| Urban heat island is one of the most notable features of urban climate in the process of rapid urbanization,and has become one of the main ecological problems faced by the cities in 21 st Century.The expansion of the city is the main reason for the generation of heat island,as vegetationandwater are gradually replaced by impervious surface,the thermal radiation properties of the underlying surface of cities have just changed,thus affecting the micro climate,and resulting in a heat island effect.On the basis of the causes of heat island,the increase of urban water,vegetation such natural cover is conducive to the city’s cooling,humidification,local circulation and climate formation,so as to ease the heat island effect.At present,the research on the mitigation of heat island effect is mainly around the vegetation,and there are few reports about the specific regulation mechanism of water body on the heat island effect.Guangzhou has abundant water resources,and compared to the vegetation,water is not affected by the seasons,and it is a more stable cover type,therefore,study on the thermal environment effect is of great significance for improving the urban thermal environment.Remote sensing technology provides the study of urban landscape with a new technical means.This paper takes Guangzhou as the study area,and land use classification is made based on Landsat remote sensing image,and GF-1 image data are further used for the extraction of urban waters,which are divided into lakes,rivers,reservoirs andponds.And the spatial distribution and temporal and spatial variation of heat island of Guangzhou are analyzed by thermal infrared remote sensing technology for surface temperature inversion.Then,combined with spatial analysis technique,the thermal environment characteristics of different types of water bodies and their effects on the surrounding thermal environment in cities are explored to reveal the specific regulation mechanism of water body to heat island effect.The main results are as follows:(1)From 1994 to 2014,with the advancement of urbanization,the land cover in Guangzhou had changed greatly.The rapid expansion of urban had resulted in the built-up area increased by 1191.4km2,with an average annual growth rate of 6.5%.Bare area shrank rapidly,a total reduction of 288.8km2.The area of vegetation decreased by 892km2,the annual reduction rate was 8%.And the water body did not change obviously.Among all water types,the total area of ponds increased by 32km2,with an average annual growth rate of 1.6%,and the increase occurred mainly in Nansha,Panyu and Huadu districts.The river area showed a decreasing trend,a total reduction of 52km2,which was mainly caused by the land reclamation and construction of embankment in Nansha District.And the area of the lake was relatively stable,and there was no obvious change.(2)The heat island effect in Guangzhou was becoming more and more obvious with the urbanization process,reflected in a large increase in heat island.From 1994 to 2014,the heat island area increased by 1.06 times,a total of 1831.8km2,about 32% of the area of Guangzhou,with an average annual growth of 5.3%.The expansion of the heat island distribution area was basically the same as that of the built-up area,which was mainly distributed in the central area,and concentrated in Yuexiu District,Liwan District,Haizhu District and Huangpu District,extension from south to north,from west to east.And the increase of heat island in northwest of Baiyun District,Southern Zengcheng District,Southern Huadu District,Southwest of Conghua District,Panyu District and Nansha District was more significant.(3)The heat island effect shows different response for different land cover types.Vegetation and water body are relatively low temperature cover types,while the bare land and built-up area are relatively high temperature cover types,the increase of the proportion of built-up area and bare land will significantly increase the surface temperature.There exists the scale effect of the response of heat island effect to land cover types,at the scale of 500 m,the correlation between water ratio and heat island effect is the strongest.The cooling effect of water body is stronger than that of vegetation.In the range of 500 m in 2014,when the proportion of the built-up area reached 100%,the heat island was formed.Considering only the influence of the water body,the heat island was not formed when the proportion of the water body reached 68%.If the vegetation effect was only considered,to 73% did not form a heat island.Considering the coexistence of water and vegetation,heatisland is not formed when the proportion increases to 71%.In 2005,when the proportion of the built-up area reached 100%,the heat island was formed.Onlyconsideringofthewaterbody,whenthe proportion of the water area increased to 61%,there is no heat island was formed.Only when the vegetation was considered,the vegetation proportion increased to 74% can do the same work.In 1994,the same when the proportion of built-up area reached 100%,there is a weak heat island phenomenon occurs.Only considering the water,the water area ratio increased to 20% will not form a heat island,only consider the vegetation,the vegetation increased to 27% can make the heat island disappear.(4)The cooling mechanism of vegetation and water body is different.At the scale of 500 m,in non-urban land,the cooling effect of vegetation is stronger than water body,but in the urban land,the cooling effect of the water body will enhance with the increase of the density of urban development,while there is an opposite situation in vegetation.The result shows that the water body has cooling effect,while water radio increase,the heat island effect will decrease,but the cooling effect will be affected by the surrounding land cover.As the radio of surrounding vegetation increase to certain extent,the cooling effect of water body will show the opposite way.(5)There exists difference in the cooling effect of different types of water,the cooling effect of facet water is stronger than that of linear water body.The cooling distance of the river is about 150 m,while the cooling distance of reservoirs,lakes such facet water is up to 250 m.The cooling effect of water body decreases non linearly with the increase of distance.At the same distance,the cooling rate of the reservoir is the highest,and the cooling rate of the rivers and lakes is similar.(6)The influence factors of different types of water’s thermal environment are different.And the thermal environment characteristics of the reservoir are mainly affected by its area,shape,location and surrounding land cover.There is a significant negative correlation between the temperature and the area,the shape index and the nearest distance from the heat island of reservoir,while a significant positive correlation with the proportion of surrounding built-up areas.The thermal environment characteristics of typical lakes are mainly affected by area and its location.For lakes,there is a significant negative correlation between the temperature and the area and the distance from the heat island.And the river is mainly affected by its width and lacation.There is a significant negative correlation between the temperature and the width of the river and the nearest distance from the heat island.(7)The main factors that affect the thermal environment around the water body are the area,width and the surrounding land cover.The cooling island intensity of waterismainlyaffectedbythewaterbodyareaandtheratioofsurroundingvegetation,while the ratio of surrounding vegetation increase 10%,the cooling island intensity will be decreased by 0.22℃.As for the cooling island efficiency,it is mainly affected by area or width.There is a threshold in water body’s area or width for effective cooling impact,the effective threshold area of facet water is 50 ha,while the effective width of linear water body is 200 m.The heat island effect of Guangzhou is becoming more and more obvious with the advance of urbanization process,while water body can form a local climate in a certain range to alleviate the heat island phenomenon because of its underlying surface properties.The thermal environment effect of water body is affected by many factors,including its own attributes(area,shape,width),location and surrounding land cover.Therefore,in urban planning,thermal environment characteristics of water shall be reasonably combined for the rational planning of cooling channels to improve urban thermal environment. |
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