The water quality of Lake Erhai has deteriorated in recent decades due to socioeconomic development in the lake basin. After the massive implementation of water environmental protection measures in Lake Erhai in 2016, the trend of water quality deterioration has been curbed and the intensity and frequency of algal blooms has decreased. However, water quality monitoring data show that pollutant concentrations in Lake Erhai still exceed acceptable values, and there is a risk of water quality standard limits being further exceeded in the future. Therefore, it is urgent to systematically study the variability characteristics of water quality in Lake Erhai to provide practical methods to predict the future evolution of water quality. Based on water quality monitoring data from 2009 to 2019, the current water quality characteristics of Lake Erhai were analyzed, and a two-dimensional hydrodynamic and water quality mathematical model was built to predict the water quality in 2025. The results showed that the total phosphorus (TP) concentration declined after 2016, mainly due to the significant reduction of TP entering the lake due to pollution interception. However, the concentrations of the potassium permanganate index (COD_Mn) and total nitrogen (TN) increased after 2016, demonstrating that the pollution control measures have had little effect on the improvement of COD_Mn and TN. The spatial and temporal distribution of pollutants showed that the water quality in winter and spring was better than in summer and autumn, and the water quality in the southern lake was better than in the northern lake. This analysis indicates that non-point source pollution remains the main source of pollution in Lake Erhai, and that rainfall is the main driving force of pollutants exceeding the water quality standard. According to the water quality predictions, without additional pollution control measures, pollutant concentrations in Lake Erhai will exceed the Class II water quality standard by 2025. This study analyzes the water quality characteristics, predicts the direction of future water quality changes, and provides a theoretical basis for the future water quality protection of Lake Erhai.

In order to facilitate works of water environment management in Yanming Lake Xi'an Shaanxi, the MIKE 21 hydrodynamic model coupled with the advection-dispersion equation was employed to develop a water quality model to investigate the characteristics of the flow field on the conditions of different wind and to simulate the variations of pollutants, which were Chemical Oxygen Demand (COD), Total Nitrogen (TN) and Total Phosphate (TP). The Back propagation Neural Net (BPPN) approach was introduced to utilize the results of simulation to evaluate water quality in Yanming Lake. Additionally, based on the distribution characteristic of pollutants, three scenarios for external pollutants' load reduction were proposed. Response Characteristics of water quality after implementation of different scenarios was conducted. The results showed that the time-average velocity value ranged from 0 to 0.01 m/s in Yanming Lake on the condition of both no wind and southwest wind. Six circulations could be induced by southwest wind in the flow field. The time-average COD concentration field could be divided into three regions, whose concentration was 12-13 mg/L, 13-14 mg/L, and 14-15 mg/L respectively. Time-average TN concentration field consisted of two regions, whose concentration was 2.85-3.00 mg/L and 3.00-3.15 mg/L separately, and TN concentration exceeded the standard of Grade V (GB3838-2002). Time-average TP concentration field had no significant zoned phenomenon and its concentration ranged from 0.040 to 0.048 mg/L. Results from evaluation of water quality indicated that water quality was in the standard of inferior Grade V. Additionally, water quality will be better improved after implementation of Scenario 2 rather than Scenario 1 and 3.