Multi-objective optimal operation for Danjiangkou Reservoir considering ecological indicators of hydrologic alteration
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Abstract:
The construction and operation of reservoirs changed the downstream hydrological regime and affected the reproduction of aquatic organisms. The current multi-objective reservoir operation models maximized the economic and ecological comprehensive benefits which only quantify a single ecological flow index. However, downstream river ecology is not only reflected by the magnitude of the streamflow, but also affected by the hydrologic alternation. Hence, it is necessary to explore an optimal operation scheme that can balance the objectives of flood control and economic comprehensive benefits and reduce the negative impact on the downstream ecological environment.Ecological indicators that could reflect the downstream ecological water demand and fluctuation were constructed according to the indicators of hydrologic alteration (IHA). The degree to which the monthly average outflow discharge met the water demand stipulated by the Tennant method was quantified as water quantity level indicator. The daily scale IHAs were filtered and simplified to reflect the flow fluctuation, and the fluctuation deviation between the outflow discharge and the natural flow constituted the hydrological alteration indicator. Water quantity level indicator and hydrological alteration indicator were selected as ecological indicators of hydrologic alteration. The greater the water quantity level indicator with the high flow grade is, the better the ecological benefit. On the other hand, the smaller the hydrological alteration indicator with the less flow fluctuation, the better for reproduction of aquatic organisms.Four objectives, i.e., water supply, power generation, outflow discharge and hydrologic alteration, were used to optimize the operation of Danjiangkou Reservoir, in which the flood control task was taken as constraint. The water transfer volume of the South-to-North Water Transfer and the north Hubei water transfer is collectively referred to as water supply, and the outflow discharge of reservoir include flood discharge and power generation. The Gaussian radial basis functions were used to fit the reservoir operation rules, and the operation model with different combination of the objective functions was optimized. The water supply, power generation and two ecological indexes under different schemes were calculated. Based on the current operation schemes of Danjiangkou Reservoir, the dialectical relationship between each index as well as the pros and cons of each scheme were analyzed and compared to find the best scheme.The application results showed that: water supply benefit was negatively correlated with power generation benefit and ecological indicators; power generation benefit was coordinated with water quantity level indicator, but contradictory with hydrological alteration indicator. In each scheme, the best multi-objective optimal operation scheme considering water supply, power generation, water quantity level and hydrological alteration simultaneously could effectively improve the two ecological indicators on the premise of maintaining the current benefits of water supply and power generation unchanged. Effectively improving the two ecological indicators was conducive to the water ecological protection and the reproduction of aquatic organisms. The outflow discharge of the best scheme was much larger than the ecological baseflow and the minimum ecological flow, and almost reached the optimal ecological flow, which could satisfy the needs of natural reproduction of aquatic organisms in the middle and lower reaches of the Hanjiang River.The water quantity level and hydrological alteration indicators based on IHA were proposed and included in the multi-objective operation model. Compared with the current operation scheme, the proposed reservoir operation model was able to reduce the negative impact on the downstream ecological environment with less data required. This practical approach could provide a reference for the multi-objective ecological operation of Danjiangkou Reservoir.
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