The impacts of water transfer on water quality under internal and external nutrient load in Chaohu Lake

Water transfer can alleviate the uneven distribution of regional water resources and improve the eutrophication status of water bodies in the receiving regions. However, few studies have quantitatively analyzed water quality changes in eutrophic lakes under water division considering the effects of internal and external nutrient load. Chaohu Lake, a large shallow lake in East China, was used as an example. Daily meteorological, hydrological, and water quality data for 2023-2024 were applied to develop a coupled hydrodynamic-water quality model using the Environmental Fluid Dynamics Code, considering seasonal sediment release of ammonia, phosphate, and nitrate. Based on this model, the seasonal contributions to nutrient loads from internal and external sources were quantified. Water transfer scenarios were employed to simulate and compare the spatial and temporal changes in the water level and water quality of Chaohu Lake. The results demonstrated that: 1) The simulated values of the water level, water temperature, and water quality parameters (including TN, TP, and DO) fit well with the measured values, thus confirming the reliability of the model. 2) The annual endogenous release of phosphorus and nitrogen accounted for 76.8 % and 75.3 % of the total load from internal and external sources, respectively. 3) Water transfer measures significantly altered the hydrological conditions of the lake, exacerbated seasonal fluctuations in nutrients, and increased sediment release, leading to an increase in TP concentration, and decrease of TN and DO. Under the four water transfer scenarios, the water transfer path of diverting water into Chaohu Lake (150 m³/s) through Zhao River and using the Baishitian River as the outlet significantly improved the water quality of the entire lake, with 5.03 % decrease in TN concentration and a minimum increase (14.29 %) of TP. This study provides a scientific basis for water diversion scheduling and eutrophication regulation in lakes.