Study On The Surface Urban Heat Island Of Shrinking Cities In China Under The Background Of Population Loss And Spatial Expansion

Under the background of rapid urbanization,more and more natural landscapes are being replaced by artificial surfaces.Temperature differences between urban areas and surrounding suburbs have become more pronounced,leading to an increasing urban heat island problem in many large cities.For shrinking cities,however,it is unclear whether population reduction will lead to a new turnaround in urban heat island effect mitigation.Although the population continues to decline,the built-up areas of shrinking cities are still expanding.And the characteristics of the expansion are unclear.Therefore,the paradox of population loss and space expansion in shrinking cities may be further manifested as the contradiction between the mitigation and intensification of urban heat island effect.Based on Google Earth Engine(GEE),this thesis takes 180 shrinking cities in China as the study area,and conducts research on the surface urban heat island(SUHI)of shrinking cities from 2001 to 2020.Based on the analysis of spatial expansion characteristics of shrinking cities,the spatial and temporal patterns and evolution characteristics of SUHI are explored when population loss and spatial expansion coexist.The random forest method is used to identify the dominant factors driving the development of SUHI in shrinking cities,and to reveal the impact of population loss and spatial expansion on surface urban heat island intensity(SUHII).The main conclusions of the study are as follows:(1)From 2001 to 2020,there is a significant trend of spatial expansion in shrinking cities in China.The area of built-up is increasing.Cropland,water,and grassland are the main streams transferred to built-up,indicating that the spatial expansion of shrinking cities is mainly achieved by occupying cropland,filling water,and encroaching on grassland.According to the calculation results of the spatial expansion index,shrinking cities generally show a low rate of expansion.The built-up area expands the fastest from 2010 to 2015,and the expansion of the urban area in the central urban agglomeration is relatively obvious.Through the analysis of standard deviation ellipse and centroid migration,the expansion of built-up areas in different shrinking cities has variability in direction and distance.(2)From 2001 to 2020,the spatial-temporal patterns and evolution characteristics of SUHII of shrinking cities in China are as follows: In terms of time,a)from the long time series,the variation of SUHII is small,and it generally shows the characteristics of weakening during the day and strengthening at night.b)From the perspective of latitudes,the SUHI is strongest in the range of 20°-25° N during the day,and significant in the range of 40°-45° N at night.c)From the perspective of climatic zones,SUHII of all climatic zones increases at night.The arid zone exhibits the characteristics of the lowest SUHII during the day and the highest SUHII at night.In terms of space,the high SUHII gradually gathers in the southern region during the day,and the SUHII of a large number of cities changes from mild(0-0.5°C)to moderate(0.5-1°C)at night.From 2001 to 2020,the SUHI of shrinking cities in China has generally eased.The analysis of the annual variation of SUHII in individual shrinking cities shows that the rising and falling trends of daytime and nighttime SUHII exist in 180 cities.It is difficult to obtain the real development characteristics of SUHI only from a single group of shrinking cities.By comparing with typical expansion cities such as Beijing,Shanghai,Guangzhou,etc.,the study finds that the SUHII of expansion cities has increased significantly in the past 20 years,while the SUHII of shrinking cities has a weak and slow upward trend.Therefore,it is considered that SUHI of shrinking cities has alleviated in general.(3)From 2001 to 2020,the SUHI area changes and thermal landscapes differed among shrinking cities.Taking Hegang and Zhuanghe as examples,in terms of the spatial distribution and evolution of the SUHI,the high-temperature area in Hegang,which is facing severe population loss and inconspicuous spatial expansion,has shrunk and the SUHI has eased.However,the SUHI area of Zhuanghe,which is facing a slight loss of population and significant spatial expansion,has obvious diffusion.In terms of the spatial migration of the SUHI area,the migration of the SUHI centroid in Hegang is related to the population change.Shrinking cities are generally more affected by the expansion of built-up areas.The calculation results of the landscape index show that at the level of patch types,the SUHI in Hegang gradually weakens,and the patches tend to be simplified.Zhuanghe is dominated by a relatively strong SUHI.At the landscape level,the distribution of SUHI area in Hegang is more compact,and it is relatively uniform in Zhuanghe.In general,the thermal landscape of shrinking cities in China presents the characteristics of weakening fragmentation,increasing inter-patch connectivity,and agglomeration of SUHI development.(4)From 2001 to 2020,the changes of SUHII of shrinking cities in China are mainly affected by population loss.The random forest regression results show that compared with the spatial expansion factors,the population loss factors can better explain the spatial-temporal variation characteristics of SUHI in shrinking cities,especially population density and population.Correlation analysis shows that almost all factors are positively correlated with SUHII of shrinking cities.Population density and population size have stronger correlations with nighttime SUHII,and the largest patch index LPI of impervious surfaces,shows the strongest correlation with daytime SUHII.