Research On Water Regime Change And Its Driving Factors In Dongting Lake Under The Background Of Three Gorges Operation

Hydrology is one of the more important elements in river and lake water resources systems,and exploring the relative importance of climate change and human activities on the relative importance of hydrological fluctuations in river and lake systems has broad environmental significance.In recent decades,the proportion of changes in the hydrological regime of the basin affected by human activities has been increasing,and the relative contribution rates of factors have also changed over time.Therefore,it is very important to clarify the relative influence of factors in different hydrological sequence stages.By quantitatively analyzing the influence of climatic factors and human factors on hydrological fluctuations,we can more deeply understand the evolution mechanism of water regimes in the basin.In order to explore the influence of climatic factors and anthropogenic factors on the hydrological changes of the river-lake system,this paper takes the typical riverlake system—Dongting Lake as an example to carry out related research,and the main work content is roughly divided into three parts.The first part of the article collects the runoff and water level data of typical hydrological sites in and out of Dongting Lake from 1990 to 2019(spanning before and after the operation of the Three Gorges Dam(TGD)in 2003).The interannual variation trends of lake inflow and outflow runoff and water level and their abrupt years.The hydrological time series was divided into three sub-time series according to the sudden change year and the actual situation of the TGD water storage,which were 1990-2002,2003-2010 and 2011-2019 respectively.The results show that under the background of the operation of the TGD,the inter-annual variation of the water entering the lake at the three mouths of the Yangtze River is the most affected.After the sudden change of water level in 2003,the average annual water level dropped by 0.35 m compared with the previous period.The change during the year is manifested in the significant reduction of the runoff in and out of the lake during the wet season(May-Aug),especially the runoff from the three inlets,with a reduction rate of 41.04%.The change of water level during the year was mainly reflected in July-November,which was 1.06 meters lower than the water level in 1990-2002.The second part organizes the climate data and land use data of Dongting Lake area from 1990 to 2019,and uses the ArcGis reclassification tool to divide the land use data into three types: agricultural cover,non-agricultural cover and water body.The changes of the water flow into the lake from the three mouths of the Yangtze River and the four waters of Hunan are defined as the effects of the dam projects.The time series was divided into two stages: 1990-2002 and 2003-2019 according to the abrupt point of the water level in 1990-2019,and the climate change(rainfall,temperature,evaporation),land Direct or indirect,positive or negative impacts of cover change(agricultural cover,non-agricultural cover)and dam construction(three inflows,four inflows)on water level changes,and quantify the relative importance of these impacts.In the early period(1990-2002),it was mainly affected by the direct positive or negative effects of climate and land cover;in the later period(2003-2019),the inflow of Sankou and Sishui had a direct effect on the water level change of Dongting Lake.The impact was relatively large,especially the change of the inflow of the three inlets,that is,the operation of the TGD(V=0.436,P=0.007).In the third part,by comparing the influence of the two stage factors on the water level in the second part,the key driving factors causing the sudden change of the water level were determined.By establishing the Dongting Lake MIKE21 hydrodynamic model,the scenarios were simulated under two different scenarios(natural scenario1990-2002 and changing scenario 2003-2019),reflecting the temporal and spatial changes of Dongting Lake water level under the action of key driving factors feature.The model results show that spatially,the water level of Dongting Lake varies significantly during the flood regulation period(Jul-Aug)and the water storage period(Sept-Nov)under the two different scenarios,especially in the East Dongting Lake.In terms of time,the average reduction of the difference water level during the flood adjustment period(Jul-Aug)and the water storage period(Sept-Nov)was about 0.50 and 0.67 m respectively;on the contrary,in the dry season(Dec-Apr),the operation of the TGD has a supplementary effect on the water level,with an increase of about 0.09 m.The results of this study provide new insights into how to quantify the influence of human activities and climate change on the hydrology of river-lake systems,and can also serve as a scientific reference for water resource planning and management decisions in other similar regions.In addition,according to the temporal and spatial characteristics of the water level of Dongting Lake under the influence of the TGD impoundment,it can provide a reference for the dam scheduling method,so as to better restore and maintain a healthy lake ecosystem.