Study On Threshold Of Water System Connectivity Index Based On Water Resources Demand In The Southern Three Outlets Of The Changjiang River

This article takes the three areas of Jingnan as the research object,and uses ArcGIS software to extract the water structure diagrams of the fifth period(1964,1978,1990,2008,and 2016),based on the three aspects of water system connectivity structure,water system connectivity form,and water system connectivity function.To analyze and comprehensively use geomorphology,landscape ecology,graph theory and other methods to conduct a full-scale study of river systems and discover changes in characteristics and laws.The correlation analysis method and multivariate regression analysis method were used to discuss the quantitative relationship between the connected form of water system and the function of water system connectivity,and the water system connectivity indicator threshold was defined based on the demand of water resources.The main findings are as follows:(1)Evolution of water structure.Based on the actual situation in the study area,the paper has rationally graded rivers.The water structure was analyzed from quantitative characteristics and structural characteristics.We can intuitively find the weakening of the water system from the five periods of the water system map,and the research results show that since 1964,the index values of the water system connectivity pattern have declined to varying degrees.The number and length of rivers and lakes are reduced,and the structural indicators such as river network density and water surface rate are weakened.Among them,the period from 1990 to 2008 was the most dramatic change in the river system pattern.During this period,the river frequency decreased from 0.56 to 0.46/km,and the reduction rate reached 17.86%.The decay rate of water structure indicators has slowed down to 2016.In addition,different grades of rivers have commonalities and differences in their changes,and humans have a stronger influence on low-grade rivers.(2)Changes in water system connectivity.Comprehensively use the graph theory method and landscape ecology,construct the evaluation index system of water system connectivity morphological indicators,and evaluate the degree of connectivity in the three areas of Jingnan.The results show that the reduction in the number of river chains and nodes in 1964-2016s exceeded 50%.This shows that with the development of urbanization,a large number of terminal tributaries,including independent tributaries,have been buried in large quantities,resulting in a large reduction in river nodes.The gamma indices for the five periods were 0.378,0.365,0.361,0.375,and 0.376,all around 1/3,indicating that the water network in this area was biased toward the tree.Among them,the 1990s water network had the lowest connectivity,and the 1964s and 2016s water systems.Good network connectivity.(3)analysis of connectivity functions between rivers and lakes.The evaluation system of water network connectivity function indicators was constructed from natural functions and social functions,and evaluated using set pair analysis.The research structure shows that:During the period from 1964 to 1978,the process of urbanization in the three areas was slow.At this time,human intervention in the river was relatively small,and the water system pattern was not greatly influenced.The natural function level in this period was "excellent" level and social function.For "good" level.During the period of 1978-1990,three areas entered the stage of urbanization.Humans began to intervene in river systems and changed the river's connectivity patterns and connectivity.During this period,the decline in natural function and social function was more obvious.Since 2008,social functions and comprehensive functions have been improved,social functions have reached a "good" level,and comprehensive functions have reached a "medium level".Although natural functions have improved,they have remained at a "bad" level.(4)Threshold value of water system connectivity index.The correlative analysis method was used to analyze the correlation between water structure connectivity index and water system connectivity function index.Four significant morphological and four functional indexes with significant correlation were screened.A linear regression equation was used to establish a linear equation.Based on the demand of water resources in the three areas,the threshold value of water system connectivity index was determined.The results show that the range of river network density is:[0.52,0.55];the range of water system ring is[0.60,0.72];the range of node connection rate is[1.32,1.77];the value of water system connectivity is The range is:[0.50,0.67].