Studies On The Relationship Between Vegetation Cover And Urban Heat Island Effect In Hangzhou

With rapid urbanization, a series of environmental problems have occurred in cities. One of the most serious problems is the urban heat island effect(UHIE). Because of the heat driving role of UHIE, it results in a great number of disadvantages to our cities. The normal life and work of citizen have been disturbed seriously by high temperature weather, and the further development of cities and people's life would also been restricted by the UHIE. Studies on the distribution and variation of UHIE would be significant to mitigate it and to improve the quality of city environment and people's life. On the basis of the research results at home and abroad, the data of remote sensing image of Hangzhou City in 1994 and 2004, together with natural, social and economic statistical data and field survey, the UHIE, the green space and the relationship between vegetation cover and UHIE in Hangzhou were investigated using the technology of remote sensing and geography information system. Suggestions for mitigation of the UHIE in Hangzhou were also proposed.1. Dynamic changes of urban heat island effect in Hangzhou. The distributions of brightness temperature (B.T.) were analyzed detailed by making use of the Landsat TM heat infrared bands in Hangzhou in 1994 and 2004 summer. Results showed that there were 3 location of urban heat island(UHI) in 1994, including the central commercial areas, southern part of Gongshu district with a heat electric power plant and the north side of the canal in Xiacheng district with a boiler factory. But the locations of UHI changed and UHI increased to 4 in 2004. The UHI in the central commercial areas had moved south to the city railway station. The locations of two other UHI did not change, but their degree and extension had been enhanced obviously. A new UHI was located in the newly development zone in Qiaosi town andXiasha town connected by the circling freeway, where material transfer centers and industrial areas were established. By analyzing the change of UHI, it was found that the areas of UHI significantly expanded with the development of Hangzhou City. The areas of high temperature and sub-high temperature places increased by 29.6% and 31.3% from 1994 to 2004, respectively. The locations of UHI were not in the centre of the city, but the station or industrial areas. The UHI expanded along the roads in and out of the city or spreaded in the form of patches in newly development zones. These results indicated that urban expansion and industrial construction were the two major driving forces of UHIE. The property of underlying surface, artifical heat and emission gas all influenced the intensity of UHI. And the green space could greatly mitigate the UHIE.2. The distribution of green space and its change in Hangzhou. The normalized difference vegetation index (NDVI) map of 1994 and 2004 were produced with TM image data, and then the distributions of green spaces in Hangzhou were analyzed. The green space information was obtained by band combination method, the changes of green spaces were analyzed by change analysis method. These results showed that there were 45992.43 ha and 40163.85 ha green space in 1994 and 2004, respectively. The area of green space had reduced by 127.82 km2 from 1994 to 2004, and about 94% of the decreased area changed into constructed land which was mainly in Binjiang, Jianggan and Xiaoshan districts. Therefore, urban expansion is the major cause of green space reduction. However, the areas planted to trees or lawns in downtown increased by 6954.21 ha in las decade. The newly established green areas were mainly distributed along the rivers, streets and the West Lake and it played a positive role to improving urban environment and the landscape.3. The relationship between vegetation cover and the urban heat island effect. Maps of the distribution of B.T. and the distribution of vegetation were compared. It was obviously discovered that vegetation cover changes and urban heat island changes coincided very well. In quantitative research, the relationship between NDVI and B.T. was studied with type sampling method, which was considered as a more scientific sampling method for the study. The results showed that the average B.T.59reduced in ther order of city - grass - shrub - arbor, but the average NDVI was opposite. It seems from the change trend that B.T. decreased obviously with the increase of NDVI from city - grass - shrub, but the decrease became smaller with the increase of NDVI from shrub to arbor. These results might indicate that mitigation effect would become less significant after NDVI approached certain high level. The result of linear curve-fitting showed that the NDVI was negatively correlated with B.T. (p<0.05). The fitted regress equation was Y = -8.9605X + 24.48 l(R2=0.4094). The scatter diagram of NDVI and brightness temperature validated the triangle Ts/NDVI models, which illustrated that both vegetation cover and humidity had an impact on the surface temperature.