Thermal Environment Change And Associated Influencing Factors Of Minjiang River Estuary Wetland In Urbanization

Estuarine region is one of the most densely populated and prosperous area around the world,and it is also a fragile area due to high-intensity anthropogenic activities.The acceleration of urbanization have inevitably resulted in a series of ecological and environmental problems on estuarine region,the thermal environment is a severe part of them.To study the spatiotemporal dynamic change of land use/land cover(LULC)and land surface temperature(LST)in Minjiang River estuary,four Landsat images acquired in 1993,2000,2008 and 2016,were used.With the help of statistical methods,the correlations between LST and different LULC classes,normalized difference vegetation index(NDVI),normalized difference moisture index(NDMI),and normalized difference impervious surface index(NDISI)were examined.The results showed that:(1)Dramatic change in land use/land cover has occurred from 1993 to 2016 in Minjiang River estuary.The built-up land area increased largely,whereas the forest/grass and cropland area declined.Remarkably,the area of wetland reduced by 17.95%(from 37.00 km~2 to 29.15km~2).The built-up land area expanded to peripheral area at the cost of cropland,forest/grass and water.Overall,the major LULC change occurred along both side of the Minjiang River,and the change magnitude gradually weakened from the sides of Minjiang River to the inland region.Geographical conditions,population growth,economic development and government policies were the main driving forces of LULC change.(2)The built-up land area in Minjiang River estuary has expanded continuously against the background of urbanization,showing a slow-rapid-steady and increasing change trajectory.Urban sprawl liked a“big cake”along the Gutai old city-Nantai island-new city zone-coastal area axis and occurred mainly in east,west and south directions.Throughout all of the 23-year period,the edge-expanding was the primary urban expansion mode.(3)The area of sub-high and high temperature zone increased markedly,while the middle temperature zone reduced,and there was no significant change in the sub-low and low temperature zone.The sub-high and high temperature zone was highly clustered in the town center,the middle temperature zone was distributed around the sub-high and high temperature zone,and the sub-low and low temperature zone was mainly observed in the area of extensive forest and Mijiang River.Moreover,the spatial distribution of the high temperature zone was consistent with built-up land expansion.The LST had a highly positive spatial autocorrelation,and the spatial clustering characteristic became more remarkable with the advancement of urbanization.And the spatial pattern of LST had a distinct scale effect,the spatial dependency of LST became weaker when grid cell size increased.(4)The LST varied with different land use/cover classes.The dominance of built-up land significantly strengthened the surface temperature,while increasing the dominance of vegetation and water could cool the surface temperature.Cropland displayed no sign of cooling effect,the LST tended to be stable as the percentage of cropland increased.Regarding the LST in the different urban expansion modes,infilling mode had the highest mean LST,followed by edge-expanding mode;and outlying mode had the lowest mean LST.(5)The LST exhibited an obvious disturbance characteristic.The land surface temperature near city center presented dramatic change,and the temperature fluctuation in suburb was relatively smoother than Fuzhou city proper.The urban expansion transformed wetland to built-up land,which exerted an influence on the fluctuation of wetland surface temperature.The analysis of correlation indicated that a significantly negative relationship was observed between LST and NDVI,between LST and NDMI.And there was a positive relationship between LST and NDISI.What's more,the NDMI and NDISI played a greater role in LST than did NDVI.(6)Measures to improve the thermal environment of Minjiang River estuary were provided,which mainly included controlling the continuous expansion of built-up land,increasing the coverage of ecological land,promoting the sustainable intensification of cropland,reducing the mission of anthropogenic heat,improving the regional ventilation channel and delimiting the ecological red line of wetland.