Hydrodynamic and water quality simulation of Yangzonghai Lake, Southwest China, using the two-dimensional CE-QUAL-W2 model.

Lake eutrophication is a major environmental concern in southwest China, where external inputs of nutrients and organic pollutants adversely impact plateau lakes like Yangzonghai (YZH) Lake. This study applied the two-dimensional CE-QUAL-W2 model to simulate the hydrodynamics and water quality of YZH Lake from 2016 to 2018, aiming to (i) assess the model’s performance for water level and surface temperature, (ii) identify spatial and temporal patterns of water quality, and (iii) determine pollutant reduction targets to meet Class II water standards. The model accurately simulated observed water levels (RMSE = 0.078 m, AME = 0.066 m, R = 0.91, p < 0.001) while moderately predicted surface temperatures (RMSE = 1.82 °C, AME = 1.53 °C, R = 0.80, p < 0.001). Total nitrogen (TN), total phosphorus (TP), and chemical oxygen demand (CODMn) exceeded Class III standards during strong thermal stratification periods, especially near major river inflows like the Baiyi and Yangzonghai Rivers. High nutrient and chlorophyll-α levels with the lowest transparency were observed during summer stratification. External inflows contributed over 70% of the total nutrient and organic matter input. Reducing external loads by 43% (TN), 26% (TP), and 10% (CODMn) annually is crucial to meet Class II standards. At the 75th percentile, 50%, 35%, and 40% reductions, respectively, are required. The internal cycling and hydrodynamics result in a non-linear relationship between load reductions and in-lake concentrations. The study demonstrates both the strengths and limitations of CE-QUAL-W2 model, offering a scientific foundation for establishing realistic pollutant reduction targets for sustainable lake management.