Study on the water environment risk conduction of water diversion during flood season at different starting water levels of the Nansihu Lake

Since the implementation of the Eastern Route of the South-to-North Water Diversion Project, it has effectively mitigated water shortages in regions like the Shandong Province. However, the contradiction between the supply and demand for water resources remains prominent. Floodwater transfer is likely to be one of the ways to solve the contradiction, but if floodwater transfer is considered, the river around the lake will open flood gates, and pollutants in the river will be leaked into the lake with the water flow, which will greatly increase the risk for the water environment in the basin. To scientifically support the flood season water environmental risk control of the East Route follow-up project and explore the risk transmission mechanism within the lake district, we employed a spatial conduction analysis model for water environment risk, focusing on the upper lake of Nansihu Lake, to quantitatively assess the water environment risk status. The probability of adjacent sub-lakes in different risk state combinations is presented the conduction patterns of water environment risk during floodwater conductions under three distinct starting water levels. The results reveal a notable consistency in risk conduction between the southern and northern parts of the upper lake. The results show that the environmental risk value of the upper lake of Nansihu Lake is the smallest when the water is transferred at high water level in a wet year and the highest in a dry year. In the case of high and medium water level water transfer, the risk is gradually transmitted from low risk to medium risk and then to high risk, while in the case of low water level water transfer, the risk is directly transmitted from low risk to high risk. For example, from Baima River to Wanfu River, the probability of low-medium risk enhanced transmission is 93%, and the probability of low-high risk enhanced transmission is 7%.