Variation in Onset and Cessation of the Rainy Season in Ibadan, Nigeria

Abstract

The need to understand changes in rainfall characteristics associated with climate variability and change in tropical Africa has become very important since the livelihoods of a majority of the population depend on climate-sensitive activities. The onset and cessation dates of rainfall is of utmost significance to agriculture in many West African countries because the sector  is largely rain-fed. The paper examines annual variations  and patterns in the onset and cessation dates; as well as the length of the rainy season in Ibadan for the period 1981-2010. A modification of the method employed by Jolliffe and Saria-Dodd was adopted for the determination of the onset of the wet season. The mean onset date of the rainy season for the study period is the 125th day in the year while the mean cessation date is the 306th day in the year.  The determined mean onset date of the rainy season for the period of study is later than that estimated for earlier climatological period. A comparative analysis of  the rainy season characteristics for two time slices within the study period, 1981-2000 and 2001-2010, show differences in mean onset and cessation dates for the rainy season and a corresponding reduction in the length of the rainy season during the latter period. The difference between the means of the length of the rainy season during the two periods was shown to be significant. The emerging pattern of the characteristics of the rainy season has important implications for agricultural practice in the region.