Environmental Engineering Reference
In-Depth Information
Chapter
26
Mediterranean environments
Mediterranean environments are controlled by a distinctive climatic regime of hot, dry
summers and cool, moist winters. This special and unique climate has great influence on
natural processes (erosion, hydrology, soil formation, ecological processes) and on
human activities (agriculture, forestry, conservation, water abstraction). Under the
Köppen system of climatic classification, Mediterranean climates are designated Cs, i.e.
temperate, with warm, dry summers. A third letter indicates the summer temperature;
thus a designates the warmest month above 22° C and b the four coldest months above
10° C. Mediterranean climates are thus Csa. Köppen also defined the Mediterranean
climate by the equation:
where winter precipitation (
R
w
) is at least three times the total amount of summer
precipitation (
R
s
).
Climates defined in this way are found in five widely separated regions of the world,
all of which occur on the western subtropical coasts of continents between latitudes 30°
and 40°. These are shown in Figure 26.1 and are in the Mediterranean region proper, in
California, Chile, South Africa and south-western and southern Australia. The total area
of the world occupied by Mediterranean environments is only about 2 million km
2
, about
half of which occurs in the Mediterranean itself: southern Europe, North Africa, the
Levant and the Mediterranean islands. Although plant species differ between each of the
five main regions, evolutionary convergence has led in each to a vegetation which is
dominated by evergreen woodland with sclerophyllous trees and evergreen shrubs. In all
Mediterranean regions much of this woodland has been replaced by agricultural land,
originally for the traditional dry-farmed crops of cereals and tree crops (e.g. the vine,
olive, carob, almond), but increasingly for high-value irrigated land use (e.g. vegetables,
citrus fruits, rice). Outside the limits of farmland, human impacts on the natural
vegetation have been severe, mainly through grazing, ranching, wood collection and
deliberate firing. The native woodland has therefore been replaced by dense scrub
(
maquis
in France;
monte bajo
in Spain) or aromatic heath (
garrigue
in France;
matorral
in Spain). In California scrub known as
chaparral
is common, whilst the term
matorral
is
used in central Chile. In South Africa the shrubby
veld
contrasts with the heathy
fynbos
.
In southwestern and southern Australia the term
mallee
is used for similar vegetation
formations. Because of the difficulty of knowing how far vegetation has been influenced
by human activities, it is difficult to delimit the exact coverage of the Mediterranean
climate. The range of the domesticated olive (
Olea europea
) is commonly used as a
biological indicator for the Mediterranean environment. Figure 26.2 shows an
approximate delimitation based on the olive, together with the distribution of two
evergreen Mediterranean oaks and two Mediterranean pines.
