A Study On Vegetation And Heat Island Effect In The Process Of Expansion Of Built-up Area In Urumqi

In order to effectively monitor the expansion process of the built-up area and correctly grasp the ecological environment changes in Urumqi over a long period of time.Based on the multi-source remote sensing image data,combined with the existing research results and experience,this paper studies the temporal and spatial evolution characteristics of vegetation and its heat island effect in the expansion process of the built-up area of Urumqi.The main conclusions are as follows:(1)From the time scale,the expansion rate and dynamics of the built-up area of Urumqi were 40.28 km~2/a and 23.98%,and both of high-speed expansion types in the past 25 years.From the perspective of spatial pattern,the built-up areas of Urumqi were scattered,mainly in Tianshan District,Shayibake District and Gaoxin Distric in1993.The built-up area gradually expanded in 2003 and it became more regular in2013.The results showed that the built-up area of Urumqi showed a“T”type expansion in the past 25 years.And it connects Jinkai District in the western of Urumqi with the Gaoxin Distric in the middle and the Midong District in the north.As can be seen from the overall characteristics of the shift of the focus in the built-up area,Urumqi built-up area shifted its center of gravity to the west by 1577.60 m in1993-2003;in 2003-2013,it was transferred to the northwest 2584.54 m;in 2013-2018,it was transferred northeast for 1232.59 m.(2)The vegetation in the built-up area showed a tendency of decreasing after increasing firstly,1993-2018,the vegetation in the built-up area decreased from419.70 to-18.20,and the mean change of NDVI was negative,it indicated that the vegetation conditions in the built-up area were gradually deteriorating.Further analysis of the vegetation coverage in the entire city area from 2000 to 2017 can be found to show a downward trend,with the average decreasing from 0.056403 in 2000to 0.045527 in 2017.According to the classification results,it can be seen that in 2000,the whole city was mainly covered with low coverage,and the vegetation coverage in the northern of the built-up area was significantly higher than that in the southern of the built-up area,and there was also a point-like low vegetation coverage area in the north.In summary,the vegetation coverage of Urumqi in the past 17 years has shown a trend of decreasing gradually from the built-up area to the northwest and the Chaiwopu-Dabancheng Valley in the eastern region.At the same time,the vegetation coverage of the Nanshan Area,the Midwest Region and the northern of the Chaiwopu-Dabancheng Valley in the southwest of Urumqi is increasing year by year.(3)Firstly,it is concluded that the surface temperature of Urumqi from 2000 to2017 is increasing as a whole.In 2000,the city's surface temperature grades were mainly composed of moderate-temperature and sub-low temperature,indicated that the city's heat island effect was not significant.In 2017,high temperature and ultra-high temperature were the mainstays,which indicated that the surface temperature of Urumqi had risen in the past 17 years.At the same time,it forms a heat islandeffecttrendextending northwestfromthe Chaiwopu-Dabancheng valley.Among them,the sub-low temperature zone and the medium temperature zone in 2000 were widely distributed,accounting for 38.72%and 36.63%of the total study area,respectively,and the proportion of the heat island area in Urumqi was 7.64%.In2017,the high temperature area,the ultra-high temperature area and the sub-high temperature area were the most widely distributed,accounting for 40.44%,22%and21.22%of the total study area,respectively.The proportion of the heat island area in this city was 83.66%,which increased 10 times compared with 2000.In summary,the surface temperature in Urumqi is generally on the rise,and the urban heat island effect is becoming more and more significant.Therefore,the causes of urban heat island effect are summarized from three aspects,namely,the change of the overall structure and fundamental nature of the urban underlying surface,the increase of artificial heat emissions,and the unreasonable planning of urban landscape layout.At the same time,suggestions and measures to alleviate the urban heat island effect were also proposed.