FLOOD FREQUENCY UNDER CHANGING CLIMATE IN THE UPPER OMO-GIBE RIVER BASIN, SOUTHWESTERN ETHIOPIA: CASE STUDY ABELTI-CATCHMENT

Abstract

The study investigated the climate change impact on the frequency of flood flows for the Abelti-catchment in the Upper Omo-Gibe river basin in the southwestern Ethiopia by using the HadGEM2-ES climate model outputs of RCP 2.6 and RCP 4.5 under CMIP5 for the future two horizons 2020`s (2021-2050) and 2050`s (2051-2080). Areal annual mean and seasonal rainfall, minimum and maximum temperature and evapotranspiration are projected and Nonparametric Mann-Kendall statistical test was used to detect trends in case of the annual mean. Then a continuous daily hydrologic model (HEC-HMS) was used to generate daily flow series for the baseline and for the future time horizons. Lastly, flood frequency was analyzed considering peak over-threshold and annual maximum data series by using General Pareto distribution and Generalized Extreme Value Distribution, respectively for the current, baseline and future. The results showed increment in trend and in percentage of all selected climate variables in both periods except rainfall, which showed variability over the catchment. The value respectively of calibration and validation R 2 ; 0.82 and 0.78 and NSE; 0.74 and 0.75 that shows satisfactory performance of the model. The streamflow is expected to increase and decrease annually in the future projection (-2.70% and 0.065%) in 2020`s and (+3.48% and +9.72%) in 2050’s under RCP 2.6 and RCP 4.5 respectively. Recurrent flood frequency increased in both directions in the future under both scenarios projection. The increase in flood magnitude in the area may have a significant negative effect on the sustainability of existed agriculture, life of the community and hydropower development projects at downstream. Moreover, it may lead to unexpected flood and flood hazards. Therefore, adaptation measures are recommended to be sought for possible flooding in the flood plain areas of the River basin at downstream.