Multi-objective optimal operation of cascade hydropower plants considering ecological flow under different ecological conditions

•A revised ecological flow method is proposed based on the variable monthly flow method and the Tennant method.•With the increase in environmental flow requirements of the river, the cascade power generation has dropped significantly.•Tradeoffs between the three optimal objectives are revealed using a decision-making support model. Maintaining ecological flow and mitigating impacts on river health are among the core requirements for sustainable hydropower development. Designing specific operational modes to fit specific ecological conditions of rivers remains a key practical challenge for hydropower operation, especially in terms of balancing the various tradeoffs among multiple objectives in the context of coordinated operation of cascade hydropower plants. Taking the Yalong River, China as an example, this study develops a decision-making support model that can quantitatively analyse the tradeoffs between the hydro-energy generation and river ecosystem projection under different hydrological conditions. The model contains three main components – calculation of ecological flow, multi-objective optimal operation, and determination of optimal schemes. For the ecological flow determination, we propose a revised variable monthly flow method (RVMF), which incorporates variations of hydrological conditions, also combines the classification concept of the Tennant method. For these objectives, we consider cascade power generation, the assurance rate of cascade power generation, and the assurance rate of ecological flow. The preliminary application in the Yalong River case has shown that (1) with the increase in environmental flow requirements of the river, the cascade power generation has dropped significantly, and the most affected is the Jinping-II hydropower plant; (2) there are clear tradeoffs between the cascade power generation and the assurance rate of cascade power generation, and the cascade power generation and the assurance rate of cascade power generation under different riparian ecological conditions; and (3) being able to quantify those tradeoffs provides crucial scientific evidence for stakeholder dialogue and informed decision making for hydropower operation modes that can achieve the best desirable balances between power generation and river health conservation.