The Research On Changes Of Land Use And River Systems And Their Impacts On Water Quality In Taihu Basin

Taihu Basin located in the Yangtze river delta has advantageous nature conditions and nature resources with developed and densely covered rivers.It is one of the regions with highest urbanization level,most developed social economy and densest population at the same time.In the process of urbanization,in order to satisfy the development of society and economy,people changed and damaged natural environment constantly.As a consequence,Taihu Basin is faced with many serious ecological and environmental problems.One of the most obvious problems is water environment problem which is caused by the changed and damaged underlying surface and river systems in the process of urbanization.Urbanization changed land use types,as a result,the impervious area increased fast.Meanwhile,in the process of expansion and development of city,occupation and transformation of rivers made the number of rivers dramatically decrease and damaged the structure of river systems.The changes of land use and river systems caused by urbanization has obvious impacts on the natural and ecological process and that is one of the most important factors of deterioration of water quality.Therefore,it is urgent to explore and study the characteristics of the changes of land use and river systems and their impacts on water quality in the process of urbanization in Taihu Basin.Therefore,the region of Wuxi City and Jiangyin City which was the typical area in Taihu basin was selected as the study area.The rules and characteristics of the changes of land use types and river systems were revealed in order to offer reference basis for land use planning and management,river protection and the prevention and control of water pollution.The main contents and results of this study are concluded as follows.1、Spatial and temporal change of land useThe technology of remote sensing interpretation was employed to interpret Landsat TM remote sensing images and classify land use types in 1991and 2010.The spatial analysis and calculation was used to analyze and study the characteristics of the changes of the structure of land use and the ways of land use transformation.Through Patch analyst,the landscape metrics of land use were calculated in order to analyze the characteristics of changes of land use pattern.The results showed that:(1)In 2010,urban land became the main land use type instead of paddy field in 1991.The structure of land use changed dramatically.(2)The largely increase of urban land area mainly came from the transformation of paddy field.(3)1991-2010,the fragmentation of landscape intensified because of the transformation among land use types frequently.2、The changes of river system in the process of urbanizationThe database of river systems in 1960s、1980s and 2010s was established through digitization of rivers in order to quantitatively analyze the change of river systems characteristics in the process of urbanization in aspects of characteristics of quantity,structure and morphology.The results showed that:(1)1960s-2010s,the number of rivers,river density and water surface ratio decreased sharply.(2)1960s-2010s,the structure of rivers tended to be simplified.(3)1960s-2010s,the index of river sinuosity changed little,as a whole,the river sinuosity is a little low in study area.(4)The reduction ratio of river density and that of river development coefficient were positively related with urbanization rate.The decrease of rivers and simplification of river structure were closely linked with urbanization.3、The response of water quality to the characteristics of land use and river systemsFive water quality parameters in 22 water quality monitoring sections including dissolved oxygen(DO),ammonia nitrogen(NH3-N),total phosphorus(TP),total nitrogen(TN)and permanganate index(CODMn)were selected to analyze the temporal and spatial variation characteristics of water quality.Then,based on the establishment of buffer zones of different spatial scales of water quality monitoring sites as water quality response unites,the correlation analysis between the indices of land use and river system characteristics calculated by ArcGIS and water quality parameters during high flow period and dry period was made.At last,redundancy analysis was employed to explore the relationship between the average water quality and the characteristics of land use and river systems.The main results are concluded as follows.(1)The temporal and spatial variation characteristics of water qualityIn temporal terms,the water quality during high flow period was better than dry period.In spatial terms,the water quality of rivers of high order was better than the rivers of low order.It is very important to control the concentration of NH3-N and TN in order to improve water quality.(2)The response of water quality to the characteristics of land use structure and landscape pattern during dry period and high flow periodDuring dry period,urban land and paddy field had obvious effects on water quality,TP was positively correlated with the percentage of urban land in buffer zones of every spatial scale while TP was negatively correlated with paddy field in buffer zones of 300m and 500m.During high flow period,the percentage of dry land was positively correlated with NH3-N and TN.The response of water quality to landscape pattern was similar to the response to land use structure.During dry period,MPSurban land and MSIurban land were positively correlated with TP while MSIpaddy field and MPSpaddy field were negatively correlated with CODMn.During high flow period,MPSdry land was positively correlated with NH3-N and TN.(3)The response of water quality to river systems characteristics during dry period and high flow periodThe response of water quality to river systems characteristics during dry period was stronger than that during high flow period,meanwhile,the response was stronger in smaller spatial scales and quite different during dry period and high flow period.During dry period,in general,water quality of rivers was quite bad in study area because of less water in river channels and slow flow speed,so it was worsen for improving water quality if there were more rivers in space unit.During high flow period,it was better for water quality if there were more rivers and more water surface area because of abundant rainfall and fast flow speed.(4)The response of average water quality to the characteristics of land use and river systemsThe response of average water quality to the percentage of urban land and paddy field was very strong.The greater the percentage of urban land was,the worsen the water quality would be while the percentage of paddy field had opposite effects on water quality.SHDI was negatively correlated with TN and NH3-N,which also suggested that the more balanced the composition of land use was,the better water quality would be.(5)The response of average water quality to the characteristics of river systemsThe average water quality was related with the characteristics of the quantity of river systems.River density(Rd)was positively with TP,which was mainly associated with domestic pollution source and insufficience of self-purification ability of rivers of low order.Water surface ratio(WP)was negatively correlated with main water quality parameters such as CODMn and TN in buffer zones of 300m,500m and 1500m.