Spatial-temporal Pattern Change And Ecological Security Assessment Of Thermal Environment In Hangzhou

Under the background of global warming and accelerating urbanization,the issues on urban thermal environment keep growing severer.People's residential environment and physical condition have consequently been put in danger,and meanwhile,implementing the sustainable development is facing greater challenges.The government can fully understand the past,present and even future situation of urban thermal environmental through studying the spatial-temporal pattern and the evolution laws of urban thermal environment and evaluating its ecological security status scientifically,which brings a great practical significance to the appropriate urban layout,urban ecological environment and sustainable development.As the provincial capital of Zhejiang Province and one of the central cities of the Yangtze River Delta urban agglomeration,Hangzhou has showed increasingly prominent problems of urban thermal environment,along with the rapid urban expansion.By 2017,Hangzhou has become one of the new "stove" cities.The rapid development of Hangzhou and its deteriorating thermal environment caused conflicts which is urgently asking for solutions—How to optimize the thermal environment while processing urban development,and make "paradise on earth" is worthy of its name.It is particularly necessary to carry out systematic and profound research on its thermal environment.Taking Hangzhou as a research area,this paper uses Landsat-5TM image in 1992,1997,2002 and 2007,the Landsat-7 ETM+ image in 2012 and the Landsat-8 OLI/TIRS image in 2017 as the main data sources.Supported by RS and GIS technologies,the spatial-temporal pattern and evolution law of the thermal environment in Hangzhou in the past 25 years are quantitatively analyzed.On the basis,the thermal environment ecological security status in Hangzhou from 2002 to 2017 were evaluated.The main contents and conclusions of this paper are as follows:(1)The image-based algorithm(IB algorithm)is used to retrieve the surface temperature of Hangzhou city in 1992,1997,2002,2007,2012 and 2017,and the meanstandard deviation method is used to define the grades of urban thermal environment.On this basis,the spatial distribution characteristics of urban thermal environment in different years in Hangzhou are analyzed by spatial statistics method from the district scale.The results show that the high temperature and sub-high temperature zones(heat island zones)are mainly distributed in the central urban areas with dense population and buildings and developed economy in Hangzhou.There are also large areas of heat islands around the economic development zones and industrial parks;the medium temperature zones are mainly distributed in the cultivated areas around city;while the low temperature zones and the sub-low temperature zones are mainly concentrated in the mountainous and waters.(2)The temporal and spatial dynamic evolution trend of urban thermal environment in Hangzhou from 1992 to 2017 was explored from three aspects: spatial distribution change,area change and gravity center migration.The results show that in the past 25 years,the distribution of heat island in Hangzhou is mainly characterized by “aggregation-central expansion-continuous transfer”;The area of heat island zones has increased sharply at first and then decreased gradually,the area of medium temperature zones does not change much,While the area of two low temperature zones change in contrast,and heat island effect is more obvious as a whole.The results of gravity center analysis show that except for several individual years,the shifting law of gravity center in the heat island zones basically the same as that of the urban gravity center,while the shifting law of gravity center in middle temperature,sub-low temperature and low temperature zones are contrary to that of urban gravity center,which indicates that urbanization is one of the main driving forces for the spatial change of urban thermal environment.(3)The method of spatial superposition analysis and spatial statistics were used to quantitatively analyze the spatial-temporal pattern changes of thermal environment ecological security in Hangzhou from 2002 to 2017.The results show that the spatial distribution of the ecological security level of each thermal environment in Hangzhou is basically consistent with the distribution of the corresponding geothermal grades.In the past 15 years,the areas of extremely high-risk zones and high-risk zones of the thermal environment in Hangzhou have shown a decreasing trend,which were mainly replaced by critical risk areas,while the areas of low-risk and safe areas have been gradually decreasing.(4)A Logistic-CA-Markov model for urban thermal environment simulation and prediction is constructed.The thermal environment pattern of Hangzhou in 2017 is simulated,and the Kappa coefficient is used to evaluate the simulation accuracy.The result shows that the kappa coefficient of thermal environment simulation is 0.8048,and the model simulation effect is good.On this basis,the logistic-CA-Markov model is used to simulate the thermal environment ecological security pattern of Hangzhou in 2022.The simulation results show that the area of ecological environment safety level of Hangzhou in 2022 is ranked in descending order: critical risk zone>high risk zone>low risk zone> extremely high risk zone>safe zone.From 2017 to 2022,the area of extremely high-risk areas and high-risk areas will be further reduced,while the area of low-risk areas and safe areas will be increased significantly,which indicate that the thermal environment of Hangzhou will be improved by 2022,but as the proportion of extremely high-risk areas and high-risk areas are still very high,it is recommended that thermal environment planning be taken into consideration in urban planning of Hangzhou.