The Evolution Trend And Regulation Effect Of The Yangtze River-Poyang Lake Relationship

Poyang Lake is the largest freshwater lake that linking to the river in China.It plays an important role in the development and protection of the Yangtze River Economic Belt.For a long time,due to the impact of multiple factors such as climate change,natural geograpy conditions and human activities,the relationship between the Yangtze River and the Poyang Lake has continued to evolve,especially in the past decade.This evolution of the river-lake relationship has greatly affected the economic,social development and ecological environment of the Poyang Lake District,which has recived the society 's wide attention.As a result,quantitative identification of the effects of climate change and human activities on the hydrological situation of Poyang Lake and analysis of the changes and trends of the new river-lake relationship under the operation of the Three Gorges Reservoir have become a research hotspot in academia.On the basis of systematically summarizing methods,achievements and existing problems of the research on the evolution and regulation effect of the river-lake relationship at home and abroad,this paper has formed the theoretical framework and technical system for the analysis on the evoltuon trend and regulation effect of the river-lake relationship.Comprehensive and systematic quantitative identification research has been done on the spatial and temporal evolution of hydrological situation of the river and lake,the pattern of occurance of floods in the river and lake,water exchange between rivers and lakes,and the regulation effect of controlling reservoirs on the river and lake.This may provide scientific support for the sustainable economic,social and environmental development of the Poyang Lake District.The main research contents and results are as follows:(1)Using hydrological evolution trend analysis methods such as mathematical statistics,Mann-Kendall test,Pettitt test and wavelet analysis,the study has revealed long-term temporal and spatial evolution characteristics and trends of water system elements such as the flow,water level,river and lake erosion and deposition,water level and discharge relationship in the river,water level relationship between the river and lake,the lake capacity for flood storage.Since the operation of the Three Gorges Reservoir,the flow of the main stream in the middle reaches of the Yangtze River has decreased significantly from September to November,while the flow has increased from December to March.The river bed at Jiujiang Station has been scoured during the dry season,and the water level was affected by the combined influence of upstream water and river erosion.The variation of outflow of the lake was less than that of inflow.However,the outflow during the dry season increased,and the maximum regorge flow rate reduced from September to November.The long-term variation of the amount of inflow of the lake was not significant,while the amount of outflow of the lake showed a slight upward trend.The water level of Poyang Lake was affected by both the Wuhe River and the Yangtze River.The water level variation near Duchang Station was the largest.The farther the distance from Duchang Station to the outlet of the lake,the smaller the impact was.The water level at the outlet of the lake correlated well with that in the main stream of the Yangtze River.After the operation of the Three Gorges Reservoir,the water level correlation has not changed significantly.When the water level at Hukou station was below 14m,the water level at the Xingzi Station significantly reduced.The jacking or intrusion effects of the Yangtze River on Poyang Lake decreased,which weakened the regulation and storage effects of Poyang Lake on the Yangtze River flood.The hydrological situation in the river and lake was affected by the combined influence of less precipitation,river bed erosion and sand mining,operation of the Three Gorges Reservoir,which sharply changed in 2003.(2)Based on the Copula function,the probability of the occurance of different magnitudes floods of the river and lake was studied.The influence of multiple factors on floods occurance under the influence of human activities was also discussed.When once in a century flood occured in the Yangtze River,the probability of floods in Poyang Lake once in 100,50,and 10 years were 19.0%,27.3%,and 53.8%,respectively.Under the condition of certain floods occurred in the Yangtze River,the possibility of low-recurrence floods in Poyang Lake was higher than that in high-recurrence floods.Poyang Lake storage reduced the probability of concurrent floods in the Yangtze River and Poyang Lake.The peak-cutting effect of the Three Gorges Reservoir reduced the probability of concurrent floods by about 7.0%.