Spatiotemporal Distribution Of Water Quality For The Grand Canal In Recent 40 Years By Remote Sensing

As the longest and oldest canal in the world,Beijing-Hangzhou Grand Canal is the world heritage of UNESCO and the ?golden waterway? second after the Yangtze River in China,which plays an important role in promoting the economic development of the north and the south.In addition,as part of the East Route of North Water Transfer Project,the section of the north of the Yangtze River is the source of drinking water for many cities,and the water quality is very important for public health and ecological environment.However,in the past few decades,with the aggravation of human economic activities,the water quality of the canal has undergone tremendous changes,and it is also facing severe water pollution problems(mainly organic pollution).To understand the temporal and spatial distribution characteristics of water quality in the Grand Canal is helpful for the overall planning of canal water environment management for the management department.Considering the lack of long-term and large-scale ground monitoring data and the lack of canal water quality research,this paper uses Landsat images from 1985 to 2019,based on Google Earth Engine(GEE)platform,designs remote sensing water quality classification algorithm,and collects ground monitoring data to analyze the water quality change rules of local time-space scale of the Grand Canal and verify the accuracy of the remote sensing water quality classification algorithm.The remote sensing water quality classification results of the past 35 years are used to study the water quality time-space change of the Grand Canal.The main achievements include the following three aspects:(1)The ground monitoring data shows that the water quality of the section of the north of the Yangtze River and the south section is significantly different.Since 2006,the water quality of the north section is mainly of Class III and Class IV,with little difference in water quality change;the water quality of the south section is significantly worse than that of the north section,but since 2006,the water quality improvement is obvious,and the water quality of Wumu and Wangting stations is mainly improved from Class V to Class IV.In 2018,the overall water quality of the Grand Canal was the worst in January and August,and the medium and heavy pollution(Class V and Class V)were mainly concentrated in Suzhou,Wuxi and Changzhou.(2)In this paper,the adaptive water pixel extraction method based on the improved water index and the adaptive water pixel extraction method based on the mixed pixel decomposition,are used to extract the water pixel of the Grand Canal.The comparison results show that the adaptive water pixel extraction method based on the mixed pixel decomposition is more suitable for the narrow and complex river situation of the Grand Canal.In this paper,a Bayesian method based on K-means++ clustering is proposed for water quality classification of remote sensing images.The algorithm combines the advantages of unsupervised classification and supervised classification,and its verification accuracy on TM,ETM + and OLI sensors is 76%,80% and 82%,respectively.It shows that the algorithm has good consistency and applicability in a variety of sensors,and has good universality in large-scale space-time.(3)The results of remote sensing water quality classification of the Grand Canal from 1985 to 2019 show that except for Suqian-Yangzhou section,the average annual pollution proportion of the other four sections of the the Grand Canal has declined in the past 35 years.However,during 2015-2019,the average annual pollution proportion of Jining-Xuzhou Section,Zhenjiang-Suzhou section and Jiaxing-Hangzhou section has not increased significantly,which needs attention of relevant departments.In addition,the South-to-North Water Diversion Project can improve the water quality of Jining-Xuzhou section and Xuzhou-Suqian section.At present,the water quality of Xuzhou Suqian section is in the best condition with the lowest proportion of pollution.The water pollution of the Grand Canal is mainly concentrated in Zhaoyang Lake section,Pizhou section,Yangzhou,Suzhou,Jiaxing and Hangzhou section,and the pollution area of Hangzhou section has obviously moved southward since 1985.The spatial and temporal remote sensing analysis of the water quality of the Grand Canal from 1985 to 2019 fills the gap in the study of the water quality change under the long time series of the Grand Canal.The spatial continuity of the monitoring results is also conducive to the search for suspected pollution sources,providing important reference for the supervision and law enforcement of the water environment management department.