Analysis Of The Spatial Pattern Changes Of Urban Thermal Environment In Zigong City And The Influencing Factors Based On Landsat Data

According to the fast development of economy in China, many cities have stepped into the stage of rapid development. Due to great changes of the land cover type and landscape pattern in the urban area, a series of environmental and ecological problems have been induced. Urban heat island(UHI) phenomenon, which was firstly reported in 1833, describes the phenomenon that the temperature in the urban area is higher than that in the surrounding suburban and rural areas. In the recent years, study on the urban thermal environment problem has been paid great attention. At present, the urban thermal environment research is not only the focus and hot spot of the scientific community, but also important to the construction and planning of the city. Zigong city is located in the south and central Sichuan province, southwestern China. Under the policy of developing western China, the size of Zigong city extends rapidly. The urban thermal environment problem has caused wide concerns.In this study, Landsat data on June 14 2001, May 21 2004, May 6 2007, August 10 2010, and June 7 2013 were collected to examine the urban thermal environment in Zigong city, Sichuan province, China. First, land surface temperatures(LST) were estimated through the radiative transfer equation method and split window algorithm, depending on the specific Landsat sensor. Second, in order to better represent details of the urban thermal environment, the estimated LST with at 120 m(based on Landsat-5 TM), 60 m(based on Landsat-7 ETM+), and 100 m(based on Landsat-8 TIRS) were downscaled to 30 m. Third, the Landsat LSTs acquired in different months and in different years were temporally normalized through a simple linear statistical approach, to remove the influences of temporal differences on the examination of urban thermal environment. Fourth, areas with different urban heat island magnitudes were extracted and the urban thermal environment was quantified based on the theory of landscape ecology. The influences of land cover and vegetation abundance on the urban thermal environment were examined.The main conclusion of this study are as follows:(1) From 2001 to 2013, the surface temperature of Zigong city showed an increasing trend.With the speeding up of Zigong city urbanization process, the land cover surrounding the city transferred from natural surface types to artificial types. The land surface temperature of the whole city increased.(2) The land surface temperature after downscaling and temporal normalization performs better in the examination of the urban thermal environment in Zigong city. Due to the extent of Zigong city is small, the Landsat thermal infrared images with low spatial resolutions contains very complicated information and they are less capable of examining the urban thermal environment.(3) The urban heat island effect showed an increasing trend. According to the different areas with different urban heat island magnitudes extracted from the land surface temperature images, the results demonstrate that: 1) from 2001 to 2013, the fraction of the area with extremely strong heat island to the urban area increased from 1.2% to 7.8%; 2) the fraction of the area with strong heat island area increased from 12.6% to 18.4%; 3) the fraction of the area with weak heat island decreased from 62.4% to 55%; 4) the fraction of the area with the medium temperature decreased from 20.8% to 16.3%; and 5) the fraction of the area with cool island decreased from 3% to 2.5%. Generally, the urban heat island of the whole city increased from 2001 to 2013.(4) The landscape pattern of the urban thermal environment in Zigong city changed from 2001 to 2013. As for the patch number and patch density, predominant type of landscape pattern in 2001 was the medium temperature area and the weak heat island area. In 2013, the dominant landscape pattern was the weak heat island area and the strong heat island area. As for the aggregation degree, the landscape elements constantly tended to be scattered from 2001 to 2013.(5) The relation between the surface temperature and various parameters were examined. There is significant negative correlation between the land surface temperature and the normalized difference vegetation index(NDVI)Land surface temperature depend on the land cover type greatly. According to the statistics of the land surface temperature of each land cover type, we found that: from 2001 to 2013, the land surface temperature ranking of the land cover type is building > bare land > vegetation > water body. The results show that the artificial constructions and the bare land have direct positive influences on the land surface temperature and increase the urban heat island effect. Therefore, scientific urban planning and effective control on the bare land is may be effective way to control the urban heat island. Vegetation and water body can significantly reduce the surface temperature of the city; thus, increasing the areas of the vegetation and water body can effectively mitigate the urban heat island effect.