Analysis of Trend and Periodicity in Long-Term Annual Rainfall Time Series of Nigeria - Hydrologic Time Series Analysis: Theory and Practice - page 250

also holds for the rainfall retreat period (Iloeje, 2001). About two thirds of the
cropped area of Nigeria is in the north, with the rest cropped areas distributed
between the middle belt and the south. According to the results of 2006
census, Nigeria is by far the most populous country in Africa with over 140
million people and population density of 138 people per km 2 .
Fig. 12.1. Map of Nigeria showing the location of the delineated
agro-ecological zones.
12.3 Methodology
12.3.1 Data Collection
Monthly rainfall data of Nigeria were collected from the Global Gridded
Climatology of Climatic Research Unit Time Series (CRU TS 2.1) presented
at a new high resolution and made available by the Climate Impacts LINK
project, Climate Research Unit, University of East Anglia, Norwich, UK
(Mitchell and Jones, 2005). The Climatic Research Unit (CRU) dataset is
composed of monthly 0.5 o latitude/longitude gridded series of precipitation;
mean monthly temperature, diurnal temperature range, wet-day frequency,
vapour pressure, cloud cover and ground-frost frequency (New et al., 1999).
The 0.5 degree resolution dataset of monthly surface-based climate parameters
cover the period 1901-2002. Amongst these parameters monthly accumulations
of precipitation are generated from available gauge datasets. Although the
time series extends back to 1901, it should be noted that the number of
available gauges varies with time, for example, in 1901 a total of 4,957