| Urban ecological problems are becoming more and more prominent with the accelerating urbanization speed recently, in which one of the research highlight is the urban heat island effect(UHIE). How to alleviate the urban heat island effect is the difficult problem need to be answered by scientists and government, and this problem will be only solved based on the fully understanding of the changes characteristics of urban heat island and its influencing factors. As one of the important cities in western China, the process of urbanization of Lanzhou city has accelerated in recent years. Meanwhile, as a typical valley city, its special topographical features have a certain influence on the heat island effect. Based on the analys is of spatial and temporal variation characteristics of UHIE and thermal landscape pattern in the study area, to explore the influencing factors of heat island effect and to analyze the quantitative relationship between UHIE and its influencing factors, will be useful for urban planning and urban management, especially for the valley cities in the future. The main conclusions could be showed briefly as follows:(1) In the recent 25 years, the area of urban land in Lanzhou City has increased continuously. Before 2003, urban land increased slowly, urban expansion intensity was weak and showed a downward trend. After 2003, the urban land expanded rapidly, the area of urban land grew largely. Urban land in the Chengguan District was the largest one, followed by Xigu District and Qilihe District; urban land in Anning District was the smallest one. But, the growth rate of urban land of Anning District was the fastest, followed by Chengguan District.(2) The spatial pattern of urban heat island effect in Lanzhou has changed greatly from 1991 to 2015 a. The non-heat-island region occupied the majority in 1991 a, 1994 a and 1999 a. The distribution area of the non-heat-island region accounted for 58.56% of the total area in 2003. In 2007 and 2015, the heat island region was the main temperature grade. Overall, the heat island region is increased with the non-heat-island region continued to decrease. The most rapid increase of the area of heat island occurred in Anning District, followed by Chengguan District,the area of heat island in Xigu District and Qilihe District changed in a small range.(3) As to the selected landscape index at the level of landscape type, the fragmentation degree of strong heat island region increased first and then decreased, patch shape was not complicated and changed a little. The area of middle zone was shrinking, and the Aggregation Index(AI) was high, but the shape was simple. At the strong green island region, the Number of Patches(NP) was the least and the Patch Density(PD) was the lowest one, but the Mean Patch Area(MPA) was not the smallest one, the AI was high, shape complexity was lower with little change. At the landscape level, the fragmentation degree of the thermal landscape increased firstly and then decreased during the study period, the patch shape complexity changed with wavy character, but the overall shape of patch was not so complicated, the aggregation degree was high and the equilibr ium degree of each type of the thermal landscape decreased gradually.(4) The profile analys is of thermodynamic field showed that the Land Surface Temperature(LST) was closely related to the characteristics of the underlying surface. The temperature of storage area and the area with larger logistics in the Qilihe District was the highest one, followed by the petrochemical zone in Xigu District and the Xiguan commercial district in Chengguan District; The lowest temperature area distributed along the Yellow River and other water bodies, followed by the area with high vegetation coverage, such as parks, botanical gardens and so on.(5) The high vegetation coverage region reduced gradually; vegetation coverage and LST was negatively correlated, but the correlation showed a downward trend. As to the impervious surface area(ISA), the distribution of ISA was increased; ISA was positive correlated with LST. From the point of water, high value area of MNDWI showed a trend of first rise and then drop. It was a negative correlation between MNDWI and LST, the correlation coefficient was lower than the correlation between LST and vegetation coverage and the correlation between LST and ISA.(6) Using the third and fourth planning map of Lanzhou City as the boundary of study area to analyze different patterns of UHIE in the different city range. The urban heat island phenomenon was obvious and had a larger range in the third planning area. As a typical valley city, the temperature of central city was lower than the temperature of surrounding mountains(fourth planning area). The high temperature was mainly distributed in the altitude range of 1633.01-1928.67 m, the slope face to south, southeast and east. The low temperature area distributed in the valley sub-regions, such as, central city and the region with elevation above 1928.67 m with the slope face to west, northwest and north. The main influence factors were vegetation and the duration and amount of solar radiation. |
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