Environmental Engineering Reference
In-Depth Information
Thornthwaite's treatment of the measurement
and classification of drought is only one of many.
It may not meet all needs, but its broadly
climatological approach lends itself to the
geographical examination of drought-prone
environments. The greatest human impact is felt
in those areas which experience seasonal or
contingent drought, although the nature of the
impact is different in each case. The influence of
the other two types of drought is limited. In areas
of permanent drought, for example, populations
are small, and may exist only under special
circumstances, such as those at an oasis, where
the effects of the drought are easily countered.
Although invisible drought may have important
consequences for individuals, it often passes
unrecognized. It does not produce the life and
death concerns that prevail in areas of seasonal
drought, nor does it have the dire economic
impacts that may be experienced by the
inhabitants of areas of contingent drought.
is often the failure of this circulation that sets up
the conditions necessary for drought. If, for
example, the ITCZ fails to move as far polewards
as it normally does, those regions, which depend
upon it to provide the bulk of their yearly supply
of water, will remain under the influence of the
dry air masses, and receive little or no rainfall.
Similarly, any increase in the stability of the
airflow following the passage of the ITCZ will
also cause a reduction in water supply. It is
developments such as these that have set the stage
for some of the worst droughts ever experienced.
DROUGHT AND FAMINE IN AFRICA
Although seasonal drought is experienced in all
of the world's sub-tropical areas, in recent years
the greatest effects have been felt in sub-Saharan
Africa, including the region known as the Sahel
where drought is much more persistent than
elsewhere on the continent (Nicholson 1989).
The Sahel proper is that part of western Africa
lying to the south of the Sahara Desert and north
of the tropical rainforest. It comprises six nations,
stretching from Senegal, Mauretania and Mali
in the west, through Burkina Faso to Niger and
Chad in the east. This region, with its population
of 33 million inhabiting slightly more than 5
million sq km of arid or semi-arid land, came to
prominence between 1968 and 1973 when it was
visited by major drought, starvation and disease.
Despite this prominence, it is, in fact, only part
of a more extensive belt of drought-prone land
in Africa south of the Sahara. Drought pays no
heed to political boundaries, reaching as it does
to the Sudan, Ethiopia and Somalia in the east
and including the northern sections of Ghana,
Nigeria, Cameroon, the Central African
Republic, Uganda and Kenya. In the 1980s,
drought also extended into Mozambique,
Zimbabwe and other parts of eastern and
southern Africa (see Figure 3.5).
Seasonal drought in the sub-tropics
Seasonal drought is most commonly experienced in
the sub-tropics. There, the year includes a distinct
dry season and a distinct wet season, associated
with the north-south movement of the intertropical
convergence zone (ITCZ) and its attendant wind
and pressure belts (see Figure 2.13).
During the dry season, these areas are
dominated by air masses originating in the sub-
tropical high pressure systems—which
characteristically contain limited moisture, and
are dynamically unsuited to produce much
precipitation. Anticyclonic subsidence prevents
the vertical cloud development necessary to cause
rain. In contrast, the rainy season is made possible
by the migration of the ITCZ, behind which, the
combination of strong convection and air mass
convergence promotes the instability and strong
vertical growth which leads to heavy rainfall. The
passage of the ITCZ—in Africa, India, SE Asia
and Australia—allows the incursion of moist,
relatively unstable air from the ocean over the
land to initiate the wet season. This is the basis
of the monsoon circulation in these areas, and it
The atmospheric circulation in sub-Saharan
Africa
The supply of moisture in all of these areas is
