Water Allocation Of Multiple Sources In Chagan Lake Based On A Hydrodynamic-water Quality-water Ecology Model

Lake drought,water shortage and water quality deterioration have become key factors threatening the global water environment health under the influence of climate change and anthropogenic activities.Multiple water sources integrated allocation is an effective measure to alleviate lake water shortages,improve water quality and restore water ecosystems.Chagan Lake is located on the Songnen Plain in Northeast China,which is one of the top ten freshwater lakes and an important fishery base in China.Irrigation discharge is one of the main sources of Chagan Lake.Driven by the national grain yield increase project in 2009,a large number of returned water from the newly-built saline-alkali irrigation area was discharged into Chagan Lake,which aggravated the eutrophication of the lake,increased the risk of water salinization and threatened the health of water ecosystem.How to scientifically determine the water quality control target for maintaining the ecological health of the Chagan Lake,i.e.integrated allocation of multiple water sources to improve ecological health,is a key scientific and technical problem.Therefore,on the basis of clarifying the evolution characteristics of the water environment,a comprehensive hydrodynamic-water quality-water ecology model coupled BLOOM module was built relied on Delft3 D.Then,the time required for lake salinity to reach the salinity limit threshold of fishery aquaculture was predicted,and the water quality control objectives of the lake to maintain the health of the lake water ecosystem were determined.Finally,the optimal multi-water source control scheme of Chagan Lake based on water quality control objectives was proposed.The conclusions are as follows:(1)Evolution characteristics of water environment in Chagan LakeHydrological elements and water quality parameters of Chagan Lake were systematically analyzed,and the water cycle processes of Chagan Lake were clarified using mathematical statistics,Mann-Kendall method,principle component analysis.Irrigation discharge accounts for about 56.31% of the water supply.Water quality evolution of the Chagan Lake was divided into four stages according to the water quality evolution characteristics and important engineering construction: Natural Stage,Deterioration Stage,Improvement Stage,and Risk Stage.In the Risk Stage,the total nitrogen(TN),total phosphorus(TP),p H and total suspended solids(TSS)of Chagan Lake had obvious spatial differences,showing an increasing trend from east to west.The multiple trophic state indexes were used to evaluate the risk of eutrophication,and it showed that the lake water was in a eutrophic state.(2)Building of comprehensive hydrodynamic-water quality-ecology model of Chagan LakeBased on the collected model database and the Delft3 D platform,a hydrodynamicwater quality-water ecological integrated model of Chagan Lake was constructed,and the hydrodynamic,water quality and water ecological parameters of the model were calibrated and verified.The results showed that the simulated results of water environmental parameters such as water depth,salinity,nutrients and phytoplankton biomass were close to the observed values,which obtained a water environment simulation model suitable for Chagan Lake.(3)Simulation and prediction of salinity trend under the development scenarios of irrigation district in Chagan LakeThe validated hydrodynamic-water quality-ecology model was used to simulate the variation trend of lake salinity under different development scenarios of irrigation districts and determine the time required to reach the salinity limit threshold of freshwater fish farming.The simulation results showed that the annual accumulation of lake salinity was 96.99 t.Under the current water withdrawal scenario,lake salinity will reach the salinity limit threshold(1 psu)of aquaculture in 2024.Assuming that the backwater of Qian' an irrigation area was completely discharged into Chagan Lake,the salinity of the lake will reach the salinity limit threshold of fishery aquaculture in 2021.(4)Determination of water quality control objective determination based on water ecosystem health in Chagan LakeUsing the validated integrated model,the competition mechanism of phytoplankton under the synergistic effect of multi-environmental factors(TN,TP,TSS and N:P)was investigated,and the water quality control for maintaining the health of Chagan Lake' s aquatic ecosystem were determined.The simulation results show that the phytoplankton in Chagan Lake was mainly energy-limited.TSS suppressed the growth of diatoms and green algae,but favored the dominance of cyanobacteria in Lake Chagan,especially in the turbid water phase(TSS ? 60 mg/L).Non-N-fixing cyanobacteria would become dominant at higher TSS concentrations and lower light intensities in the turbid water.N-fixing cyanobacteria favored lower N:P ratios and higher light intensities in the clearwater phase(where TSS ?60 mg/L).According to the principle of minimum effect of harmful cyanobacteria,the water quality control target of Chagan Lake should be maintained at or below the thresholds: TN ? 1.5mg/L and TP ? 0.1 mg/L.(5)Multiple water allocation scheme to achieve water quality control objectiveThe integrated model was used to simulate the time and the ratio of multiple water sources for Chagan Lake to reach the water quality control objective under 18 multiple water source control scenarios that combined irrigation discharge control with improvement of hydrodynamic conditions in different hydrological years.The water quality improvement rate(IPWQ)was used to evaluate the simulation results and to compare and select the multi-water source control schemes of Chagan Lake in combination with the western river-lake connection project in Jilin Province.The optimal multi-water source control scenario was determined under which the backwater of Da'an Irrigation District was not discharged into Chagan Lake at all.These findings can provide scientific basis and decision support for the development planning of Chagan Lake water quality management and surrounding irrigation areas,as well as the Jilin West River-lake Connection Project for the ecological water supplement planning of Chagan Lake.At the same time,it also fulfill the requirements of "one lake,one policy" in China.