Rainfall trends in Nigeria, 1901–2000

There is the need to evaluate changes in the spatial and temporal patterns of rainfall in order to improve water management strategies of a given region. In this study, standard tests are used to examine the existence of trend in annual and monthly rainfall of Nigeria over the last century. Rainfall variability index was estimated as standardized rainfall departure while autocorrelation spectral analysis is used to obtain the periodicities inherent therein. Rainfall spatial distribution was highly latitudinal dependent (r2 > 0.90) and had no clearly linear relations with the longitude. Rainfall variability index showed that 1950s was the wettest decade (+0.84) while 1980s was the driest (−1.19), with the two decades between 1970 and 1990 being drier than any other comparable period in the last century. Observed rainfall changes varied between −3.46 and +0.76 mm yr−2. About 90% of the entire landscape exhibited negative trends but only 22% showed significant changes at 5% level. There was a sharp difference between changes in rainfalls in 1931–1960 and 1961–1990 periods. Annual precipitation reduced by 7% between the two periods. While more than 90% of the landscape showed no significant rainfall change in the first period, about 57% of Nigeria showed a significant (P < 0.05) decrease in the second. The dominant peaks can be classified into four distinct rainfall cycles with periods 2–3, 5–7, 10–15 and 30 yr. These cycles may be associated with the stratospheric Quasi-Biennial Oscillation (QBO), the El-Nino Southern Oscillation (ENSO); the sunspot cycles and the Atlantic Multi-Decadal Oscillation (AMO) sea surface temperature, respectively.