Using The SWAT Model To Estimate The Relationship Between The Yangtze River And Poyang Lake In The Three Gorges Reservoir Region

Human activity since the twentieth century has accelerated the evolution of river-lake relationship,especially with the construction and operation of regulatory water projects such as the Three Gorges Reservoir(TGR).It is driving a new round of adjustment in river-lake relationship.Water leakage from reservoirs during flood season increases the pressure of flood control in the basin,and water storage in reservoirs during dry season intensifies the drought situation.The change of river-lake relationship under the operation of TGR has attracted much attention and controversy on the aggravation of flood and drought pressure in Poyang Lake basin.Therefore,quantitative assessment of the river-lake relationship and analysis of the change pattern provide important information for optimizing reservoir dispatching scheme and formulating flood and drought prevention policy in the basin.The scientific basis has a profound influence on the economic development and ecological construction of Poyang Lake and the whole Yangtze River basin.In order to quantitatively evaluate the relationship between the Yangtze River and Poyang Lake under the operation of TGR,this paper constructs a large-scale Yangtze River basin SWAT((Soil and Water Assessment Tool)Distributed hydrological model with simulations with and without runoff in the Yangtze River basin under the NO TGR scenario,from the energy point of view,using the water level and runoff to calculate the energy difference(Fe)to quantitative river-lake relationship.The main elements and findings of the study are as follows.(1)To validate the applicability of SWAT model in the large-scale Yangtze River basin,the coefficient of determination R2 and the efficiency coefficient NS were chosen as the evaluation indexes.The main streams of the Yangtze River,Huatan,Cuntan,Yichang,Hankou,Jiujiang,and Datong at calibration period(1981?1990)and validation period(1991?2000)evaluation indicators R2 and NS above 0.80,mostly The absolute value of PBIAS is less than 10%;the R2 and NS at Huhukou is more than 0.90,and the absolute value of PBIAS is less than 5%.It is concluded that the SWAT model is applicable to the Yangtze River basin.(2)Construction of the SWAT model.Using the model to control the consistency of meteorological and other factors,we set up TGR scenario and NO TGR scenario to simulate runoff(2010?2018).Under the operation of the TGR,the trend change of the hydrological process in the mainstem of the Yangtze River tends to slow down,with the summer peak decreasing and the winter valley increasing,and Poyang Lake as the the confluence of river and lake action,where runoff fluctuations are far less cyclical than in the mainstem of the Yangtze River,and where the trend peak of runoff at the mouth of the lake is around June each year.The troughs are around February,during which there are also irregular small peaks.(3)The water level and runoff of the Hukou is selected to establish the energy difference(Fe)index to quantify the river-lake relationships and analyze its changes.Under the two scenarios,the trend of river and lake interactions is roughly the same.From June/July to October/November,Fe.is less than 0,which is dominated by the Yangtze River;from November/December to May/June,Fe is greater than 0,which is dominated by Poyang Lake.Under the operation of the TGR,Fe is divided into-0.1560 and-0.1147 in August and September 2010,with only continuous low values(<-0.1).The Yangtze River has a strong role and severe flooding occurred in Poyang Lake.In November and December 2013,Fe is 0.1180 and 0.1436 respectively,with only continuous high values(>0.1).Poyang Lake had a strong role,and a rare drought occurred in Poyang Lake.In the inter-annual change trend,Poyang Lake strengthened and the role of the Yangtze River weakened compared with the case of no reservoir.From the perspective of the four months in the average month,Fe is less than 0 during the discharge period of the reservoir,and the relationship between the rivers and lakes shows the role of the Yangtze River.The role continues to increase,but the impact is not large;Fe is less than the NO TGR scenario during the flood period,and the role of the Yangtze River continues to increase.In the flood period of the Poyang Lake,the Yangtze River action hinders the outflow of the Poyang Lake,which will increase the flood control pressure of the Poyang Lake Basin;during the storage period,Fe rises from-0.0522 to 0.0399,and the river-lake relationship shows that the Yangtze River role transitions to the Poyang Lake role in October.If the reservoir advances or prolongs the storage time,it reduces the flow of the main stream of the Yangtze River,and the increase and decrease of the outflow of the lake mouth further exacerbate the drought in the Poyang Lake basin;the Poyang Lake effect in the dry season is weaker than that in the case of NO TGR,the peak value of Fe is 0.0754,and it continues water storage,the flow of the Yangtze River is reduced,and the water level is reduced,which forms a "pulling effect"on the Poyang Lake,which is one of the reasons for the increasing drought in the Poyang Lake Basin.However,if the reservoir is properly discharged during this period,the runoff of the Yangtze River will be increased to reduce the dryness of the Poyang Lake.The drought in the water season has a certain positive effect.