Environmental Engineering Reference
In-Depth Information
moisture is a dominant influence on the development and productivity of the region's
vegetation, and hence on the entire ecosystem. It is an additional limitation on
ecosystems that rainfall totals can vary greatly from year to year (interannual variations,
measured by the 'interannual coefficient of variation', can reach 25-35 per cent), and that
available records of rainfall show significant periods of wetter and dryer rainfall.
In addition to the three unfavourable factors of the summer drought, the variability of
rain from year to year and the general unpredictability of rainfall, a further important
characteristic is the intensity of rainfall from Mediterranean lows, falling on bare and dry
soils in autumn. A raindrop may reach 6 mm diameter in size, giving a terminal velocity
(maximum sustained speed) of 10 m s
−1
. The amount of work done and the erosion
caused by such storms, infrequent though they are, is out of all proportion to the
relatively small amounts of precipitation involved. This is discussed later in this chapter
under the Medalus project.
The question of climate change is a vital one, given the present interest in the two
environmental processes of 'desertification' and 'global warming'. During the glaciations
of the Pleistocene, the higher mountain ranges experienced ice caps (Alps, Pyrenees,
Sierra Nevada, Tauros mountains), although the rest of the Mediterranean was outside the
area of direct glaciation. The Pleistocene seems to have been dry in the Mediterranean
(steppe vegetation), becoming wetter by 8000 BP in the early Holocene (postglacial).
Since then it has become difficult to identify climatic cycles because of changes brought
about by cultural and land-use events. It is difficult to distinguish natural climatic trends
from anthropogenic (human) effects. However, it is possible to analyse climate records in
the historical period, and these show persistent trends over long periods of time. For
example, the mean rainfall in south-east Spain halved between 1890 and 1940. There
have also been significant wetter and dryer periods on a regional scale. Although the
evidence is still debatable, the so-called Mediterranean Oscillation gives periods of lower
rainfall in the western Mediterranean associated with higher rainfall in the eastern
Mediterranean, and vice versa. Some see these trends as entirely random, whilst others
detect clear cycles, even though the causes are as yet unknown.
SOIL FORMATION AND DISTRIBUTION
Soils form an integral part of all ecosystems. They reflect the influences of their specific
site (the factors of soil formation: climate, vegetation, topography, geology, time) and
they strongly influence many surface processes (infiltration, overland flow, surface
erosion). Also, they have important effects on land use through the supply of a rooting
medium, water and plant nutrients for any cultivated crops.
The Mediterranean climate exerts a powerful influence on soil-forming processes. In
the moist winter season, rates of weathering and leaching are at a maximum. Minerals in
rocks and unconsolidated parent materials are subjected to chemical weathering along
cracks and fissures in the subsoil. The weathering processes of hydrolysis and hydration
are carried out by rainwater charged by carbon dioxide (CO
2
) both from the atmosphere
and from soil air, whose higher content of CO
2
comes from the activities of soil fauna
and soil micro-organisms. pH values for rainwater of 5·5 readily attack soil minerals, and,
where the parent rock is limestone, cause rapid dissolution by carbonation. Simultaneous
