Change in urban wetlands and their cold island effects in response to rapid urbanization

Abstract

The cold-island effect of urban wetlands has received increasing attention in recent years due to its important role in the alleviation of urban heat islands. Hangzhou, a representative rapidly urbanizing city with rich wetlands in China, was selected as a case study for researching the changes that the urban wetlands have undergone and their impact on the urban thermal environment. Land surface temperature (LST) was acquired from the thermal infrared data of Landsat 5 Thematic Mapper (TM) images in 1990, 1995, 2000, 2006, and 2010, using the single-channel method. The results are as follows: 1) considering the changes in land use, the urban wetlands located to the west of Hangzhou have decreased significantly during 1990–2010 because of rapid urbanization. In the Xixi Wetland, the change in land use was relatively small and most of the water body and vegetation were preserved. However, to the east of the Xixi Wetland, large areas of water body and vegetation have been replaced by built-up land as a result of the urbanization process; 2) considering the change in LST, it was found from land surface temperature retrieval that the changing spatial pattern of the thermal field was highly correlated with land use changes. Low temperature regions of the eastern Xixi Wetland were gradually eroded by high temperature regions, and the centroid of the heat island in East Xixi was found to be constantly shifting westward. In addition, the difference in LST between the Xixi Wetland and East Xixi has increased; 3) considering the impact factors for this area, land use structure and patch shape were found to have a significant impact on LST, shown by the results of multiple linear stepwise regressions. Increasing the size of the wetlands in urban planning is considered to be the most effective measure in alleviating the urban heat island effect. Moreover, reducing the spatial complexity of landscape patches also contributes to the alleviation of the urban heat island effect.