Abstract: The water storage and power generation of small hydropower on small and medium-sized rivers not only brings about economic benefits but also affects the environment. The ecological flow calculation and ecological operation of small hydropower are particularly important to refrain from dehydration in the downstream. With Hengxi Hydropower Station on Yangxi River as a case study, we first of all calculate the runoff according to the relation between rainfall and runoff depth, and further calculate the ecological flow of wet year, flat year, and dry year respectively using the modified flow duration curve (FDC) method. Subsequently we calculate the ecological operation in consideration of ecological flow, power generation, and irrigation. Results reveal that we could accurately calculate the runoff of Hengxi Hydropower Station by using the relationship between rainfall and runoff depth. In the absence of operation, the inflow of Hengxi Hydropower Station could not fully meet the requirements of ecological flow calculated by the modified FDC method at each level year. The modified FDC method is applicable and rational. After ecological operation, the dam-crossing discharge of Hengxi Hydropower Station could meet the demand of aquatic ecology and maximize the benefit of power generation. The research results offer reference for promoting the sustainable development of hydropower, balancing the benefits of power generation and ecology, as well as the healthy management of small and medium-sized rivers.

Key words: small hydropower, ecological flow, ecological operation, flow duration curve, Hengxi Hydropower Station