Eco-flow Assessment of Hydropeaking Rivers in the Xiangjiang River Basin

Abstract Hydropeaking refers to short-duration and high-frequency fluctuations in river discharge that are caused by the operation of hydroelectric power plants. The impacts of hydropeaking can be overlooked when environmental or ecological flow (“eco-flow”) assessments rely on long-term flow characteristics and lower-resolution hydrological observations. This study evaluated how hydrological eco-flow methods and temporal resolution influence whether critical eco-flow thresholds are achieved under hydropeaking conditions. The Tennant, Flow Duration Curve (FDC), and Minimum Monthly Flow (MMF) methods were applied to estimate eco-flow thresholds in three hydropeaking rivers in the Xiangjiang River Basin, China, using historical discharge records. High-frequency discharge data from 2023 were then used to characterise hydropeaking and calculate Guaranteed Rates (GRs), defined as the proportion of time that discharge met or exceeded the derived thresholds. The results showed pronounced differences in short-term flow variability between control and hydropeaking-influenced reaches, with substantially higher peak frequencies at hydropeaking sites. Eco-flow thresholds also varied among the three methods, with the MMF method generally producing higher thresholds than the Tennant Method. Guaranteed Rate estimates varied substantially among temporal resolutions at hydropeaking-influenced reaches. For example, at XH1, immediately downstream of the Centianhe Hydropower Dam, the Tennant-based Guaranteed Rate was 52.74% using hourly data, compared with 97.81% using daily average data and 43.84% using fixed-time observations at 08:00. The results indicate that both threshold-setting method and temporal resolution should be considered when assessing eco-flow threshold attainment in hydropeaking rivers. Our work suggests that the Tennant and MMF methods provide complementary information on minimum-flow and seasonal low-flow conditions, while high-frequency discharge monitoring should be considered where substantial within-day flow variability occurs.