| As a man-made system for regulating,storing and controlling water resources,reservoir is an effective way to realize the rational development and utilization of water resources.The total water resources of lakes and reservoirs are large,supplying more than 50% of drinking water for urban and rural areas in our country.The quality of the reservoirs is very important for urban drinking water and urban ecological environment.In recent years,due to excessive nitrogen and phosphorus nutrients due to urban development and water pollution,some diversion reservoirs in southern China have been threatened by eutrophication,and cyanobacteria blooms have broken out successively,which have brought many adverse effects on the function of reservoir water ecosystem and the quality of water supply.As one of the methods to study the changes of hydrodynamics and water quality of lakes and reservoirs,the hydrodynamic water quality model of lakes and reservoirs can provide comprehensive decision-making for the improvement of specific water quality and the control of algal blooms.Xikeng Reservoir is an important water supply reservoir in Shenzhen.due to the long-term pumping of river water with high nutrition level into the reservoir,the eutrophication of the reservoir is serious and water bloom occurs from time to time,which affects the quality of water supply and the safety of drinking water in Shenzhen.The formation of water bloom depends not only on the eutrophication caused by nitrogen and phosphorus nutrients,but also on many factors such as hydrology,meteorology and water dynamics.In view of the importance of reservoir water supply,it is necessary to explore the important impression factors of reservoir water quality and the mode of operation and management,adopt a scientific model to improve water quality and water environment,and the model can provide technical support for improving management.In this paper,through the establishment of EFDC three-dimensional hydrodynamic water quality model,aiming at Xikeng Reservoir,the meteorological data,discharge data and water quality data in 2019 were used to simulate the changes of water level,nutrient concentration and cyanobacteria cell concentration in the whole reservoir.The main research results of this paper are as follows:(1)The three-dimensional hydrodynamic and water quality model of EFDC is constructed,and the simulated water level,dissolved oxygen,total nitrogen,total phosphorus,chlorophyll a and ammonia nitrogen accord with its realistic annual variation law.The overall error between the simulated value and the monitoring value is less than 30%,which proves that the EFDC model is feasible and effective in revealing the temporal and spatial variation characteristics of water quality in Xikeng reservoir.(2)Based on the results of the model simulation,the characteristics of the temporal and spatial changes of water quality,hydrodynamics,and algae growth in the Xikeng Reservoir were analyzed,and the correlations between the temporal and spatial changes and the shape of the reservoir,water diversion,nutrient input and wind drive were obtained.The special reservoir shape and water diversion mode are important influencing factors of hydrodynamic conditions and water quality distribution in the entire reservoir area,creating good conditions for the outbreak of cyanobacteria;and the high level of nutrient input is the direct driving factor for the outbreak under good dynamic conditions in the reservoir area.(3)According to the key influencing factors of algal bloom in the reservoir area,three kinds of scenario simulations are designed from three aspects of nutrients,water regulation and biological algae control.The water quality indexes of multi-point such as reservoir center,water intake,water outlet and reservoir bay are analyzed.At the same time,reducing the level of nitrogen and phosphorus nutrients in the reservoir by 50% can reduce the level of nutrients in the reservoir and effectively alleviate the problem of cyanobacterial blooms(reduced by13.25%);Single control of nitrogen to reduce the ratio of nitrogen to phosphorus in storage can also control the growth of algae.When the N/P in storage water is less than 20,the effect is more obvious,and the concentration of Chl.a is reduced by more than 30%,but it will cause the accumulation of phosphorus in the reservoir and increase the risk of cyanobacteria outbreaks in the future.Through reservoir operation,Increase the diversion flow of Dongjiang River,and the average water age in the reservoir area is reduced by about 10 to 15 days.An increase of 30% discharge has a promoting effect on algae(an increase of about 4.7%),while an increase of 50% water inflow reduces the average Chl.a concentration by 1.32%.Biological predation scenario simulation shows that the concentration of Chl.a varies between-48% and + 178%,and the descending effect of organisms in the reservoir is significant.The biological predation method represented by algae-eating fish to control algae is applied in the water diversion reservoir.In the long run,it is feasible and beneficial for the prevention and control of cyanobacteria blooms.The algae control schemes corresponding to the three scenario simulations all improve the cyanobacteria bloom in the reservoir to a certain extent,and have strong practical significance,feasibility and maneuverability.EFDC model well simulates and predicts the changes of reservoir water quality and the dynamics of cyanobacteria blooms.The quality of inflow water is the key to affect the water quality of Xikeng Reservoir.The influence of inflow of Xikeng Reservoir on the hydrodynamics of the water body can change the concentration of cyanobacteria in the whole reservoir and improve the accumulation of cyanobacteria in the bay of the reservoir.The research results provide an effective decision-making basis for algal bloom outbreak and water quality evaluation of closed diversion reservoirs such as Xikeng Reservoir. |
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