Study On Change Of Hydrological Processes And Water Environment In Typical River Network Region In Taihu Lake Basin Under Urbanization

As one of the most intense process changing the surface profound,urbanization have made regional water cycle changed greatly,and most significance is the change of flooding and water environment,which have made great effect on sustained socio-economic development and a healthy ecological environment.Therefore,study the impact of human activities on the regional hydrological processes and water environment of the urbanization background is an important and fundamental scientific question.Yangchendianmao region,located in Taihu basin,has the biggest developing speed and is one of areas with highest urbanization level in China.It is characterized by plain river network,and has many lakes across the region with larger water surface ratio.During the process of high speed of urbanization and expanding of construction areas,impervious area has increased a lot and streams structure and connectivity has decreased by a great deal which has significant impact on the river evolution and hydrological process within the watershed.Past studies have shown that streams are gradually dominated by trunk rivers;capacity of flood regulation decreased;water levels in rivers and lakes has been keeping high in flood season,and the phenomenon that mild precipitation events cause huge economic loss has become frequent;water quality and environment in the watershed has arouse public's attention.Therefore,the researches on the change of hydrological process and water environment under the background of urbanization are of great significance theoretically and practically.At present,hydrospheric atmospheric research under environment changs has become a heated issue of water science in the 21st century.Simultaneously,it has been received extensive attention in governments and research institutions.As the most profound and intense process of human activites to the Earth's surface varied,urbanization have changed the regional water cycling greatly.Of which changes most significant in flood disasters and water environment.Furthermore,this influnse is a greater impact to social and economic sustainable development and a healthy ecological environment.Therefore,nowadays,it is an important fundamental scientific questions to study the impact of human activities on the regional hydrological processes and water environment in rapid urbanization.(1)Variations of underlying surface and river system under the influence of urbanization.With the rapid development of urbanization,river system presented a degenerative trending,and drainage structure showed simplification.During the past 20 years,construction land was greatly increased with the high urbanization process.The ratio of urban population and industrial-service industrial showed an obvious increasing,and the changing tread coincided with the construction land during the past 50 years.Besides,there were significant correlations between urban population and the other two indicators,indicated these indicators could characterize the urban development in different view.The integral variations of drainage density and surface water rate in Yangchengdianmao region showed a degenerative trending from the 1960s to 2010s.The growth factor of tributary presented a downward treading,and the river network showed a trunk treading with the urban development.The river area-length ratio and box dimension had different changing tread during the three periods.The regional differences of river length and drainage density were remarkable in different zone of Yangchengdianmao.In short,the maximum changing tread of river length and drainage density occurred in rapid urban area,the biggest main river area-length rate appeared in concentration areas,and the bigger river network growth factor always occurred in low urbanization.The overall box dimension presented downward treading,especially in highly urban areas.(2)Analysis of temporal-spatial variation characteristics in precipitation and water level long series.Rainfall shows generally a weak upward trend,but the incidence of extreme rainfall increases,water levels rise significantly.Precipitation and summer precipitation in flood season displayed a unobvious upward trend for recent 50 years,Contribution rate of high-level of rainfall events and numbers showed an increasing trend.In addition,annual precipitation and flood season precipitation have differences in the spatial distribution,rainfall high-value centers located in the southwest of highly urbanized area,which indicates higher urbanization level incurs higher flood risks.Each time characteristics mutations of the water level was about in 1987 when early development of urbanization took place,the water level of each year shows upward trend,closer to the water site,greater the tendency to increase the city of water level,highest level linear trend showed slight downward trend in Changshu city,Kunshan and Suzhou that obtained higher urbanization level,which may be related to its water projects in areas with high level of urbanization.Monthly average water level fluctuation is large in the study area.Highest values after urbanization rises not obviously in flood season;Monthly minimum water level in the different sites shows different trends:higher urbanization level,higher water level,the highest monthly water level is bigger in the area where level of urbanization is fastest,and is small on the contrary;In the spatial distribution,the monthly minimum and maximum linear trend that distribution declines from area of high level of urbanization such as Suzhou,Changshu and Kunshan to the surrounding area in space.From the change trends of contribution rate of each grade,the low level of water level in flood season shows a downward trend from the overall,while the grade level shows a upward trend,indicating that the probability of extremes water level values increases,flood risks increases.(3)Study on variation characteristics of water environment and its influence mechanism.The poor water quality of study area has gradually improved in the decade.The main contaminants in the river system of the study area are TN and NH3-N.The all contamination indexes of study area showed a trend of first climb up and then decline in ten years.From the perspective of comprehensive pollution concentration index and spatial distribution,poor water quality of the river is mainly concentrated in urban areas where the more vigorous human activities,while the better water quality areas are far away from urban areas.However,the water quality of study area has improved in recent years.Based on the method of remote sensing inversion,this paper proceed the comprehensive water quality analysis of the whole area named Yangchengdianmao in 2010,showed that the surface water's quality in the southern area is better than northern area,the eastern area's surface water quality is better than western area,and the water quality in lake is better than in river,but the water quality of Yangcheng Lake surround the city is poorest in all lakes,according to the spatial distribution of surface water.And from the aspect of the relationship between land use and water quality,the patterns of land use and landscape nearer to river banks have more influence to river water quality,and the patterns have greater influence to dry season than wet season.There exists a correlation between land use pattern and different types of contamination indexes,from the analysis of the relationship between water quality and the social and economic industry,we found that,the more developed the tertiary industry,the the worse the water quality,the smaller concentrations of the other pollutants.water quality and the tertiary industry is more developed,there are signs of improvement,also showed with the development of urbanization,change in water quality to deteriorate good development trend.Through analysis of the relationship between the water level and water quality in the district found a relatively low level of urbanization area,the greater the river water,better water quality;and urbanization level is relatively high regional relations is not very clear,which is probably due to the urbanization The higher the degree of river water quality are more serious potential source areas.The study area based on 2001 and 2010 river healthy water environment to analyze and evaluate the results obtained from the health of the rivers in the study area to moderate pathological transformation of river health status as urban development to a certain extent there is a gradual trend of improvement.