Researches On Water And Suspended Sediment Discharge Variation Processes Of Dongting Lake And Their Responses To Anthropogenic Activities

Lakes are important constitution of earth's hydrosphere.Not only do lakes have functions of water supply,navigation,flood control and maintaining regional and global ecosystem stability and diversity,but also play an important role in the socio-economic development as the vital places of human settlement.However,under the impacts of global climate change and anthropogenic activities,most large lakes of the world are undergoing shrinking in the era of Anthropocene.Therefore,the hydromorphological processes of lakes,particularly the variations of water discharge and suspended sediment discharge(SSD)that directly linked with the lake evolution,have attract the world's attention.Dongting Lake,one of the two lakes connecting directly with the middle reaches of the Yangtze River,is the second largest fresh water lake in China.The lake is of vital significance to buffer flooding along the middle-lower Yangtze River while it is recharged by both basin tributaries and the Yangtze River.Due to its unique location,huge water storage capacity and complicated relationship with the Yangtze River,Dongting Lake plays an irreplaceable role in basin ecosystem stability maintenance and flood control along the middle-lower Yangtze River.Hence,analyzing variations in water discharge and SSD of Dongting Lake and their possible driving factors are of great academic value and practical significance to understand the evolution of Dongting Lake,the relationship between river and lake and morphological evolution of the middle-lower Yangtze River.Here,based on the hydrologic data of 10 gauging stations from 1951~2014,we analyzed annual and seasonal variation characteristics of Inflow and Outflow water discharge and SSD of Dongting Lake watershed by using Mann-Kendall trend/abrupt changing test,K-means clustering,wavelet analysis,and coefficient of variation.Then,we discussed the potential influencing factors of water discharge and sediment variation of Dongting Lake and their mechanisms,including basin precipitation,morphology development,and a series of hydraulic engineering.The major conclusions are as follows:The entire time series of 1951~2014 was divided into three sub-periods,namely1951~1970s,1970s~2000s and 2000s~2014.The results indicated that there were obvious stepwise decreasing trends in the annual Inflow and Outflow water discharge and SSD of Dongting Lake,with the inflow SSD showing the most remarkable declining magnitude.Obvious seasonal fluctuations were observed in Inflow and Outflow water discharge and SSD of Dongting Lake during the study period.Over 70%of annual water discharge was transported during the flood season.As for the SSD,Inflow and Outflow respectively exhibited 90%and 50%of the total during the flood season.Seasonal differences in water discharge and SSD of Dongting Lake over the recent 60 years exhibited a tendency of“less flooding during the flood season and more drying during the dry season”.The relationship of water and sediment exchange between Dongting Lake and the Yangtze River changed dramatically during the study period.From 1951~1979s,the average annual water exchange coefficient(I)was less than 1,showing that Dongting Lake played a main role of flood division;from 1970s~2000s,the coefficient of I fluctuated around 1,indicating a relatively stable water exchange mode;and from2000s~2014,the coefficient of I was greater than 1,suggesting that Dongting Lake played a main role of water replenishment to the Yangtze River.Generally,Dongting Lake played an important role in flood division during the flood season with the coefficients of I less than 1,while served as a water supplier during the dry season to the Yangtze River with coefficients of I greater than 1.As the Inflow water discharge presented significant stepwise decreasing trend from 1951~2014,water replenishment from Dongting Lake to the Yangtze River have weakened.Compared to 1951~1970s,the coefficients of I decreased obviously over the last two periods.Meanwhile,sediment budget of Dongting Lake showed stepwise decreasing trends.During the flood and dry season,the sediment budget,respectively decreased approximately49.84%,96.26%and 45.06%,48.42%compare to those of 1951~1970s.Moreover,Dongting Lake shifted from a deposition state with 16751×10~4t/yr sediment gaining during 1951~1970s to an erosion state with 40.03×10~4t/yr sediment losing during2000s~2014.The deposition-erosion pattern variation of Dongting Lake showed that Dongting Lake transformed from accumulation to source.Variations of water discharge and SSD and deposition-erosion pattern of Dongting Lake are dominated by both the lake basin and the Yangtze River.Specifically,impact factors within Dongting Lake basin include natural factors(such as watershed developmental stage and basin precipitation)and anthropogenic activities(such as reservoir construction and land use change).Influence factors from the Yangtze River mainly consist of the Lower Jingjiang Cutoff Project,the Gezhou Dam(GD)and the Three Gorges Dam(TGD).The Hypsometric Integral(HI)value of Dongting Lake basin ranges from 0.05 to 0.21,indicating that the geomorphologicacl evolution of Dongting Lake basin has reached old stage with poor sediment yield production,which essentially dominated the decreasing of Inflow SSD from the lake basin.Besides,basin precipitation should be responsible for the annual changes of water discharge at Four Waters and the seasonal characteristics of Outflow SSD.From 1951~1970s,annual average soil erosion of Dongting Lake basin was 2×10~8 t,which is the primary reason that resulted in a large amount of sediment flowed into Dongting Lake.From1970s~2000s,the Inflow water discharge and SSD of Dongting Lake respectively decreased by 19.60%and 47.75%,compared to 1951~1970s while the Outflow water discharge and SSD decreased by 19.42%and 49.31%separately.The Jingjiang Cutoff Project implemented in 1967~1972 is the main driver for the water discharge and SSD decreasing.Meanwhile,the rapid and large-scale construction of reservoirs and mitigation of soil erosion in the Dongting Lake basin enlarged the declining magnitude of Inflow water discharge and SSD of Dongting Lake.From 2000s~2014,the drastic reduction of Inflow sediment and the transformation of relationship between Dongting Lake and the Yangtze River were closely linked with the operation of TGD.High sediment retention rate(139×10~6 t/yr)within the reservoir and low sediment delivery ratio are the primary drivers for the drastic Inflow sediment reduction and the deposition-erosion pattern transformation at Dongting Lake.Meanwhile,due to severe channel bed erosion in Jingjiang reaches and enlarged topographic gradient between Dongting Lake and the Yangzte River,Outflow water discharge and SSD increased significantly after 2007.In addition,large-scale sand mining in Dongting Lake,as high as 9.3×10~8 t during the season time,accelerated the lake pattern transformation from deposition to erosion.