Abstract. The potential impact of climate change was investigated on the hydrological extremes of Nyando River and Lake Tana catchments, which are located in two source regions of the Nile River basin. Climate change scenarios were developed for rainfall and potential evapotranspiration (ETo), considering 17 General Circulation Model (GCM) simulations to better understand the range of possible future change. They were constructed by transferring the extracted climate change signals to the observed series using a frequency perturbation downscaling approach, which accounts for the changes in rainfall extremes. Projected changes under two future SRES emission scenarios A1B and B1 for the 2050s were considered. Two conceptual hydrological models were calibrated and used for the impact assessment. Their difference in simulating the flows under future climate scenarios was also investigated.The results reveal increasing mean runoff and extreme peak flows for Nyando catchment for the 2050s while unclear trend is observed for Lake Tana catchment for mean volumes and high/low flows. The hydrological models for Lake Tana catchment, however, performed better in simulating the hydrological regimes than for Nyando, which obviously also induces a difference in the reliability of the extreme future projections for both catchments. The unclear impact result for Lake Tana catchment implies that the GCM uncertainty is more important for explaining the unclear trend than the hydrological models uncertainty. Nevertheless, to have a better understanding of future impact, hydrological models need to be verified for their credibility of simulating extreme flows.

The potential impact of climate change was investigated on the hydrology and hydrological extremes of Nyando River and Lake Tana catchments, located in two source regions of the Nile River basin. Climate change scenarios were developed for rainfall and potential evapotranspiration (ETo), considering 17 different General Circulation Model (GCM) simulations to better understand the range of possible future change. Projected changes under two future emission scenarios for the 2050s were extracted from an ensemble of selected GCM experiments. The future climate change scenarios were constructed by transferring the extracted climate change signals to the observed series using a frequency perturbation downscaling approach, which accounts for the effect on rainfall and ETo extremes, its dependence on the return period of rain storm depth, and the correlation between rainfall and ETo changes. Two conceptual hydrological models were calibrated and used for the impact assessment. Their difference in simulating the flows under future climate scenarios was investigated. The results reveal that the hydrological models project increasing runoff extremes for Nyando catchment towards the 2050s while unclear trend is observed for Lake Tana catchment for cumulative volumes as well as high and low flows