Temporal And Spatial Characteristics Of Urban Land Cover In Semi-arid Areas And Its Influence On Land Surface Temperature

Since the 21st century,the rapid urbanization process had a profound impact on the macro structure of the regional ecosystem and the components of the inner surface cover structure in the city.With the continuous growth of urban population and the ever-increasing scale of development and construction,urban ecological environment and climate problems have become increasingly prominent,which greatly affects the quality of urban residential environment and the well-being of residents.The ecological environment in the semi-arid area is relatively fragile,and the process of large-scale urbanization has a profound impact on the regional ecosystem structure and the urban surface level structure,as well as the urban thermal environment pattern.Therefore,the research on the evolution of the surface spatial structure and the effect of thermal environment in the semi-arid urban agglomerations is the major strategic demand for national new urbanization and the realistic demand for the regional healthy urbanization.At present,the researches on the urbanization process and its eco-environmental effects in the semi-arid area are mostly limited to individual urban scale,and the study on the ecological environment impact on the urbanization process of the urban agglomeration is still weak.Under the background of promoting urbanization with urban agglomeration as the main form,the study on the spatial expansion of urbanization and its impact on the regional ecosystem in the semi-arid ecological fragile zones,the analysis of the temporal and spatial characteristics of surface cover structure and its components in urban agglomerations,and the reveal of the thermal environment effect of spatial process of urbanization,have important theoretical and practical significances to explore the impact of human activities on the fragile ecological environment in semi-arid areas,to formulate scientific and rational urban development and ecological environment protection plan,and to promote the construction of regional ecological civilization.This study is to solve the following key scientific issues:in view of the urban areas under severe human activities disturbance in the semi-arid area,through the establishment of urban land cover change research system,the spatial-temporal changes of urban land cover are accurately described at different levels,and their influence on the spatial-temporal evolution of urban surface thermal environment is discussed.Meanwhile,from the perspective of urban surface coverage,the measures for improving the surface thermal environment are put forward.This study,taking the semi-arid area of Hohhot-Baotou-Ordos(HBO)as an example,on the basis of the construction of the research system of the urban land cover change,selecting the 2000,2005,2010 and 2015 ecosystem macro structure data and Landsat image data,with the help of RS,GIS and SPSS technology,using decision tree threshold classification model and exponential coupling mode,combining with V-I-S endmember selection and the latest least squares pixel mixed decomposition model,researched the surface expansion of urban,the land cover structure of urban and the surface coverage components of urban,discussed the temporal and spatial evolution of urban surface temperature and revealed the thermal environment effect of urbanization process in urban agglomeration.From the point of view of urban surface coverage,the measures for improving the surface thermal environment are put forward.The main conclusions are as follows:(1)The macro-structure analysis of the regional-scale ecosystem shows that in 2015,the macro-structure of the ecosystem of Hohhot-Baotou-Ordos(HBO)was dominated by grassland and desert.From 2000 to 2015,the change of the settlement ecosystem was the most intense,with the largest increase in area.It was the dominant type that affected the ecosystem service function in the region,and the ecological environment effect caused by the change became the focus of attention.(2)The analysis of spatial expansion of urban agglomeration shows that the urbanization process of Hohhot-Baotou-Ordos(HBO)is faster in 2000-2015 years,and the period from 2005 to 2010 is the most significant.There are obvious differences in the pattern and process of urbanization in different cities,the extensional expansion,is the main form of Hohhot,Baotou is mainly filled with filling type,and Ordos with enclave expansion pattern as the main.(3)The water impervious surface presents a continuous increasing trend,with the most dramatic changes,and becomes the dominant urban surface structure type in the 2000-2015 years.The vegetation presents a trend of fluctuation and increase,reflecting attention was paid to the green space construction in the urbanization process in the semi-arid area.After the transformation of bare soil into urban area has gradually changed to impervious surface and vegetation,it has promoted ecosystem services and residents' well-being in general.Therefore,in semi-arid area bare soil should become the surface type of urban construction.(4)Different from the humid and semi-humid areas,the expansion of the urban space in the semi-arid areas,especially the expansion of the bare area,and the treatment of the bare land inside the city,generally lower the surface temperature of the region,and have a positive effect on the regional thermal environment.(5)The surface temperature of land cover types in different cities varies greatly,and the degree of contribution to the surface thermal environment of cities is also different.The urban surface temperature shows the rule of bare land>water surface>urban green land>water.The bare land in the surrounding and inner regions of the semi-arid areas becomes the local "heat island" center of the city,which contributes greatly to the city heating up,and is also an important factor causing the deterioration of the urban thermal environment.Based on the urban boundary change scenario analysis,the surface temperature is increasing at the highest temperature and the surface energy polymerization ability is constantly enhanced.It shows the phenomenon of urban heat island.(6)Surface-covered impervious components and bare components have a temperature polymerization effect(positive feedback),the former is less than the latter,and the green component has the cooling effect(negative feedback),and the three are not linearly related to surface temperature,but are Expressed as accelerating and accelerating decline.