Environmental Engineering Reference
In-Depth Information
Figure 28.9
Changes in soil properties
after conversion of tropical forest to
pasture.
Source: After Park (1992).
valleys. Some of this sediment accumulates in the river channels, in reservoirs or even on
farmland if floods have occurred. Previously navigable rivers have become silted up in
Madagascar, where deforestation and soil erosion are major problems. During the wet
season the coastal waters change colour as vast quantities of sediment are transported to
the sea.
Climate
. Deforestation can affect climate on the local, the regional and perhaps even
the global scales. The local effect is obvious, as surface characteristics, such as the
albedo, are being changed. The dark green of the rain forest is replaced by lighter greens
of growing crops or light browns of bare soil. Solar radiation is able to reach the surface
without much interruption and moisture is more easily evaporated from the exposed
surface. As less moisture is stored in the sponge-like forest floor, there is a loss of water
via surface run-off rather than as evaporation back to the atmosphere. It is believed that
this reduction of moisture can reduce the rainfall regime, leading to drier conditions. Soil
temperatures which were previously relatively stable will fluctuate more widely. Heating
of the forest soils can speed up the processes of hardpan formation and nutrient leaching
which quickly render them useless for agriculture.
On a regional scale, extensive deforestation can change surface temperatures and even
regional air circulation through the albedo change and alter the hydrological properties.
Increased run-off, increased evaporation, decreased soil water storage and increased
temperatures can lead to a cycle of drying. Cloud cover may be reduced
HURRICANE MITCH
human impact
September is the month that normally experiences the most severe hurricanes in the Gulf
of Mexico and the Caribbean, and by October and November their intensity and
frequency are in decline
However
in early October 1998 an easterly wave developed
