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the seasonality of rainfall distribution in Nigeria for two periods, 1930-1961
and 1962-1993, and Alli (2010) studied rainfall trends and cycles for 20
rainfall stations scattered over Nigeria since 1960. Adefolalu (1986) analyzed
70-year period (1911-1980) rainfall data from 28 meteorological stations to
examine trends in precipitation patterns. The results indicated a general decrease
of dry season contribution to annual rainfall. In Nigeria, with more than 70%
of the populace engaged in agriculture that is mostly rainfed, rainfall is the
most important climatic variable owing to its manifestation as a deficient
resource (droughts) or a catastrophic agent (floods). Therefore, for the purpose
of sustainable water resources planning and management, trends and
periodicities in long-term rainfall must be examined. The main aim of this
study is to detect significant temporal and spatial trends and periodicities in
the long-term (1901-2000) annual rainfall time series of Nigeria. This chapter
is organized as follows: after presenting the background of this study, an
overview of the study area is presented in Section 12.2 followed by
methodology in Section 12.3, results and discussion in Section 12.4 and
conclusions in Section 12.5.
12.2 Study Area
Nigeria is located in West Africa between latitude 4
o
-14
o
N and longitude
2
o
-15
o
E and encompasses a total area of about 925,796 km
2
. The climate of
Nigeria is more varied than that of any other country in West Africa. This is
the result of the great length from south to the north (1100 km), which covers
virtually all the climatic belts of West Africa. The climate is dominated by the
influence of three main wind currents. These are the tropical maritime (MT)
air mass, the tropical continental (CT) air mass and the equatorial easterlies
(Ojo, 1987). The first air mass (MT) originates from the southern high-pressure
belt located off the Namibian coast, and along its way picks up moisture from
over the Atlantic Ocean and is thus wet. The second air mass (CT) has the
high-pressure belt north of the Tropic of Cancer as its origin. This air mass is
always dry as a result of little moisture it picks along its way. The first two air
masses, MT and CT, meet along a starting surface called the Inter-Tropical
Discontinuity (ITD). The third air mass (equatorial easterlies) is a somehow
erratic cool air mass, which comes from the east and flows in the upper
atmosphere along ITD. This air mass penetrates occasionally to actively
undercut the MT or CT and gives rise to squall lines or dust devils (Iloeje,
2001). The entire country is grouped into three ecological zones based on
latitude, which are Sahel (11
o
-14
o
N), Savannah (8
o
-11
o
N) and the Guinea
(4
o
-8
o
N) zones as shown in Fig. 12.1. The climate is semi-arid in the north and
humid in the south and also humid strip along the coast with an average
annual rainfall of more than 2000 mm. Rainfall commences at the beginning
of the rainy season around March/April from the coast (in the south), spreads
through the middle belt, reaching its peak between July and September, to
eventually get to the northern part very much later. The reverse of the situation
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