Fjölrit RALA - 05.12.1999, Blaðsíða 30

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Desertification in the Mediterranean Europe
• Climate and bioclimate is characterised by large moisture deficits, temporal
variability and írequent extreme events.
• Landscapes are rugged, with steep slopes, large elevation differences and are
highly dissected by torrential steams.
• Surface geology favours formation of soils which are sensitive to drought and
erosion.
• Hydrology is characterised by the scarcity of surface and ground waters, and
by the need to bring water ffom elsewhere to satisfy demand.
• Soil formation rates are much slower than soil loss, resulting in inadequate
rootable depth and water storage capacity on sloping land.
• Out of phase rainfall and vegetative periods.
• Four millenniums of human land use and ffequent abuses of land.
Physical desertification
Desertification can be characterised as physical or chemical depending on the proc-
esses involved. Physical degradation occurs on sloping land and is very extensive. The
dominant physical process is accelerated soil erosion which occurs on marginal lands
which have lost more than 60% of vegetative cover (Thomes 1988 ) and their are lo-
cated within the semi-arid and dry-subhumid zones. Accelerated erosion may lead to
reversible or irreversible desertification.
Desertification is reversible when, soil moisture has been depleted beyond the tol-
erance level of the economically and environmentally valuable plants, but the rootable
soil depth has not been decreased below critical thresholds. Main processes responsi-
ble for this type of desertifícation are soil erosion and surface structure deterioration.
Both result in low rates of water infiltration and high rates of surface mn-off and hin-
der seed germination. Reversible desertification can also occur when overgrazing has
caused the occupation of the land by plants of low economical and environmental
value. Hilly lands on marl and on Leptosols (shallow bails) are very sensitive to this
type of desertification, but human action can mitigate them. Extensive areas in Spain
have suffered this type this type of desertification.
Irreversible desertification is the terminal stage of accelerated erosion that has per-
manently reduced the rootable space and the water storage capacity of the soil below
the tolerance levels of economically and environmentally valuable plants. Lands with
Lithosols (very shallow soils) on limestone and southem slopes are the most vulner-
able throughout the Mediterranean Europe.
The extent of physical desertification is not easy to determine accurately, because
dependable indices are still being developed. An approximate estimate can be made
on the basis of the CORINE (1992) soil erosion risk map (Figure 1). Many areas of
high potential risk are vulnerable to desertification. Seriously threatened lands by wa-
ter erosion cover about 50% of the EU Mediterranean. However, not all these lands
are located in the semi-arid and dry-subhumid zones.
Using rootable depth and climatic aridity, it is possible to approximately estimate
on a soil Map of Europe (Commission of European Communities 1985) the areas
threatened by desertification as in Figure 2 (Yassoglou 1998).