Analysis And Attribution Of Hydrological Connectivity Of Dongting Lake Based On Sentinel Images

Hydrological connectivity refers to the hydrologic cycle in which matter,energy,and living things are carried by water.The hydrological connectivity of flood wetlands plays a key role in maintaining river and lake ecosystem stability,improving water quality,and ensuring water security.Due to the influence of human activities,hydrological change,and climate change,the hydrological connectivity of floodplains has changed in different degrees.The change in hydrological connectivity will directly or indirectly affect the functions of flood wetlands such as drought resistance,flood control,biodiversity protection,and water quality improvement.Therefore,quantifying the hydrological connectivity of river and lake interconnection systems and clarifying the driving factors affecting hydrological connectivity will help us better und erstand the hydrological ecological processes of wetlands and provide policy support for wetland management and conservation.Dongting Lake,as an important lake freely connecting the Yangtze River,not only plays an important role in regulating and storing floods in the middle and lower reaches of the Yangtze River but also plays an important role in maintaining the stability of the ecological environment in the basin.In recent years,with the impact of human activities,climate change and hydrological changes on the basin,the hydrological connectivity of Dongting Lake has been damaged,the spatial and temporal distribution of water resources has been uneven,flood disasters have occurred frequently,and water pollution and water ecology problems have become increasingly prominent.Improving the current hydrological connectivity of Dongting Lake will solve the water ecological problems of Dongting Lake.At present,the analysis of the hydrological connectivity of Dongting Lake and the analysis of the driving factors of the hydrological connectivity is still in the initial stage.To quantify the hydrological connectivity of Dongting Lake,Sentinel-2 remote sensing images and geostatistical methods were selected to quantify the hydrological connectivity of Dongting Lake.To quantitatively measure the contribution of hydrological and meteorological factors to the connectivity,the hydrological driving factors were divided into the total inflow of the three inlets of the Yangtze River and the total inflow of the four tributaries of the Hunan rivers.Then structural equation model(SEM)was selected to quantitatively analyze the effects of hydrology(three-inlets flow,four-tributaries flow)and meteorology(precipitation,evaporation,and temperature)on hydrological connectivity.Finally,the hydrodynamic model was used to verify the effect of the increase of discharge on the water level and hydrological connectivity of Dongtin g Lake.The main conclusions of this paper are as follows:(1)Analysis of the rule of hydrological connectivity for many years.In order to quantify hydrological connectivity,this study used sentinel-2 image with high temporal and spatial resolution to extract water.The overall accuracy and Kappa coefficient of sentinel-2 remote sensing image extracted from water by atmospheric correction were 82% ～ 96% and 0.80 ～ 0.98,respectively.After CAST 1.0 and Origin processing,the trend maps of the whole lake’ s cultural connectivity and the hydrological connectivity in the east,west and south of Dongting Lake were obtained.At the same time,the connectivity threshold of Dongting Lake was divided according to the hydrological connectivity curve of Dongting Lake,combined with hydrology(flow,water level and water area)and meteorological law.The analysis of the connectivity trend showed that the hydrological connectivity showed significant seasonal changes: the hydrological connectivity gradually increased from spring to summer,and reached the highest level in July-August,followed by a gradual decrease in hydrological connectivity and reached the lowest level in winter.For the cultural connectivity of dongting Lake,the north-south connectivity played a dominant role from May to October,and the east-west connectivity played a dominant role from November to the next April.In east Dongting Lake,the north-south connectivity plays a leading role,while the restricted hydrological connectivity is east-west connectivity.In south and West Dongting Lakes,eastwest connectivity is the dominant connectivity and north-south connectivity is the restrictive connectivity.After a comprehensive analysis of the discharge,water level,water area,and hydrological connectivity,the hydrological connectivity threshold are obtained: When HC > 0.82,it is high hydrologic connectivity;When 0.70 < HC < 0.82,it is medium hydrological connectivity;When HC < 0.70,it is low hydrological connectivity.Based on multiple linear regression analysis of the effects of different lake bodies on hydrological connectivity,it is found that the south Dongting Lake has the greatest influence on east-west connectivity of Dongting Lake(48%),and the east Dongting Lake has the greatest influence on north-south connectivity of Dongting Lake(55%).(2)Analysis results of driving factors of hydrological connectivity.For quantitative analysis of the driving factor’s contribution to the connectivity of Dongting lake,this study used Pearson correlation matrix and the linear regression model analysis hydrometeorological factor relations with connectivity.According to Pearson correlation matrix and the least squares linear regression model,we build structural equation model,to reveal the direct and indirect effects of hydrological factors(three-inlets flow,four-tributaries flow)and meteorological factors(rainfall,temperature,and evaporation)on connectivity: The results are as follows: N-S HC was mainly explained by Tflow(r = 0.49,p < 0.001)and F-flow(r = 0.28,p < 0.05),the weather has no significant effect on north-south connectivity;T-flow(r = 0.44,p < 0.001),temperature(r = 0.33,p < 0.05),and F-flow explained E-W HC,rainfall and evaporation had little effect.Therefore,hydrological factors have an important effect on hydrological connectivity changes,and the three inlets discharge of the Yangtze River has the most significant effect.(3)Effect of the increase of the three inlets flow on water level and connectivity of Dongting Lake.To verify the effect of the discharge of the three outlets of the Yangtze River on the hydrological connectivity,the linear relationship between the hydrological connectivity and water level was analyzed,and the hydrodynamic model was used to simulate the effect of increasing the three inlets discharge on the water level of Dongting Lake.By comparing the water level of Dongting Lake under two scenarios of increasing the three-inlet inflow and natural conditions(keeping the three-inlet inflow unchanged),it is found that increasing the three-inlet inflow can increase the water level of Dongting Lake and improve the hydrological connectivity.The water level of Dongting Lake from June to August and from September to November increased by 0.5 m and 0.67 m compared with the natural condition after increasing the inflow of the three inlets of the Yangtze River.Therefore,the hydrological connectivity level of Dongting Lake can be improved by optimizing the regulation of the Three Gorges and increasing the flow of the Yangtze River into Dongting Lake.According to connectivity analysis and structural equation model,the hydrological connectivity of Dongting Lake presents significant seasonal characteristics,and the inflow discharge of the Yangtze River has the greatest impact on the hydrological connectivity of Dongting Lake.The results of the hydrodynamic simulation show that the water level and hydrological connectivity of Dongting Lake can be improved by increasing the discharge of three inlets.The method adopted in this study can be used as a method to analyze the hydrological connectivity of similar basins and provide a reference for the improvement of hydrological connectivity and the healthy development of flooded wetlands.