The trend analysis of rain over threshold the 100 mm on the Bia basin at south east Cote d'Ivoire

• PROBLEM STATEMENT

Recent flooding in the Bia watershed has affected the Ayamé 1 and 2 hydroelectric dams functionnality and caused damage in the Aboisso area. Studies have revealed that the threshold for observing these floods is estimated at 100 mm of rainfall. It therefore appears necessary to verify the link between the 100 mm rainy days trend and the floods recurrence on the Bia basin.

• OBJECTIVE

This study aims at analyzing the trend of rain over a threshold 100 mm in the Bia watershed.

• METHODOLOGY

The frequency of rainy days over a threshold 100 mm was constituted from 1941 to 2015 from the daily rainfall available over the same period at the stations Adiaké, Bianouan, Aboisso, Ayamé and Agnibilékro. The linear regression between these rainfall frequencies and time allowed us to assess the trend of rainfall days over threshlod 100 mm at each station. The slope of the regressions allowed us to appreciate the increasing or decreasing trends while the the t-test probability p applied to the regression slope highlights the highly significant (p < 0.01), significant (0.01 ≤ p <0.05) and non-significant (p ≥ 0.05) trends.

• RESULTS

The daily rainfall over threshold the 100 mm frequency varies from 15 days in Agnibilékro to 120 days in Adiaké. The trend in these rainfall thresholds is characterized by an average decrease of 0.5% per decade. On an interannual scale, this negative trend was very significant in Adiaké and Bianouan, then significant in Aboisso and insignificant in Ayamé and Agnibilékro. An intermonthly observation, the decrease was very significant in May, June and July in Aboisso, Adiaké and Bianouan respectively.

• CONCLUSION

The flooding recurrence in the Bia basin is not related to an increase in the frequency of threshold rains (100 mm) of flooding, it could be related to the vulnerability of the basin.