Simulations For Nitrogen,Phosphorus And Suspended Sediment Of Poyang Lake

Poyang Lake is located in the middle reaches of the Yangtze River,connecting the river Ganjiang,Fuhe,Xinjiang,Raohe and Xiushui,then the water in Poyang lake enters into the Yangtze River through the lake outlet.It is two of the largest Yangtze-connected lakes,and has important functions in extreme hydrological harmonization,freshwater resource supply,fishery resource protection,water environment purification,agricultural irrigation,and other aspects.Since the impoundment of the Three Gorges Reservoir(TGR)in 2003,it protects the middle and lower reaches of the Yangtze River from flood disasters,improves downstream navigation,and alteres the relationship between the river and lake(Poyang Lake and Dongting Lake).The being altered relationships between the river and the lake has led to decline in the water level,which triggered a series of changes in the hydrodynamic,sediment,and water quality variations in the lake.In order to study the distribution of nitrogen and phosphorus in Poyang Lake,explore the quantitative impact of upstream reservoir scheduling on the hydrology,hydrodynamic and water quality of Poyang Lake,alleviate the water pollution of Poyang Lake,a two-dimensional hydrodynamic,water quality,and sediment mathematical model of Poyang Lake was established to simulate of the hydrodynamic,suspended sediment,TP and TN concentration in the lake.The impact of upstream reservoirs’scheduling around the lake on the input water and sediment was calculated quantitatively,which provides scientific basis for water environment management of Poyang Lake.The main conclusions were listed as follows:(1)The certainty coefficients of calculated water level at Hukou,Xingzi,Duchang,Tangyin,and Kangshan station are all above 0.9,and the Nash efficiency coefficients are greater than 0.8.The spatial distribution of the suspended sediment concentration in Poyang Lake is mainly concentrated in two regions:one is concentrated in the deep channel of the lake,especially in low or middle water level periods,where the suspended sediment concentration is higher than that in other regions;The another region mainly occurs in the delta region of inflow river during the wet season,and a high sediment concentration area will be formed near the inlets.As the flow velocity decreases,suspended sediment gradually besinked and then the concentration in the water flow decrease.Therefore,the concentration of suspended sediment becomes lower as it approaches the center of the lake with relative larger water depth.(2)TN and TP in the lake show significant seasonal variation characteristics,with lower summer and higher winter.The concentration at Hukou station is impacted by the Yangtze main flow,and the seasonal characteristics are not obvious.The spatial distribution of TN and TP is relative higher in the West and low in the East region,and high in the South and low in the North.The inflow of the Zhangtian River and Tongjin River is smaller than that of the other five rivers,resulting in smaller water pollution.And the water near eastern part of Poyang lake is not heavily affected by urban and rural domestic sewage discharge,industrial pollution,and agricultural non-point source pollution from the upstream,while the western and southern lake areas are relatively more affected by the discharge of different pollutants from the upstream.Therefore,the inflow basins of the southern and western lake regions are main pollutant control areas.(3)The predicted peak flow at Jiujiang stations has reached about 57000 m~3/s without reservoirs in Yangtze River,which is about 25%higher than the peak value of the measured data.Meanwhile,the decline of flow discharge has lagged behind in the period after the floods.The peak water level in the flood season is higher than that under the effect of reservoir impoundment.when the water level fallen,the"river phase"appeared later than the actual situation without dams,with a time difference of nearly two months,which indicating that the upstream controlled reservoir regulation is one of the key impact factors for the advance of the dry season in Poyang Lake.(4)The reservoirs have the greater trapping effect on suspended sediment and a subsequent significant impact on the suspended sediment concentration in the downstream.The regional average concentration of suspended sediment in March and May without reservoirs is between 0.4 kg/m~3 and 0.5 kg/m~3,and the sediment from Ganjiang and Fuhe River is the main reason for the increase of suspended sediment concentration in the lake area.Due to the storage of the Three Gorges Reservoir in the upstream,the frequency and sediment concentration of the Yangtze main channel flowing backward into Poyang Lake have been reduced to some extent.The spatial distribution of TP and TN under different scenarios are basically consistent:in the first half of the year,the water quality types in the tail section of each tributary,the southern lake area,and the western lake area are basically in categories IV-V during the dry season or normal season,which indicated water quality was relatively poor,while the water quality in the lake center and northern lake area was relatively good.In terms of time,TN decreased to a certain extent throughout the year after changing the upstream flow,while TP concentration only significantly decreased in the flood season.In addition,the reverse flow of the Yangtze River caused an increase rapidly in the concentration of TN and TP in the lake during special period.