Study On The Water Quality Features And The Water Quality Improvement Scheme Of The Yilong Lake

Located in Shiping County,Honghe Hani Yi Autonomous Prefecture,Yunnan Province,Yilong Lake has a water area of about 31km² and an average water depth of about 2.4m,making it the smallest lake in area and the least amount of water among the nine plateau lakes in Yunnan Province.In recent years,driven by multiple factors such as resource-based water shortage in the river basin,exogenous input such as production and life in the land area,and increasing endogenous enrichment in the lake,the water quality in Yilong Lake area has been at the level of class V.or inferior class V for a long time,and the degree of eutrophication of water bodies is serious.How to scientifically and effectively manage the ecological environment of Yilong Lake so that its water quality can get rid of deterioration as soon as possible and continue to improve has become a problem that governments at all levels and scientific researchers in Yunnan Province are very concerned about.Based on historical monitoring data and on-site monitoring results,this paper systematically analyzes the spatiotemporal evolution characteristics of water quality in Yilong Lake,identifies the main influencing factors of eutrophication succession of water in Yilong Lake in recent years,and studies the hydrodynamic,water temperature,water quality change process and temporal and spatial evolution characteristics in Yilong Lake area based on the constructed three-dimensional hydrodynamic and water environment mathematical model of Yilong Lake,analyzes and demonstrates the feasibility of water diversion from the outer basin to improve the water quality of Yilong Lake,and draws the following four understandings and conclusions.（1）Affected by the hydrological situation of the river basin,the seasonal fluctuation characteristics of the water level in Yilong Lake area during the year are obvious,and the inter-annual fluctuation range is large.In 2016～2021,the annual average change of water level ranged from 1412.53～1413.73m,of which the highest in 2018 and the lowest in2021.The water level of the whole lake showed a decreasing trend month by month in the first half of the year,falling to the annual minimum of 1412.86m in June and rising to a maximum of 1413.27m in September.The change of lake water level can be divided into three stages,January～April is the flat water period,May～July is the dry period,and August～December is the rich water period.（2）The analysis of the monitoring results of Yilong Lake in 2016～2021 shows that the p H value and DO concentration of Yilong Lake are Class III.water standards,NH3-N concentrations and TP concentrations are Class IV water standards,COD_Mnand BOD₅are Class V water standards,and COD and TN are inferior Class V.standards.The annual average changes of water temperature,transparency,conductivity,p H value and DO of Yilong Lake were 19.9～21.37°C,34.11～50.02cm,42.49～63.71m S/m,8.26～8.70,5.78～8.38mg/L,respectively.The annual mean variation ranges of COD_Mn,COD,BOD₅,TN,NH₃-N and TP were 8.33～14.93mg/L,36.97～68.08mg/L,3.66～6.42mg/L,1.82～2.56mg/L,0.12～0.24 mg/L,and 0.05～0.07mg/L,respectively.The sensitive period of water quality of Yilong Lake is generally in July～August every year.It is a phosphorus restricted water body,and the nitrogen-phosphorus ratio ranges from 15～90.The annual average concentration of Chl-a in the water body of Yilong Lake showed an increasing trend year by year,the concentration range was 39.6～177.7ug/L,the eutrophication index range of the water body of Yilong Lake was generally 60～70,and the nutritional status of the water body was moderately eutrophic.Through Pearson correlation analysis,ρ（Chl-a）was significantly positively correlated withρ（WT）,p H,ρ（BOD₅）,ρ（TN）,ρ（TP）,and negatively correlated with W＿cond and SD.（3）The three-dimensional hydrodynamic-water temperature-water quality coupling mathematical model of Yilong Lake was constructed using EFDC,and the rate verification results showed that the model could better reflect the evolution characteristics of water quality and eutrophication in Yilong Lake over time.The measured average value and the simulated average value of each water quality parameter are relatively close,the relative error of water temperature is controlled within 10%,the relative error of dissolved oxygen is controlled within 25%,the relative error of ammonia nitrogen is high,but it can better react to the change trend,and the relative error of total phosphorus,total nitrogen and chlorophyll A monitoring points is quite different,and the overall is within the acceptable range.Based on the coupled hydrodynamic-water temperature-water quality model,the endogenous nitrogen and phosphorus fluxes in the lake were determined by monthly rate from the perspective of nitrogen and phosphorus nutrient balance in the lake.The results showed that the annual average endogenous phosphate flux was about 3.21mg/m²/day,the ammonia nitrogen flux was about 5.33mg/m²/day,and the nitrate flux was about 0.53mg/m²/day.（4）Considering the adjustable water volume,the sensitive period of water quality of Yilong Lake,the available water diversion into the lake mouth and other factors,different diversion scenarios were designed,and the water quality improvement effect of different diversion scenarios on Yilong Lake was calculated by mathematical model.The results showed that the highest average reduction concentrations of ammonia nitrogen,total nitrogen,total phosphorus and chlorophyll A in the whole lake were 0.04mg/L,0.25mg/L,0.0026mg/L and 18.38mg/L when the water diversion flow was 3m³/s and the water replenishment was carried out in July～August when the water quality was poor,corresponding to scenario 3（the water replenishment point was the water replenishment point in Lulai Village）,scenario 1（the water replenishment point was located at the city river water recharge point）,scenario 1 and scenario 1,respectively.Therefore,based on the water quality improvement effect of the whole lake,scenario 1 was selected as the optimal solution for water quality improvement.The research methods and results have certain application and reference value for scientific understanding and quantitative solution of the effect of water transfer in the outer basin on the improvement effect of water quality in Yilong Lake and the scientific management of lake water environment.