An environmental flow assessment of a river's blocking effect on a lake in a river-lake system: application in the Yangtze-Poyang system.

Hydrologic regimes are essential to riverine, lacustrine, and wetland ecosystems, and every component of a hydrologic regime has a specific ecological environmental function. In an outflow lake-river system, water levels are reduced by a reservoir constructed at the river, which interferes with the river-lake interrelationships by impairing the river's blocking effect. This increases the lake-river hydraulic gradient and accelerates the lake's drainage to the river, resulting in shrinkage of the lake and damaging environmental issues. To respond to these issues, we propose an environmental flow assessment that considers the river's blocking effect on the lake. This novel methodology consists of four steps: data preparation, assessment of the lake's environmental water level, assessment of the river's environmental water level, and environmental flow assessment. We estimated the river's environmental water level through a hydraulic correlation between the lake and the river, and found that the river's blocking effect could be sustained. The Yangtze-Poyang system was selected as a case study to illustrate the methodology's procedures and applicability. The impoundment of the Three Gorges Reservoir, during the fall retreating season, decreased the Yangtze's water level and weakened the Yangtze's blocking effect on Poyang Lake. Poyang Lake's environmental water level, which ranges from 11.71 to 15.81 m in the month of October, was used to estimate the Yangtze's environmental water level as falling in a range of 11.95 to 16.17 m, which corresponds to an environmental flow range of 16,822 to 32,371 m3/s. This study offers a solution for reservoir-induced accelerated lake drainage, which may be helpful in mitigating the negative impacts of reservoirs and sustaining natural rive-lake interactions.