Abstract: The optimal operation chart of a hydropower station is a useful tool to guide its optimal operation, and the remaining benefit is a key factor restricting the effect of an operation scheme. Based on the information of historical flows at the dam site, this paper calculates the long-term remaining benefit for the reservoir storage at year end. And, from the magnitude and frequency of the inflows in each period, we calculate optimal expected benefits for different reservoir water stages at the beginning of each period in a reverse order starting from year end, using the dynamic programming method. Then, optimal decision charts are made for different typical inflow conditions in each period based on the different reservoir stages at its beginning, so that the charts of all the periods can serve as an optimal operation atlas for long-term power generation of the reservoir. A case study of the Nam Ou VII hydropower station in Laos shows this new method offers long-term operation schemes that are simple and on clear principles, achieving a benefit of 1.03% higher than traditional operation and 0.87% higher than function-based operation. This demonstrates that for long-term optimal operation of reservoirs, it is of great significance to make use of historical runoffs and flow predictions and to consider the impact of reservoir decision on remaining benefits.

Key words: reservoir operation, optimal operation atlas, remaining benefits, long-term, runoff forecast