Rainfall Variability In Ghana During 1961- 2005

Abstract

This study uses daily rainfall data for 40-45 years to investigate the extent and nature of vari-ability in the onset, cessation, annual rainfall and length of the rainy season (LRS) over four stations in Ghana (Tamale, Kumasi, Axim and Accra). The study shows that generally, the early onset and late cessation dates that occurred in the 1960s and early 70s and which resulted in long rainy seasons almost everywhere in Ghana have now changed significantly, the onset oc-curring rather late with early cessation since the 1980s. However, cessation for Accra was late from the 90s resulting in slightly longer rainy season. Pentade series of annual rainfall and rainy days also show a significant downward trend at all the stations but particularly more so at Axim. All these have serious consequences for agricultural practices and water resources man-agement, particularly for hydropower generation in the country. Classification of the annual rainfall into wet and dry years and consideration of moisture availability shows significant dif-ferences between years of abundant and deficient rainfall. It was found that wet (dry) years have early (late) onset and late (early) cessation resulting in longer (shorter) rainy season. Also, rain-fall amount and length of the rainy season during dry years are reduced by about 50% and more than 30 days, respectively at all stations. In general, main rainfall begins about 2-months after the moisture influx has attained a maximum value over all stations. The study also shows that dry spells of 10 or more days between rain events occur more frequently in the southern parts of Ghana than the north.