcloth of an inherently
sensitive physical environment, there has been a long history of often quite drastic human
modification for agriculture, water supplies, urban and residential development, and
tourism. These impacts have gone on for millennia in the Mediterranean region itself, and
for several centuries in other Mediterranean areas.
The slopes of Mediterranean landscapes are rarely covered 100 per cent by plants; thus
bare soil is open to erosion processes and the production of loose sediment, which is
easily carried downslope. Plant cover is the major control of run-off and sediment yield,
although another important variable is the intensity of rainfall, with low-intensity rain
producing much smaller amounts of sediment and run-off. A vegetation cover of about 50
per cent seems to mark an approximate threshold between extreme and low erosion rates
(Figure 26.13). Cover seems to be more important than biomass (size, or strictly weight,
of vegetation), so that bushes and low clumps of plants are just as effective as trees. The
main effect of the vegetation, of course, is to break the fall of raindrops, thus preventing
both rain splash and surface sealing. Surface crusts and seals are a common feature of
soils in both arid and semi-arid regions. The force of the raindrop breaks the aggregates at
the soil surface into loose mineral particles of sand, silt and clay, which on drying
become repacked into a dense and impermeable surface 'skin'. If there are free chemicals
such as salts, lime or gypsum in the soil, they provide a chemical cement which is
extremely tough.
There are several additional influences of the Mediterranean vegetation on erosion
rates. It is common to see shrubs growing on low mounds of soil. It was previ-
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