Abstract
This study examines the behavior of land surface temperature (LST) and surface urban heat island (SUHI) from MODIS data over Ahmedabad city, Gujarat state (India), from 2003 to 2018. Summer and winter LST patterns were analyzed, both daytime and nighttime. Ahmedabad, one of the fastest growing metropolitan cities in India, is characterized by a semi-arid climate. The investigation focuses on the SUHI variations due to warming or cooling trends of both urban and rural areas, providing quantitative interpretations by means of multi-sensor/source data. Land cover maps, normalized differential vegetation index, surface albedo, evapotranspiration, urban population, and groundwater level were analyzed across the years to assess their impact on SUHI variations. Moreover, a field campaign was carried out in summer 2018 to measure LST in several rural and urban sites. During summer daytime, the rural zone exhibits a higher average LST than the urban area, resulting in a mean negative SUHI, typical of arid cities, while a slight positive SUHI (mean intensity of 0.4 °C) during winter daytime is present. An evident positive SUHI is found only during summer (1.8 °C) and winter nighttime (3.2 °C). The negative SUHI intensity is due to the low vegetation presence in the rural area, dominated by croplands turning into bare land surfaces during the pre-monsoon summer season. Higher LST values in the rural area than in the urban area are also confirmed by the field campaign, with an average difference of about 5 °C. Therefore, the impact of the rural LST in biasing the SUHI is evident, and a careful biophysical interpretation is needed. For instance, within the urban area, the yearly intensity of the summer daytime SUHI is not correlated with the evapotranspiration, while the correspondent summer daytime LST exhibits a significant negative correlation (−0.73) with evapotranspiration. Furthermore, despite the city growth across the years, the urban area does not generally reveal a temporal increase of the magnitude of the heat island but an enlargement of its spatial footprint.
Highlights
Environmental and local climate effects of land cover and land use changes over the years are clearly visible in the urbanization process
We considered a total of 736 clear sky MODIS images for the 16-year period of 2003 to land use and land cover (LULC) maps were obtained from the annual global land cover time series of the Climate Change Initiative (CCI)-European Space Agency (ESA) having a
The results found within the city area considering the biophysical factors of ET and with Potential Drivers (WT) and their relationships with summer daytime urban land surface temperature (LST), whose values are not biased by the rural area as in SUHI intensity (SUHII), are very interesting
Summary
Environmental and local climate effects of land cover and land use changes over the years are clearly visible in the urbanization process. The conversion of natural areas to impervious surfaces results in a decrease of the evapotranspiration and in an enhancement of absorption and trapping of solar radiation in the urban areas, causing the well-known urban heat island (UHI). The UHI pattern can be influenced by external factors (urban area location and climate conditions) and intrinsic factors (land use and land cover, city size and growth, human activities) [7]. Changes of the intrinsic factors mainly affect the land surface temperature (LST) dynamics over the urban area, and potentially explain the surface urban heat island (SUHI) variations. Heat released from energy consumption, changes in materials, and the albedo of pavements and building rooftops [8,9]
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