DRY AND WET SPELL VARIABILITY OVER NIGERIA

Abstract

Wet and dry spell events have critical influence on the duration of rainy season, mostly, in terms of false onset or early or late cessation of the season(it also often intervene between the onset and retreat period). This study examines the seasonal, annual and spatiotemporal variability of wet and the dry spells for selected weather stations in Nigeria. The dataset used are observed daily rainfall series from 23 synoptic stations for the period 1975 - 2013. The analysis focused on the three climatic zones; Guinea coast, Savannah, and Sahel. Results show that the main contribution in the wet and dry comes from those lasting for 2 days in all the climatic zones. The contributions of shorter wet and dry spells appear to be more important in the Guinea Coast, Savannah and Sahel. These trends are of no significant in most stations. The standardized precipitation index reveals that wet and dry rainfall years is not absolute in all climatic zones, that is, a typical wet year in the Guinea Coast may not necessarily be wet in Savannah or/and Sahel. Furthermore, the frequency of wet spells in dry year composite is higher (around 60 events) for longer wet spell duration in Guinea Coast while the reverse is the case for Savannah and Sahel. The frequency of dry spells in dry years composite is severe and can reach 300 cases when compared with the wet years. Findings from this study therefore emphases the need for accurate forecast of wet and dry spells as well as their variability, which is critical for planning agriculture activities, that largely dependent on the monsoon rainfall.