| Dongting Lake,the second largest freshwater lake in China,is now the only lake directly connected with the main stream of the Yangtze River in the middle reaches of the Yangtze River.It plays an important role in maintaining the balance of flood storage in the middle reaches of the Yangtze River,avoiding disasters and alleviating disasters,as well as the balance of erosion and siltation.Since the 1980 s,with the construction of Gezhouba Water Conservancy Project and a series of cascade reservoirs in the upper reaches of the Yangtze River,the sediment discharge in the Jingjiang reach has sharply decreased,which has resulted in the long-distance top-down erosion of the Jingjiang River.Especially after the Three Gorges Project was put into operation in 2003,the hydrological situation of Jingjiang River has changed greatly.The wetland ecosystem of Dongting Lake is facing a series of ecological problems,such as the deterioration of ecological environment,the decrease of biodiversity and the weakening of ecological regulation function.(1)Based on the research results of archaeology,geology and other disciplines in Dongting Lake area in recent decades,the Dongting Lake-Jianghan Plain is regarded as a dynamic whole,and the relationship between Jingjiang River and Dongting Lake and the evolution process of Dongting Lake since Holocene are discussed.At the beginning of Holocene,the Dongting Lake area should be a river network staggered state.Only in the present Yuanjiang Estuary and the East Dongting Lake area,there are long and northeast-trending overflow lakes.In the middle Holocene,the climate tended to be warm and humid,and the sea level rose.The riverbed in the middle reaches of the Yangtze River became thicker,the area around the East Dongting Lake area is gradually backed up and developed into a larger lake area.In the late Holocene,the evolution of the Dongting Basin was controlled by both natural and man-made processes.With the gradual change of tilting tectonic subsidence,the continuous subsidence of Dongting Basin,the decline of Gumengze and the southward migration of Jingjiang River,the relationship between Jingjiang River and Dongting Lake has undergone a major turning point since the pre-Qin and Han Dynasties.Lishui has been diverted into Dongting lake,and the southern bank of Jingjiang River has begun to divide into Dongting lake.During the Yuan,Ming and Qing Dynasties,human activities,such as blocking caves and building dikes and reclaiming lakes and farmland,made the water flow accumulate in Jingjiang River and channel bed silted and raised continuously,resulting in the break and diversion of the Jingjiang River.In the second half of the 19 th century,with the collapse of Ouchi and Songzi,four outlets were diverted into Dongting Lake,and the water surface area of Dongting Lake reached its peak again.Over the past 100 years,reclamation of lakes by human beings has led to rapid shrinkage of Lake surface.The whole Dongting Lake is obviously divided into three parts: east,South and west.(2)Taking 1980-2002 as the natural state before the operation of the Three Gorges Reservoir,the influence of the construction of the Three Gorges Dam on the hydrological situation of the three outlets was analyzed and measured based on IHA method.Except for the slight increase in Xinjiangkou from February to March,the monthly mean flow value of each station in the three outlets showed a significant downward trend,and the decline in flood season was larger.The maximum daily flow value is in a continuous decline state,and the trend is relatively flat after 2003.There is no obvious trend change for the days of break-off in Mituoshi station and Kangjiagang,while Shadaoguan and Guanjiapu stations show an remarkable upward trend.The average increase(decrease)rate of discharge showed a significant decrease trend.The closer to the lower Jingjiang reach,the greater the average change rate of discharge at the entrance.(3)In recent decades,the annual runoff and diversion ratio of the Three Outlets of Jingjiang River have been declining continuously.However,after the Three Gorges Project was put into operation,the decline of annual runoff and diversion ratio in the Three Outlets of Jingjiang River slowed down significantly.There are obvious differences in attenuation rates of diversion volume and diversion ratio at different outlets.The closer to the lower Jingjiang reach,the greater attenuation degree of diversion volume and diversion ratio.Before and after the operation of the Three Gorges Project,the critical flow of the main stream of Shadaoguan station,Guanjiapu station and Kangjiagang station has no obvious trend change,while the critical flow of Mituosi station has decreased.When the main stream flow of Jingjiang River is 10 000 m3/s,the diversion capacity of the other four stations has no obvious change before and after the operation of the Three Gorges Project,except that the Kangjiagang Station is in the state of cut-off.Under the condition of 20 000 m3/s mainstream flow,the diversion capacity of Songzikou increased,Taipingkou decreased,Ouchikou was stable,and the overall diversion capacity of the Three Outlets did not change significantly before and after the Three Gorges Project.Under the condition of 40 000 m3/s mainstream flow,the shunting capacity of Taipingkou and Ouchikou decreased significantly.The average flow of Mituosi Station decreased from 1950 m3/s to 1674 m3/s,Guanjiapu station decreased from 3760 m3/s to 2617 m3/s by 30.4%,and Kangjiagang Station decreased from 303 m3/s t0 161 m3/s,with a decrease of 46.86%.The Three Gorges Reservoir has increased the flow in the dry season of the main stream during the operation period,but the Shadaoguan,Mituosi,Guanjiapu and Kangjiagang stations are basically in a state of cut-off.Only the Xinjiangkou Station has significant diversion capacity,so the water supply dispatch during the dry season can not effectively alleviate the seasonal water shortage in the Dongting Lake area.