The sustainable development of water resources has been an important issue particularly under the condition of global warming. The future temperature and the amount of rainfall have to be considered when the future water supply policy is determined. In this paper, the future water supply policy is evaluated using the future predicted rainfall. The forecasted total amount of rainfall in year 2100 has not been changed much from the assessment of Intergovernmental Panel on Climate Change in 2008, but the monthly pattern in the Han River Basin, South Korea has been shifted significantly. Historically, Korea has had the high rainfall during the summer and the spring has experienced drought often. The different rainfall pattern should lead the different reservoir operation policy to supply enough water to users and prevent flood and drought. The water supply system developed using system dynamics approach simulated the different water supply policies under the climate change scenarios. As the results, the reservoir operation has to be changed to prevent flood in spring and drought in winter.