Study On The Evolution Of Urban Thermal Environment And Its Influencing Factors In Xi’an

In recent years,the issue of urban heat island effect has been concerned by more and more people.It is not only a hot topic in academic circles,but also a problem to consider in building eco city.However,with the accelerated process of economic development and urbanization,the population of the city is increasing,and the scale of the city is expanding.The green vegetation initially covered on the urban surface gradually disappeared and replaced by cement and asphalt pavement.Great changes have taken place in land cover and land use.The continuous increase of urban construction land and the degradation of vegetation cover area bring about a series of environmental problems,which results in the aggravation of heat island effect.As the economic,cultural and political center of Shaanxi Province,the starting point of the new Silk Road,while the rapid development of Xi’an City,the thermal environment of the city is also deteriorating and many ecological problems also follow.Based on this,this paper selects the Xi’an city administrative region as the study area,through the comprehensive comparison of various methods on city thermal environment,thermal environment of city of Xi’an area using thermal infrared remote sensing monitoring method.Landsat TM and OLI remote sensing data in 1995 and 2015 were used as data sources,and data were processed in a series of processes,including correction,cutting and enhancement.At the same time,the surface temperature is retrieved by the radiative transfer equation method,and the heat island intensity was divided into different grades by means of mean standard deviation method and then the temporal and spatial variation of the thermal environment in Xi’an was analyzed.Otherwise,the maximum likelihood classification method was used to extract the land use types.Based on these,the land use change in Xi’an is monitored from the land use change range,speed of change and transformation analysis,this study also analyzed the relationship between land use and thermal environment.At the same time,Fragstats software and SPSS software were used to analyze the changes of landscape pattern of Xi’an thermal environment,and the correlation between landscape pattern index and surface temperature was analyzed.Finally,the improvement measures were proposed from the aspects of planning urban construction,optimizing urban ecological spatial layout and controlling urban heat source emission.The main conclusions obtained in this study are as follows:(1)With the rapid development and expansion of the city,the thermal environment in Xi’an changed significantly in the 20 years from 1995 to 2015.The surface temperature always showed an increasing trend,and the grade and distribution of the surface were also different.On the whole,the distribution of thermal field in Xi’an is expanding from the center of the city to the periphery,and its spatial form is constantly changing from punctate or continuous to diffuse.The strong cold island area and the cold island area are mostly distributed in the water body and its surrounding area,and the middle area is mostly distributed in the surrounding area,and the area is reduced.While the strong heat island and the heat island area are concentrated in the urban built-up area,the area is obviously increased,the surface temperature is obviously higher than the surrounding area,and the urban heat island effect is obvious.(2)In the past 1995-2015 years,the area of cultivated land and woodland in Xi’an has been obviously decreased and distributed,and the area of construction land in Xi’an is growing rapidly.The trend is obvious from the downtown of Xi’an to the surrounding districts and counties,and the area along the river construction land is also obviously increased,and the average surface temperature is also obviously rising.The trend of surface temperature is roughly the same as that of construction land.The conversion of land use and cover types is mainly based on the conversion of cultivated land,mainly to grassland,woodland and urban construction land.Urban expansion is at the expense of farmland and other ecological land,thus causing the thermal environment effect of the study area.(3)In 1995,the middle area was the dominant patch in the study area,which accounts for a much higher proportion than other regions.It and the strong cold island area of perimeter-are fractal dimension were higher,indicating that the shape of the patch was complex and the degree of fragmentation was high,while the other three types of landscape were relatively regular.In 2015,the middle area was still a dominant patch,but the degree of differentiation was high.The shape of the heat island and the cold island and the middle area was complex.The degree of fragmentation in the cold island and the heat island was higher,and the other areas were the second.(4)By studying the change rule of landscape pattern index,the dynamic change of urban thermal environment landscape pattern in Xi’an and its influence on the thermal environment are analyzed.The results show that with the passage of time,the landscape pattern changes dramatically,the degree of fragmentation increases,and the heat island effect intensifies.(5)On the basis of studying the law of Xi’an thermal environment evolution and its influencing factors,the paper considered from three aspects to improve the thermal environment,including planning urban construction,optimizing urban ecological spatial layout and controlling urban heat source emission etc.so as to provide favorable support for Xi’an the construction of ecological city.