Analysis of Trend and Periodicity in Long-Term Annual Rainfall Time Series of Nigeria - Hydrologic Time Series Analysis: Theory and Practice - page 267

ecosystems and environment. In this study, parametric and nonparametric
statistical tests such as Least Square Regression test, Mann-Kendall test, rainfall
variability index, autocorrelation and spectral analyses are used for detecting
temporal and spatial trends and periodicities in 100-year period time series of
annual rainfalls in Nigeria.
The observed annual rainfall variability index showed slight differences
for the three zones, namely Guinea, Savannah and Sahel. Rainfall variability
characteristics, exhibited by the Savannah zone and the mean rainfall over
entire Nigeria, are very similar. Generally, it has been wet before the 1970s,
whereas there has been dramatic reduction in annual rainfall totals over Nigeria
in the post-1970s. The analysis of rainfall trends showed that more than 90%
of the entire landscape was generally drying, while only 10% of the total area
was getting wetter during the 20 th century. The annual rainfall has been reduced
significantly over 20% of the landscape. The amount of annual rainfall has
been declined by 50 to 350 mm in 63.7% (590,000 km 2 ) portion of the entire
country during the 20 th century. The 100-year period Nigerian annual rainfall
time series has an underlying sinusoidal signal embedded in random noise.
The signatures of QBO, ENSO and sunspot cycles are evident in the annual
rainfall series.
There is a need to extend the present study to include monthly and seasonal
analyses so as to capture intra-annual trends and distribution of rainfall over
Nigeria. Analysis of each calendar month and season will allow the
identification of time characteristics peculiar to each month and season, which
may be masked in annual total rainfall analysis. Furthermore, future related
studies should investigate country-wide changes in other rainfall characteristics
such as frequency of events, onset of rains, cessation and length of growing
season.
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