Abstract: Based on the review of the successive four assessment reports of the IPCC WGII on climate change impacts on streamflow from 1990 to 2007, it is summarized that the First (FAR) and Second (SAR) Assessment Reports were regarded as the first generation studies, featuring the impacts of mean climate change on streamflow and the adaptation to it, while the Third (TAR) and Forth (AR4) Assessment Reports as second generation studies, emphasizing the impacts of anthropogenically forced and natural climate changes on streamflow and the adaptation to them. The progresses and issues in the research methodologies of the impacts of climate change on hydrology and water resources are analyzed. It is pointed out that the impacts of decadal and multi-decadal variability of climatic variables on streamflow can be identified, but changes in interannual and daily variabilities are not taken into account in the conventional methodology of hydrological impact studies. As the results, the impacts of climate change on extreme events, such as droughts, floods and irrigation water demands are underestimated. As for further studies, the paper comes to conclusion suggesting to emphasize the interdisciplinary science study of hydrology and climatology in the next IPCC Assessment Report.

Key words: climate natural variability, anthropogenic climate change, streamflow, IPCC Assessment Report