Research On The Influencing Factors Of Spatiotemporal Dynamic Changes Of Water Turbidity In The Yangtze River Basi

The dynamic monitoring of water resources in the Yangtze River is an important basis for water resources protection,utilization and sustainable development.Turbidity is a key index of water quality,which can reflect the quantity and size of suspended particulate matter in water,so as to comprehensively evaluate water quality.Traditional water quality monitoring has high cost,low efficiency and difficulty,while satellite remote sensing has the advantages of macroscopical,rapid and cost-effective,which can dynamically monitor the spatio-temporal variation characteristics of large-scale water quality,and has obvious advantages in water resources investigation and monitoring.However,in recent years,remote sensing monitoring of water quality mainly focuses on monitoring and evaluating water quality changes in large lakes or parts of rivers,while remote sensing monitoring of water quality in the whole basin and at large spatial and temporal scales is relatively rare.In order to make up for this gap,this paper studied the temporal and spatial dynamics of water turbidity in the Yangtze River Basin from 1986 to 2021,providing a basis for water resources management and management in the Yangtze River Basin.In this study,245,495 Landsat remote sensing images from 1986 to 2021 were used to construct the baseline of water turbidity in the Yangtze River,and the spatial and temporal dynamics of turbidity and its influencing factors were discussed.The results showed that:(1)Based on the Google Earth Engine(GEE)platform and the Landsat data and the measured data of synchronous stations,the correlation coefficient(R2)of the remote sensing inversion model of turbidity in the Yangtze River Basin in the whole basin is 0.68.The root mean square error(RMSE)is 7.83 NTU,which can meet the accuracy requirements of turbidity inversion in the Yangtze River Basin.(2)The average turbidity of the Yangtze River Basin in the past 36 years was higher in the upper reaches(41.7NTU),lower in the middle reaches(30.9NTU)and higher in the lower reaches(37.6NTU),and the turbidity of rivers(60.1NTU)was much higher than that of lakes and reservoirs(29.6NTU).Turbidity in the Yangtze River basin decreased from 1986 to 2021(the overall rate of decline was 2.22NTU/10a),and the decrease was most significant in the Yangtze River trunk basin.Local areas have inconsistent trends,such as Zhimen to Shigu and Hangjiahu area has a significant upward trend.(3)In terms of seasonal trend,turbidity in the Yangtze River Basin showed a trend of "low in summer and high in winter",with an average value of 40.41±34.88 NTU in wet season and 47.54±18.51 NTU in dry season.However,the turbidity of rivers and lakes showed the opposite trend in the seasonal trend.The turbidity of lakes and reservoirs was "low in summer and high in winter",such as Dongting Lake,Poyang Lake and Taihu Lake,with the mean values of 23.24 NTU in wet season and 34.94 NTU in dry season,respectively.The main reason for this phenomenon is that the lake water level rises in wet season,and the suspended substances settle after being diluted by rain,so the turbidity level is low in wet season.However,in wet season,the river velocity is accelerated and the sediment in the river is re-suspended,so the turbidity level is higher in wet season.(4)In the long time scale(1986-2021),the average turbidity of the whole Yangtze River Basin did not increase with the increase of annual rainfall,but showed an average trend of decline due to the effect of soil and water conservation caused by the increase of average forest coverage rate(42% to 46%)over the past 36 years.The seasonal trend showed that the turbidity of water in river basins was higher in summer and lower in winter,while that in lake and reservoir basins was higher in winter and lower in summer.The results showed that natural factors,including precipitation and natural vegetation(woodland,grassland and shrub),could explain 58% of the turbidity variation.However,the contribution of human activities,including impervious surface,cultivated land and wasteland,to turbidity variation is 42%,and the cultivated land and wasteland are significantly positively correlated with turbidity.The study of the spatial and temporal dynamics of turbidity in the Yangtze River basin not only has important practical value for the sustainable development of the water environment in the Yangtze River basin,but also provides reference and reference for remote sensing monitoring of the water environment in inland water bodies.