Decadal Assessment and Distribution of Rainfall Anomaly Index (1991 – 2020) for Benin City, Edo State, Nigeria

Abstract

This study was designed to provide valuable insight into the temporal patterns of rainfall in Benin City, Edo State, Nigeria using rainfall data from 1991 – 2020 (30 years) collected from Nigerian Meteorological Agency (NIMET), airport station, Benin City. The data were assessed based on 10 years interval (decade) identified as decadal A (1991-2000), decadal B (2001-2010) and decadal C (2011-2020). The data was analysed descriptively using charts and graphs. Also, Rainfall Anomaly Index (RAI) was determined for each decadal. Findings from the study reveal that rainfall pattern changes significantly based on statistics for each decadal. In decadal A, rainfall usually began in the month of July to October, June to September in decadal B while May to September in decadal C with rainfall going above the annual precipitation (2679 mm) for the City. The rainfall anomaly over the city revealed that there was a composite nature in which some dry years were mixed with wet years and vice versa and this occurred in all decades. RAI revealed that decadal C recorded the highest number of years (7) of intense rainfall compared to decadal A and B. The trend for the average annual rainfall showed a significant trend based on the decade. The average annual rainfall increased with time (decade) as the trend rose from 1886.9 mm in decade A to 1890.0mm in decade B and 2078.8 mm in decade C. The year of greatest positive value was 2016 (decadal C), with an average RAI of 6.53 classified as extremely humid. Based on these findings, the study concludes that the climate in Benin City has significantly changed.