Impacts of climate change on Blue Nile flows using bias-corrected GCM scenarios

Abstract. Global circulation models (GCMs) depict different pictures for the future of Nile basin flows in general and for the Blue Nile sub-basin in particular. This study analyses the output of 17 GCMs included in the 4th IPCC assessment report. Downscaled precipitation and potential evapotranspiration (PET) scenarios for the 2081–2098 period were constructed for the upper Blue Nile basin. These were used to drive a fine-scale hydrological model of the Nile Basin to assess their impacts on the flows of the upper Blue Nile at Diem, which accounts for about 60% of the total annual Nile yield. All models showed increases in temperature with annual values ranging from 2°C to 5°C. All GCMs also showed increases in total annual PET varying from +2 to +14%. GCMs disagreed on precipitation changes with values between −15% and +14%, but more models reported reductions (10) than those reporting increases (7). The ensemble mean of the 17 GCMs showed no change. Compounded with the high climatic sensitivity of the basin, the annual flow changed by values ranging between −60% to +45%. The increase in PET either offsets the increase in rainfall or exacerbates its reduction and the ensemble mean flow is reduced by 15%. The results were also used to study the linkages between temperature, rainfall, PET and flow. Simple relationships are devised that can be used to estimate the impacts of climate change and facilitate comparison with output from other hydrological models and GCMs.