The Temporal Trend Analysis of Extreme Rainfall Events in Ga-West District of Accra, Ghana

摘要

The increasing frequency and impacts of flooding in the world have been blamed on global warming and climate change together with other anthropogenic factors. However some studies maintain the recent increase in flooding globally is mainly due to increasing extreme precipitation or rainfalls. In Ghana also, some researchers, technocrats and ordinary people believe that the recent increase in flooding in the city of Accra is resulting from increasing occurrence of extreme rainfall events, attributed to climate change. But this view was contested by other researchers who attributed the increasing flooding events purely to anthropogenic factors. This study, therefore, analyzed the temporal trend of extreme rainfall events from 1970 to 2009 to ascertain whether extreme rainfall events have been increasing significantly over the period under review in Ga West District to warrant the increasing flood events in the study area. A Manual Mann-Kendall Statistical Trend Test Table was used to analyze extreme 24hour maximum rainfall events which were extracted from secondary rainfall data procured from Ghana Meteorological Agency(GMet) for the Airport Weather Station as proxy data. Since the major raining season occurs from April to July in southern Ghana, the annual number of days of these extreme 24hour maximum rainfall events for the four months were extracted from the raw data for each year, from 1970 to 2009 for temporal trend analysis at the p=0.05 level of significance. The hypothesis of the study was as follows: i) Null hypothesis (H 0 ): there was no monotonic trend in the extreme 24-hour maximum rainfall events in the study area. ii) Alternative hypothesis (H 1 ): there was a monotonic trend in the extreme 24hour maximum rainfall events in the study area. The study found that Z=0.0058 which was less than Z (1- p / 2 ) = 1.96 at p=0.05 significant level. The null hypothesis (H 0 ) was, therefore accepted and the alternative hypothesis rejected. The acceptance of the null hypothesis shows that there was no statistically significant increase in extreme 24hour maximum rainfall events in the study area. The study, therefore, concluded that since there was no statistically significant increasing trend in extreme 24 hour maximum rainfall events, extreme rainfall events could not be the reason for the increasing flooding in the study area as some believed, but rather anthropogenic factors, or a combination of both.