Analysis of Trend and Periodicity in Long-Term Annual Rainfall Time Series of Nigeria - Hydrologic Time Series Analysis: Theory and Practice - page 248

Several studies on analysis of rainfall time series have been carried out at
different temporal scales and in different parts of the globe. Existing analyses
of rainfall time series show for some areas a positive trend and a tendency
towards higher frequencies of heavy and extreme rainfall in the last few
decades (Houghton et al., 1996). In the Canadian Prairies, a significant increase
in the amounts and number of rainfall events have been reported (Akinyemi
et al., 2001). Ati et al. (2009) reported significant increase in rainfall over nine
stations in northern Nigeria between 1953 and 2002. Increasing trends were
also reported for Ibadan in southwest Nigeria (Oguntunde et al., 2011). Many
researchers analyzed the precipitation patterns in several parts of Europe.
Brazdil (1992) described fluctuations of precipitation in Europe using a time
series of annual areal precipitation totals. Some of the results of precipitation
analysis suggest that spatial and temporal non-uniformity in trend exists,
which make generalization over large areas difficult if not impossible.
Significant positive and negative trends have also been observed in the USA
(Karl et al., 1995; Trenberth, 1998; Kunkel et al., 1999), Australia (Suppiah
and Hennessey, 1998; Plummer et al., 1999), South Africa (Mason et al.,
1999), the United Kingdom (Osborn et al., 2000) and Italy (Brunetti et al.,
2000; 2001). Besides the increase in precipitation intensity, there are some
indications that the overall percentage of the earth's surface affected by either
drought and/or excessive wetness has increased (Dai and Trenberth, 1998).
Gemmer et al. (2004) analyzed the annual rainfall series of 160 stations in
China. They observed a spatial clustering of the trends in certain months,
including district trend belts in east and northeast China.
Recent studies have shown that Africa has been drier over the last few
decades (Nicholson et al., 2000; L'Hóte et al., 2002; Oguntunde et al., 2006).
Furthermore, there are two schools of thought regarding the recent trends in
Sahelian rainfall. Some researchers believe that the Sahelian drought continued
till the end of the 20 th century (L'Hóte et al., 2002), while others argue it may
have ended in the 1990s (Ozer et al., 2003). Ojo (1987) examined the
characteristics of rainfall variations between 1901 and 1985 in West Africa
and discussed such characteristics as periodicities and variabilities. The study
found no observable regular patterns in trends, periodicities and persistence of
hydrologic consequences of rainfall variations to allow for predictability of
these consequences in relation to rainfall variations. The role of rainfall trends
in poor growth performance of sub-Saharan African nations relative to other
developing countries, using a new cross-country panel climatic dataset in an
empirical economic growth framework was examined (Barrios et al., 2010).
The results showed that rainfall has been a significant determinant of poor
economic growth for African nations but not for other countries.
In Nigeria, studies on rainfall analyses have been reported for different
periods and locations within the country (e.g., Adefolalu, 1986; Tarhule and
Woo, 1998; Bello, 1998; Olaniran, 2002; Ogolo and Adeyemi, 2009; Ati et al.,
2009; Alli, 2010; Oguntunde et al., 2011). For example, Bello (1998) compared