Climate, Climate Variability and Impacts in the Mediterranean Area: An Overview
17
due to scattering of solar radiation by the leave and needle structures. The more
active the vegetation is the stronger are also the water absorption bands of which
even the weaker one at 0.94 Ilm appears. Fig. 4d finally shows that the general
slopes of the spectra of different plants are not very different from each other though
the amplitudes and the angular dependency of the renectances are.
The emittance of vegetated surfaces follows, because of the water content of the
plants, nearly the Planck formula for Black Body radiation. Integrated over the
whole wavelength range it is proportional to about (0.985 ± 0.005) oT4, where T is
the absolute thermodynamic surface temperature in Kelvin and 0= 5.6703 * 10 8 W
m,2 K4 the Stefan-Boltzmann constant. The emittance of bare soils is smaller and
is proportional to £OT4. E, the emissivity, takes values around 0.9 and is very
variable with soil type and moisture.
2.4 Contemporary Human Drivers
Land management and irrigation are major human impacts on land surfaces which
may affect land surface processes and in turn the Mediterranean water cycle and
regional climate. While in ancient times regional climates have been affected by
human activities such as large scale deforestation (the wood was needed for
constructions, heating and cooking) and the intensification of agriculture (see
section 3) the modification of land-surface processes with the potential to affect
regional climates has shifted to more complex causes and actions during the last
century among which economical reasons and more recently the European
subvention policy can be identified. Only few of them are discussed here.
During the first half of the century in southern Portugal forests have been cleared
in favour of crops to make the country more autarkical. The result now is
dramatically increased erosion due to nash noods. In most Mediterranean countries
the introduction of heavy machines for land treatment has changed the agricultural
practice from the former terracing to agriculture on nat or soft hilly land especially
in estuaries which are preferred for growing agricultural products because here the
land is nat and the soils are fertile. Land which does not meet the conditions for
machine work is abandoned. In Italy the Pontine swamps have been drained since
1935 and converted to agricultural land. Here and in other coastal zones, the
demand for irrigation is growing firstly for competitive reasons - higher harvests -
and secondly because apparently the dry spells seem to extend throughout the
summer months. Most of the irrigated water remains in the uppermost soil layers
and is very quickly evaporated thus not being much helpful to reduce the soil water
deficit. Because of the deep pumping in these regions salty sea water is entrained
into near coastal aquifers which makes the water useless for agriculture. In addition
industry is preferably settling in coastal areas and draws on the fresh water
resources.
Especially in Portugal, Spain and Egypt rivers are dammed for large water
reservoirs. In Morocco an irrigation system was constructed for about I million
17
due to scattering of solar radiation by the leave and needle structures. The more
active the vegetation is the stronger are also the water absorption bands of which
even the weaker one at 0.94 Ilm appears. Fig. 4d finally shows that the general
slopes of the spectra of different plants are not very different from each other though
the amplitudes and the angular dependency of the renectances are.
The emittance of vegetated surfaces follows, because of the water content of the
plants, nearly the Planck formula for Black Body radiation. Integrated over the
whole wavelength range it is proportional to about (0.985 ± 0.005) oT4, where T is
the absolute thermodynamic surface temperature in Kelvin and 0= 5.6703 * 10 8 W
m,2 K4 the Stefan-Boltzmann constant. The emittance of bare soils is smaller and
is proportional to £OT4. E, the emissivity, takes values around 0.9 and is very
variable with soil type and moisture.
2.4 Contemporary Human Drivers
Land management and irrigation are major human impacts on land surfaces which
may affect land surface processes and in turn the Mediterranean water cycle and
regional climate. While in ancient times regional climates have been affected by
human activities such as large scale deforestation (the wood was needed for
constructions, heating and cooking) and the intensification of agriculture (see
section 3) the modification of land-surface processes with the potential to affect
regional climates has shifted to more complex causes and actions during the last
century among which economical reasons and more recently the European
subvention policy can be identified. Only few of them are discussed here.
During the first half of the century in southern Portugal forests have been cleared
in favour of crops to make the country more autarkical. The result now is
dramatically increased erosion due to nash noods. In most Mediterranean countries
the introduction of heavy machines for land treatment has changed the agricultural
practice from the former terracing to agriculture on nat or soft hilly land especially
in estuaries which are preferred for growing agricultural products because here the
land is nat and the soils are fertile. Land which does not meet the conditions for
machine work is abandoned. In Italy the Pontine swamps have been drained since
1935 and converted to agricultural land. Here and in other coastal zones, the
demand for irrigation is growing firstly for competitive reasons - higher harvests -
and secondly because apparently the dry spells seem to extend throughout the
summer months. Most of the irrigated water remains in the uppermost soil layers
and is very quickly evaporated thus not being much helpful to reduce the soil water
deficit. Because of the deep pumping in these regions salty sea water is entrained
into near coastal aquifers which makes the water useless for agriculture. In addition
industry is preferably settling in coastal areas and draws on the fresh water
resources.
Especially in Portugal, Spain and Egypt rivers are dammed for large water
reservoirs. In Morocco an irrigation system was constructed for about I million