(4)Based on the analysis of water level changes at the same discharge in different years,and interpreting remote sensing images of the three outlets in dry and high water periods,the plane morphological changes of the three outlets are illustrated.After the Three Gorges Project was put into operation in 2003,the low-water channels in the three outlets were all in the state of erosion;the floodplain channel in the Songzikou was relatively stable,but the floodplain channel in the Taipingkou and ouchikou was in the state of continuous siltation,but the rate of siltation was slowed down.The scouring degree of the low-water channel in the upper Jingjiang main stream is basically equivalent to that of the three outlets,which has not yet had a significant impact on the diversion capacity of the three outlets.After the Three Gorges Project was put into operation,the main factor for the significant attenuation of the three outlets diversion was the sharp decrease of Jingjiang River inflow from July to October and the siltation of high-water channels in Taipingkou and Songzikou.(5)In the past 60 years,the annual sediment discharge and ratio at the three outlets have shown an obvious downward trend.From 1956 to 1980,the annual sediment discharge of Xiangjiang,Zishui,Yuanshui and Lishui fluctuated sharply.Except for the Xiangjiang,the other three rivers increased first and then decreased.After 1980,the annual sediment in all four rivers showed a significant downward trend.The main reason for the gradual decrease of sediment deposits in Dongting Lake is the sharp decrease of sediment transport in the main stream of the Yangtze River,resulting in the decrease of sediment inflow from the three outlets.At present,the Dongting Lake Basin is gradually transiting to erosion.The influence of the Three Gorges Dam on the sediment deposition of Dongting Lake is quantitatively described by constructing two artificial neural network models.The results show that the Three Gorges Dam only explains 27.9% of the decrease in annual sediment discharge at Yichang Station and 16.9% of the decrease in sediment deposition at Dongting Lake.The construction of cascade reservoirs in the upper reaches of the Yangtze River and the implementation of soil and water conservation projects play a major role.(6)In the past 60 years,due to the sediment deposition in the lake basin and the replenishment of the Three Gorges Reservoir during the dry season,the annual lowest water level of the East Dongting Lake has gradually risen and the annual water level variation has obviously decreased.After the operation of the Three Gorges Project,the average water level of East Dongting Lake increased significantly from January to June,the average water level of July and August in flood season was relatively stable,and the average water level of September and October decreased significantly.The rise of water level in dry season is mainly caused by sediment deposition in the lake basin and the rising water level by replenishment of the Three Gorges Reservoir.The fall of water level from September to October was caused by the autumn impoundment of the Three Gorges Reservoir.In the elevation range of 20-21 meters,the annual flooding time keeps increasing.The flooding time in the elevation area above 22 meters increases first and then decreases.With the increase of elevation,the variation range of annual flooding time decreases significantly.The regulation of the Three Gorges Project increased the submergence time of low beach land below 22 meters,decreased the submergence time of beach land above 22 meters,and generally reduced the submergence time of beach land above 23 meters for more than 20 days,resulting in the continuous decline of the minimum elevation of vegetation coverage and the synchronous downward movement of the distribution pattern of vegetation.The application of the Three Gorges Project has led to an increase in the frequency of water level fluctuations and a decrease in stability and predictability,which is not conducive to the formation of stable habitat environment and the development and utilization of resources by organisms.Influenced by the regulation of the Three Gorges Reservoir,the timing of water withdrawal at 24-27 m elevation in 2003-2016 was 12,17,19 and 16 days earlier than that in 1981-2002,respectively,and the outcropping time was approximately advanced to the beginning of October,the middle of October,the end of October and the middle of November.The optimum growth elevation of Carex decreased from 25 m and 26 m to 24 m and 25 m,which resulted in the decrease of distribution elevation and area of Carex.The rising time of 21-24 m elevation in 1980 s was greatly advanced,and the rising time was further advanced after 2003,but the advance range decreased with the increase of elevation. |
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