Ecological flow assessment and guarantee of Longyangxia and Liujiaxia Cascade Reservoirs
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Abstract:
Assessing ecological flow is an important measure to enhance the ecological function of reservoirs and maintain the healthy life of rivers. The ecological flow process proposed using cutting-edge methods generally includes components such as base flow, high flow pulses, and floods. The various flow regime is crucial for the ecological health of rivers, but the complex ecological flow process makes it difficult to put it into practical application. How to face practical needs and coordinate the scientificity and practicality of the ecological flow process is a difficult problem that urgently needs to be solved in this field. In 2022, the Ministry of Water Resources of the People's Republic of China launched the pilot work of ecological flow assessment and guarantee for constructed water conservancy and hydropower projects, aiming to strengthen the management and supervision of ecological flow. The Longyangxia Reservoir and Liujiaxia Reservoir in the main stream of the Yellow River have been included in the list of pilot projects. Longyangxia Reservoir and Liujiaxia Reservoir were constructed long ago and there is no ecological flow release task in these two reservoirs scheduling currently. Taking Longyangxia Reservoir and Liujiaxia Reservoir as examples, a method for ecological flow assessment based on practical needs for cascade reservoirs is proposed. Firstly, the Tennant method and QP method are used to calculate the reasonable range of ecological flow. When using the Tennant method, the range of ecological flow during periods of low water is chosen as 10% to 40% of the annual average natural flow, while during periods of high water, the range of ecological flow is chosen as 30% to 60% of the annual average natural flow. When using the QP method, the frequency is selected as 90%. Then, the flow control indicators in the main affected river sections of cascade reservoirs approved by the government and the demand range for outflow from the existing scheduling tasks of cascade reservoirs are sorted out. After coordinating the needs of all parties, the initial value of ecological flow is proposed. Next, the guarantee capacity of ecological flow is analyzed. By establishing a simulation model for cascade reservoirs operation, the reservoirs' ability to guarantee the ecological flow is simulated. Meanwhile, a long series of measured reservoir outflow data is used to calculate the historical guarantee rate of ecological flow. If the guarantee capacity is insufficient, it is necessary to adjust the ecological flow. For cascade reservoirs, it is also necessary to analyze whether the ecological flow discharged from upstream reservoirs can meet the ecological flow requirements of downstream reservoirs. After coordination analysis, the recommended ecological flow value is finally obtained. Results show that the ecological flow of Longyangxia Reservoir is 400 m3/s from May to June, 200m3/s in other time periods. The ecological flow of Liujiaxia Reservoir is 285 m3/s. The long-term guarantee rates of two reservoirs are not less than 90%. The Longyangxia Reservoir and Liujiaxia Reservoir both have good ecological flow guarantee capabilities. The simulated guarantee rates are both 100%. The historical guarantee rates during the 1990-2019 hydrological year were 95% and 97%, respectively. However, ecological flow guarantee still faces risks under extreme low water conditions, such as the historical guarantee rates were 81% and 82% in the 2002 hydrological year, respectively. Implementing ecological flow in Longyangxia Reservoir and Liujiaxia Reservoir will face two prominent issues: ecological flow guarantee during sensitive periods under extreme low water conditions and the impact of interval cascade power stations. May to June (sensitive periods) is the concentrated spawning period for fish in the section from Longyangxia Reservoir to Liujiaxia Reservoir, which requires a large flow to stimulate spawning and form suitable spawning grounds. However, historical guarantee rate in May to June was much lower than other periods, indicating the need to increase the priority of ecological flow release during sensitive periods in the future. There are 11 hydropower stations built between Longyangxia Reservoir and Liujiaxia Reservoir. Although the storage capacity of these hydropower stations are relatively small, it may still affect the daily flow process. It is suggested to promote the ecological flow assessment work of the cascade power stations in this section in the future.
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