Environmental Engineering Reference
In-Depth Information
(Grii ths, 1972; Mussa, 1978). In East Africa rainfall is inl uenced by the
movement of trade winds over the Indian Ocean and additionally by the
El Niño Southern Oscillation (Lovett, 1993). These major air and ocean
current circulations mean that the semi-arid regions in East Africa are
subject to highly variable rainfall concentrated in one or two rainy seasons
a year separated by relatively long dry seasons (Van Keulen and Seligman,
1992). While parts of East Africa have a single rainy season that stretches
from November/December to April/May (Grii ths, 1972; Peberdy, 1972),
most of the semi-arid region in Tanzania experiences a short break in
the rains so that there are, in fact, two rainy seasons (Kingdon, 1971;
Hamersley, 1972). These peaks in rainfall correspond with the passage of
the ITCZ as it tracks the sun's movement in the sky caused by the earth's
axial tilt (Kingdon, 1971; Grii ths, 1972; O'Brien, 1998). The equatorial
location of Tanzania also means that mean monthly temperatures vary by
only 3-4°C (Grii ths, 1972; Mussa, 1978). The seasons therefore broadly
divide into a hot and dry season from December until March, a cooler
rainy season in April and May followed by a long cool dry season with
the short rains in November corresponding with a rise in temperature
(Hamilton, 1989; Lovett et al., 2002). In addition to large-scale inl uences
on climate, the position of lakes and mountain ranges as well as altitude
can create localized weather conditions (Kingdon, 1971; Grii ths, 1972;
Maro, 1978, Hamilton, 1982; Lovett, 1993), so that rainfall is highly
variable spatially as well as temporally.
Semi-arid regions are subject to both inter-annual and intra-annual
variations in rainfall. This means that rainfall l uctuates between years
but also during seasons (Mortimore, 1998). Large volumes of rain can fall
over quite short periods so that while seasonal and yearly averages may
be maintained the patterns of rainfall between seasons and years might be
very dif erent. The short rains in November are particularly variable and
may not appear at all in some years (Grii ths, 1972). Failure of the rains
occurs when the trade winds fail to meet in the right place at the right time
(Walter, 1939). Rainfall can also l uctuate by as much as 60 per cent from
the average between years (Peberdy, 1972), with both single and multi-year
droughts a common occurrence. However, long-term studies of rainfall
patterns in sub-Saharan Africa have not shown conclusively a downward
trend in the amount of rainfall received in these regions overall, although
sub-Saharan Africa has been subject to a period of particularly low rain-
fall since the 1980s. This period of aridity is not unprecedented but rises in
sea temperature caused by anthropogenic climate change could contribute
to a situation of long-term aridity (Nicholson, 1993, 2001). It is also pos-
sible that an increase in aerosols in the atmosphere from pollution may be
causing reduced rainfall by reducing surface solar radiation (Ramanathan
