Abstract:

Objective: To explore the patterns of evolution of ground surface temperature in urban area,quantitatively explain the difference in vegetation cooling under different vegetation coverage conditions and provide a basis for improving urban ecological environment and rational planning of urban green space. Method: The study was focused on the highly urbanized area within the 5^th ring road in Beijing. We divided the study area into extremely low temperature zone,low temperature zone,sub-low temperature zone,medium low temperature zone,sub-high temperature zone,high temperature zone and extremely high temperature zone and then explored the spatial and temporal changes of ground surface temperature during 1999-2017 based on the ground surface temperature and vegetation coverage images from the five-phases Landsat remote sensing images. Linear regression was used to further analyze the correlation between vegetation coverage and surface temperature at 300,600,900 and 1 200 m grid scales. Result: From 1999 to 2017, the temporal change of the thermal environment within the 5^th ring road in Beijing is divided into two stages. The area of the high temperature zone and the extremely high temperature zone gradually increased from 1999 to 2011, and the thermal environment has improved from 2011 to 2017 The area proportions of the high temperature zone and the extremely high temperature zone decreased by 0.96% and 0.71%,respectively,and the those of the extremely low temperature zone and the low temperature zone increased by 0.45% and 1.19%,respectively. The spatial pattern of the thermal environment changed significantly with the urban development of Beijing. In 1999, zones with high annual surface temperatures are concentrated within the 2^nd ring road,and gradually transferred outward after 1999. By 2011, high temperature and extremely high temperature zones were concentrated in the area south of the center between the 3^rd and the 5^th ring road (the area proportions of the high- and extremely high-temperature zones reached the highest,70.73% and 78.92%,respectively). The overall thermal environment within the 5^th ring road has improved in 2017. During the study period,the vegetation coverage of the whole study area generally showed a decreasing-to-increasing trend with the lowest value of 31.84% in 2005. The zones with high vegetation coverage are mainly distributed in the area between the 4^th and 5^th ring roads. We observed the overall negative correlation between the surface temperature and the vegetation coverage (P < 0.001). Moreover,this relationship is significant under the condition of 40%-60% vegetation coverage at all grid scales throughout the study period. Moreover,the cooling effect increased with the vegetation coverage when compared with the same grid scale. Conclusion: The areas of high temperature and the extremely high temperature gradually increased from 1999 to 2011. The high temperature area continued to transfer outward from within the 2^nd ring road,while the thermal stress was alleviated during 2011 to 2017. The vegetation coverage generally decreased first and then increased,and the vegetation coverage was higher between the 4^th and the 5^th ring roads. The increase in vegetation coverage can reduce the ground surface temperature,and a stable cooling effect is shown when the vegetation coverage reaches 40%-60%. Therefore,it is plausible to alleviate the urban thermal stress by appropriately increasing the vegetation coverage and improving the cooling effect of urban green space.

Key words: urbanization, thermal environment, temporal change, spatial pattern, vegetation cooling