Changes Of Pattern And Connectivity Of River Systems And Its Impacts On Storage And Self-purification Functions In The Taihu Plain

Changes of water cycle and water environment under the influence of climate change and human activities are one of the hot spots in current water science research,and changes of watershed underlying surface and river systems caused by urbanization and other human activities are also received extensive attention at home and abroad.In recent decades,fruitful scientific achievements concerning river systems evolution,connectivity assessment,and river functions were obtained by domestic and foreign scholars.Nevertheless,there were few quantitative descriptions concerning the characteristics of river systems pattern evolution in terms of its quantity,morphology,and structure in previous studies.Meanwhile,domestic and foreign studies about connectivity assessment of river systems were still in the primary stage,relative theory and method were imperfect,successful applications were limited as well.Moreover,there were few studies about river functions,especially the quantitative analysis and empirical study in regard to the impacts of river systems evolution on river functions.Therefore,carrying out the study concerning pattern evolution and connectivity change of river systems and its impacts on main river functions under the background of urbanization,the results have higher scientific value and important theoretical significance and can enrich the contents and methods of related studies in a certain extent.Taihu Basin is one of the regions with the most developed economy and the highest urbanization level in China.Meanwhile,it is one of the regions with the densest rivers and the largest number of lakes in China.However,in recent decades,the distribution pattern,connectivity status,and main functions of river systems were made significant changes under the strong interfere of urbanization and other human activities.These changes caused many water security problems such as flood disaster and water pollution,and it became the serious threat for the sustainable development of Taihu Basin.Therefore,it is necessary to carry out the study concerning pattern evolution and connectivity change of river systems and its impacts on storage and self-purification functions under the background of urbanization.The results have higher application value and important practical significance due to its support for flood control and water environment protection in Taihu Basin.Based on the reasons above,the Taihu Plain(the plain river network regions in central Taihu Basin)was selected as the study area.Meanwhile,RS and GIS techniques,and multidisciplinary methods including river geomorphology,hydrogeography,landscape ecology,and spatial statistics were employed,and such a research process as "evolution of river systems pattern? change of river systems connectivity?changes of storage and self-purification functions" was followed.Finally,the essential characteristics of the pattern evolution and connectivity change of river systems under the background of urbanization were revealed,and the influencing mechanism of the evolution of river systems on main river functions was analyzed.Main contents and results of this research are concluded as follows:(1)Evolution of river systems patternFirstly,based on topographic maps,remote sensing images and water resources survey data,and field investigation and verification in the Taihu Plain,the data extraction and order design of river systems were carried out.Secondly,based on the quantitative characteristic,plane morphology and spatial structure of river systems,river density,water surface ratio,river sinuosity,main river area length ratio,river development coefficient and box dimension were selected to describe their characteristic quantitatively.Meanwhile,based on ArcGIS platform,cartogram,gravity center model,and spatial autocorrelation analysis were employed to analyze the temporal and spatial evolution of river systems pattern in the Taihu Plain during the 1960s-2010s.Finally,in combination with land use change and other urbanization process,three indicators including population urbanization level,spatial urbanization level and spatial urbanization level were selected to discuss the impacts of urbanization on the evolution of river systems in recent 50 years.Results indicated as follows.First,the number of river systems decreased drastically,and the plane morphology of river channels changed into uniformity significantly and the spatial structure of river network tended to be simplified.Then,there was large difference in the distribution pattern of river systems,and the change of the distribution pattern of river development coefficient was significant during rapid urbanization period.Meanwhile,the characteristic of the spatial pattern of river systems evolution was diverse,but the protection of river systems should be different.Moreover,the impacts of urbanization on the changes of river systems were significant,but the decrase of water surface ratio was mainly influenced by urban land expansion.(2)Change of river systems connectivityThrough collating and analyzing the domestic and overseas relevant research results,the concept and connotation of river systems connectivity was discussed,and the river systems connectivity assessment method in plain river network regions was proposed.The indicators including circuitry(? index),link node ratio(? index),proximity index(y index),and hydrologic connectivity index were used to assess the structural connectivity and functional connectivity.Meanwhile,spatial analysis of GIS,moving average,liner trend estimation,Mann-Kendall test,moving t-test,Yamamoto method,and other methods were employed to analyze the temporal and spatial change of river systems connectivity in the Taihu Plain over the past 50 years.Then,the impacts of river systems changes on structural connectivity in representative region were analyzed.The functional connectivity impact assessment method including river network connectivity index and longitudinal connectivity index was also established.Finally,the impact of floodgate construction on river functional connectivity of representative rivers was revealed.Results indicated as follows.First,the structural connectivity of the arterial river systems had a gradually improving trend in the Taihu Plain,but the degree of improvement was very limited.Meanwhile,The functional connectivity of river systems was also improved slightly in the Taihu Plain,but its spatial difference was significant.Then,the structural connectivity of river systems was declined significantly by the dramatic decrease of the tributaries,but the negative effect was partially alleviated by the changes of the arterial river systems.Moreover,there were significant cumulative effects in the impacts of the floodgate construction on the functional connectivity of river systems.(3)Impacts of river systems evolution on storage and self-purification functionsWith the propose of characteristic indicators of the storage function of river systems in plain river network regions including storage capacity,potential storage capacity,storing ability,potential storing ability,and the rising rate of flood water level,the temporal and spatial changes of the storage function of river systems was analyze in the Taihu Plain,and the impacts of river systems evolution on the its storage functions were also revealed from both static and dynamic aspects.Meanwhile,through the analysis of temporal and spatial changes characteristics of five water quality indexes including DO,NH3-N,TP,TN,CODMn,characteristic indexes of self-purification function of river network and river channel including the over standard rate and the degradation rate of water quality were eatalished,and impacts of river systems evolution on the self-purification function and that of river channel morphological characteristic on the self-purification function were then analyzed.Results indicated as follows.First,the static storage function of river systems was visibly degenerated,and the degeneration of the static storage functions of lakes and tributaries were particularly significant.Meanwhile,the degeneration of the dynamic storage function of river systems was more rapid,but its spatial difference was significant.Then,water surface ratio was an important influence factor of the storage function change,but the impacts of others characteristics of river systems were also significant.Moreover,the water quality of river network was always very poor over the past decades,and TN was the key pollutant,and the temporal and spatial difference of water quality was significant.Besides,water surface ratio was also an important influence factor of the self-purification function of river network,but the impact of main river area length ratio was significant.Finally,the self-purification functions were various among rivers with different morphological characteristics,and the impact of fractal dimension of river channel on its self-purification function was significant.