(3)Based on the concept and method of characteristic index such as the index of jacking strength(JSI),the index of intruding strength(ISI)and the coefficient of water exchange(WEC),the study systematically analyzed the complex water exchange relationship and intensity between the river and lake,and discussed the influence intensity of main driving factors.During the flood season of Poyang Lake from July to September,the annual average jacking strength was 27.5%,the annual intruding strength was 8.3%,and the water exchange was mainly the situation that the Poyang Lake diverted floods from the Yangtze River.The water from Poyang lake flowing into Yangtze River was the major state during non-flood season(Dcecmber to April)with the annual mean JSI 8.8%,while the water from Poyang lake flowing out of Yangtze River is the major state when the incoming water from the middle reaches of the Yangtze River is 15.7 times more abundant than that from the Wuhe River.When the incoming water from the middle reaches of the Yangtze River is 6.8 times less than that from the Wuhe River,the water from Poyang lake flowing into Yangtze River is the major state.Other situations were in a "stable" state.The differences of coming water from Yangtze River and Poyang Lake is the dominant factor which drives the evoluation of river-lake relationship.Since the Three Gorges was put into operation in 2003,the JSI at Hukou Station has increased 6.0%,and the peak clipping effects of the Three Gorges has decreased JSI and ISI by 4.8%and 2.3%,respectively.(4)The study has constructed a two-dimensional coupled hydrodynamic model of the complex river-lake relationship,so as to quantitatively identify the impact of the operation of the Three Gorges Reservoir on the natural hydrological processes of the river and lake and their relationship.During the water storage period of the Three Gorges Reservoir(September to November),the average flow and the water level at Jiujiang Station in the middle reaches of the Yangtze River decreased,especially in October,with the average flow decreasing by 3340 m3/s;During the Three Gorges Reservoir water replenishment period(Decmber?May),the average flow rate and the water level increased.The replenishment effect was obvious during the dry season from January to March,and the average flow rate increased by 800-1550 m3/s.The Three Gorges clipped the peak of the flood.The operation of the Three Gorges Reservoir changed the water exchange process between the river and lake.The average outflow rate of Poyang Lake increased by 14.6%in September,while the outflow rate decreased by 9.6%in October.The outflow rate decreased by 0.1-5.0%from December to May,and the outflow rate increased by 2.7-7.4%from June to July,while the outflow rate decreased by 4.6%in August.The intrusion flow decreased from July to October.The water level of the lake area is significantly affected by the incoming water from the main stream of the Yangtze River and the water exchange between the river and lake.The water level in the increasing phase was relatively high,while the water level in the decreasing phase is relatively low.The speed of the falling was accelerated,and the time of the dry season arrived in advance,meanwhile the dry season duration was also prolonged.The hydrological rhythm has become a rapid transition between floods and droughts.The water storage of the Three Gorges Reservoir reduced the water level in the lake area from outlet to Kangshan.However,in the dry season,the Three Gorges Reservoir only increased the water level in the lake area from outlet to Duchang.The average water level at Xingzi Station decreased by 1.04m in October,and the average water level from January to March increased by 0.17-0.32m.The surface area and capacity of the lake area changed accordingly.The variation characteristics of hydrological rhythm in the lake area in recent years have become a normal trend,which has a great impact on the water supply and ecological environment of the lake area.(5)The hydrodynamics model and the simulation of dynamic lake capacity were employed to study the joint regulation effects of united control of the Poyang Lake Water Control Project and the Three Gorges Reservoir on the hydrological situation of the river and lake.After the scientific regulation of the Poyang Lake Water Control Project,the flood has less impacts on the hydrological situation.The regulation could effectively restore and scientifically adjust the river-lake relationship in the dry season,which improved the water resource utilization and freshwater ecological environment.It also played a certain role in the replenishment of the mainstream of the lower reaches of the Yangtze River.At the end of storage period of the Poyang Lake Water Control Project(September 1st to 15th),the average decreasing outflow rate at Hukou Station accounted for 5.6%of the Datong Station's simultaneous flow,while the water level at Xingzi Station increased by 0.78m.The recovery period of the river-lake relationship(September 16th?the end of October)was the main water storage period of the Three Gorges Reservoir.The water level at Xingzi Station was 2.59m higher than the current situation.The water level and decline rate of the lake area could be basically restored to the situation before the operation of the Three Gorges Reservoir.During scientifical adjustment period of the river-lake relationship(November?February),the average outflow rate at Hukou Station increased by 714m3/s in November,and the water level at Xingzi Station increased by 2.72m compared with the current situation,which is 0.7m higher than the water level before the operation of the Three Gorges Reservoir.The dry season of the main stream of the Yangtze River was from December to February,and the outflow rate changed slightly,while the water level at Xingzi Station increased by 2.94m on average,which was 2.36m higher than the average water level before 2003.