Study On Thermal Environment Of Different Land Cover Types In City

Heat island effect is one of the problems caused by high-speed urbanization,which has seriously damaged the ecological environment and directly threatened the future survival of mankind.With the continuous advancement of urbanization in China and the continuous expansion of cities,artificial impervious surface has largely replaced other natural surfaces such as green space,resulting in an increase in land surface temperature and the increasingly serious urban heat island effect.Therefore,it is necessary to explore the influence of different land cover types on the change of thermal environment and optimize the layout of urban land.In this paper,the urban areas of Xi' an,Xianyang and the Xi Xian New District are taken as the research areas.Based on Landsat images,the results of land surface temperature and land cover of summer in the study area for many years are obtained,and the land surface temperature is classified.The spatial analysis methods such as standard deviation ellipse,overlay analysis,distance analysis,landscape pattern analysis and mathematical statistics methods such as linear fitting are used to explore the dynamic changes of thermal environment and the land cover in Xi ' an,Xianyang City and Xi Xian New District of 2005,2010,2014 and 2019.In the study,the land surface temperature characteristics of different land cover types are analyzed,and the land cover types that can affect the land surface temperature change in summer are identified.The cooling effect of land cover types is quantitatively analyzed,and specific suggestions on how to configure these land types to improve the thermal environment are given.The main conclusions are as follows :1)During 2005 – 2019,the proportion of high temperature areas and above that remained unchanged gradually increased,and the distribution of urban heat island in the study area tended to be stable.The distribution of low-temperature region changed fromeast-west to northwest-southeast.The northwest-southeast distribution of high temperature region and extra-high temperature region had a trend of close to east-west distribution.The other temperature regions remained the northwest-southeast distribution throughout the year.The direction of low temperature region is gradually enhanced,and the trend of other temperature regions is weakened,especially in the extra-high temperature region.The center of each temperature region is located in the Wei Yang.The center of the low temperature region tends to move to the main city of Xianyang,and the other temperature regions' centers tend to move to the main city of Xi ' an.2)In 2005,the largest land cover type in the study area was vegetation,followed by construction land,accounting for 60.54 % and 33.34 % of the total area,respectively.In 2019,construction land became the largest land cover type,accounting for 49.94 % of the total area of the study area,and the area ratio of vegetation decreased to 45.08 %.The area of urban construction land in the region increased significantly,mainly occupying the vegetation coverage areas.3)In 2005 and 2010,the order of mean land surface temperature in the study area from high to low was bare land,construction land,vegetation and water.In 2014 and 2019,the order was construction land,bare land,vegetation and water.The average land surface temperature of vegetation and water was lower than that of the study area,and it was below the middle temperature region all year round.The average land surface temperature of construction land was always higher than that of the study area,and it was above the high temperature region all year round.The average land surface temperature of bare land changed significantly,showing a trend of transferring from the extra-high temperature zone to the middle temperature zone.4)Construction land can significantly improve the land surface temperature of the surrounding environment.Water and the area covered by palnts with canopy(vegetation 2)can significantly reduce the land surface temperature of the surrounding environment.The area covered by plants without canopy(vegetation 1)and bare land have no significant effect on the land surface temperature change of the surrounding environment.The cooling distances of vegetation 2 for construction land,bare land and vegetation 1 were 330 m,180 m and 210 m,respectively.The cooling intensities within the cooling distance were 3.84 °C,2.19 °C and 1.66 °C,respectively.The cooling distances of water for construction land,bare land and vegetation 1 were 180 m,150 m and 210 m,respectively.The cooling intensities within the cooling distance were 4.32 °C,3.94 °C and 2.93 °C,respectively.Vegetation 2 had a larger cooling distance than water,and water had a higher cooling intensity than vegetation2.In the specific planning,woodlands with large area,diverse shapes and aggregation should be arranged as far as possible,and water bodies should increase the area as much as possible under the premise of maintaining the integrity of a single patch,and the woodlands and water bodies should be dispersed among construction land combined with different cooling distances.