In order to climate change, it become increasingly important to enhance the operation of existing dams. By operating dams more flexibly, we can maximize their functions, improve flood control capabilities and increase hydropower generation. However, there is currently a lack of consideration for advanced dam operation, including pre-release, post- flood operation and flexible dam operation. This study used long-term ensemble rainfall prediction in multipurpose dams to determine target drawdown water level for prerelease operation, and flexible dam operation for hydropower production. We investigated how these new rules improve overall dam operation including flood control safety, water usage and hydropower generation. By adjusting the reference water level based on ensemble rainfall prediction, we can effectively manage the dam water level while monitoring flood risk and water usage capacity. Additionally, maintaining a high water level in the dam when no flood is expected ensures the efficient operation of the dam-type power plant, leading to increased hydropower generation. If the maximum hydropower intake discharge is small comparing to reservoir storage, the minimum discharge can be selected as the hydropower intake rate to maximize hydropower without affecting water usage.