Rainfall Trends and Variability over Onitsha, Nigeria

Analysis of trend and variability in rainfall can provide the necessary information required for water resources planning and management in any geographical setting. Therefore, the present study applied standard tests to investigate rainfall trends and variability using monthly, seasonal and annual series over Onitsha, Nigeria between 1971 and 2008. Rainfall variability index revealed the year 1983 as the driest (-2.38) and 1997, the wettest (+2.0), with more dry years observed between 2000 and 2008. Variability was relatively low annually as compared to seasonal and monthly series. September (15.8%) has the highest contributions to total annual rainfall, while January contributed the least (0.6%). Seasonally, about 40% of the annual rainfall was received in June-July-August (JJA), while the lowest rainfall was during December-January-February (DJF) (3.75%). Trends were mostly insignificant on monthly basis with 5 of the 12 months exhibiting negative trends, while only January depicted positive significant trend (p< 0.05). Similarly, only JJA exhibited insignificant upward trend while other seasons showed downward trends that are also not significant. On the annual basis, an insignificant negative trend was observed for the period under study. Hence, farmers and water resources managers may need to develop appropriate management strategies which include construction of more water storage and diversion structures such as reservoirs and dams to combat recurrent flooding during summer seasons and potential future water scarcity in the area. Keywords. Onitsha, rainfall, trend analysis, variability index