Environmental Engineering Reference
In-Depth Information
Figure 3.1
Rainfall fluctuations in five regions of
Africa, 1901-87: expressed as a per cent departure
from the long-term mean
some degree of aridity in their climatological
makeup. Along the desert margins in Africa, for
example, annual precipitation is low, ranging
between 100 and 400 mm, but, under normal
conditions, this would allow sufficient vegetation
growth to support pastoral agriculture. Some
arable activity might also be possible, if dry-
farming techniques were employed. Drought
occurs with considerable regularity in these areas
(Le Houerou 1977). The problem lies not in the
small amount of precipitation, but rather in its
variability. Mean values of 100 to 400 mm are
based on long-term observations, and effectively
mask totals in individual years, which may range
well above or below the values quoted (see Figure
3.3). Weather records at Beijing, in drought-prone
northern China, show that the city receives close
to 600 mm of precipitation in an average year.
However, the amount falling in the wetter years
can be 6-9 times that of the drier years. Only
148 mm were recorded in 1891, for example,
and 256 mm in 1921, compared to a maximum
of 1,405 mm in 1956 (NCGCC 1990). Rainfall
variability is now recognized as a major factor
in the occurrence of drought (Oguntoyinbo
1986), and a number of writers have questioned
the use of 'normal' values in such circumstances.
In areas of major rainfall variability the nature
of the environment reflects that variability rather
than the so-called normal conditions, and any
response to the problems which arise from
drought conditions must take that into account
(Katz and Glantz 1977).
Some researchers claim that the drought in
sub-Saharan Africa has been intensifying as a
result of climatic change. Bryson (1973), for
example, has identified changes in atmospheric
circulation patterns which could intensify and
prolong drought in the Sahel. There is, however,
no conclusive evidence that the current drought
is anything other than a further indication of the
inherent unreliability of precipitation in the area
(Nicholson 1989).
Source:
After Nicholson (1989)
the result of low average rainfall, and is a
permanent feature of the climatology of a region
(see Figure 3.2a). The deserts of the world, for
example, are permanently arid, with rainfall
amounts of less than 100 mm per year. In
contrast, drought is a temporary feature,
occurring when precipitation falls below normal
or when near normal rainfall is made less effective
by other weather conditions such as high
temperature, low humidity and strong winds
(Felch 1978).
Aridity is not a prerequisite for drought. Even
areas normally considered humid may suffer from
time to time, but some of the worst droughts ever
experienced have occurred in areas which include
The human response to drought
Each generation has its images of drought. In
