The Influence Of Urban Development On Structure And Connectivity Of River Systems And Its Coupling Relationship With Water Resource In Jiaxing City

With the rapid development of urbanization the morphological structure of river network water system was damaged dramatically.The urbanization effect on the hydrological and water environment was widely concerned by many scholars.Jiaxing City of Zhejiang Province is located in Taihu Basin of downstream of Yangtze River.There are dense river network and high urbanization level.However,with the urban land expanded rapidly,the activities of human have changed the earth's surface strongly.And urbanization is also one of the most important influencing factors of river network change in recent years,such as the patterns of river system were changed,the connectivity of rivers weakened as well because the low grade river channels were silted,blocked and landfilled.At present,under rapid urbanization,understanding the significant changes of urban land use types,the changes of rivers drainage structure and connectivity is important to solve the problems of local water environment and the sustainable development of regional social economy.Taking Jiaxing City as an example,the mathematical statistics method was applied to evaluate quantitatively the Jiaxing City river network area drainage connectivity situation via building river system connecting characterization of indicators under the integrated use of RS and GIS and related theories such as landscape ecology,river geomorphology multidisciplinary in the paper.The five land use types of town,paddy field,dry farm,water area and forests in three periods were obtained based on the three remote sensing images of Jiaxing City in 1991,2001 and 2016,the change characteristics of the underlying surface were analyzed.Based on the topographic maps in the 1960 s,1980s and 2010 s,the river systems were extracted by GIS,and then drainage structure and temporal variation characteristics of connectivity were analyzed from the aspects of quantity,structure and connectivity features over the past 50 years.The results show as follows.(1)There are great changes of land use/cover types of decrease agricultural land and water area and increase of the urban land.The proportion of urban area increased rapidly during 1991-2016,which was mainly gained from paddy fields.The annual rate of change on urban land and water are the largest,which means that the areas are most affected by urbanization.(2)The river system in Jiaxing City shows a trend of drastic reduction in recent 50 years.The density of river network,water surface rate,river network development coefficient and box dimension are all decreasing.And the reduction rates of most river system indexes in the 1980's and 2010's are higher than that of 1960's and 1980's.However,the degradation of the river systems varies in different urbanization level.(3)The hydrologic connectivity indexes which include actual ring ratio(? index),node connection rate(? index),river system connectivity(? index)and hydrologic connectivity index were used to assess the structural and functional connectivity.The tributary stream in Jiaxing City showed a trend of decrease in the number of nodes and river chain.The river system connectivity index had also significantly declining trend,which are consistent with the reductions in the number of node and chain river.It showed that reduction of tributary streams affect obviously the connection level of river network structure.The structural connectivity of the arterial river systems had a gradually improving trend,while the degree of improvement was very limited.The functional connectivity of river systems was improved slightly in the Jiaxing City.The structural connectivity of river systems was declined significantly by the dramatic decrease of the tributaries,while the negative effect was slightly alleviated by the increase of the arterial river systems.(4)There are certain difference in the water resource environment and urban comprehensive development in Jiaxing City during the year 2005–2016.The evaluation index of urban comprehensive development showed an obviously increasing trend.Meanwhile,the comprehensive index of water environment has a larger fluctuation which was presented a “slight increase within steady trend” in recent years.Compared to the indices of the water resource environment and city comprehensive development,the relationship presented the disparity from expansion to shrink,which means a transition from the urban development lag(2005–2011)to water environment lag(2012–2016).According to the perspective of coupling coordination degree,the coupling value of the water resource environment and urban development(0.595–0.994)were slightly higher than the coordination value(0.569–0.864)which both showed a trend of rising volatility.The coupling degree of the water resource environment and urban development was developed from the phase of grinding to the high level.This situation indicates that the water resources environment and the urban development level had strong interaction.The coordination degree of water resources environment and the level of urban development had been gradually improved from barely coordinated development to well coordination.The improvement of the state indicated the trend of coordinated development.