The Chennai Metropolitan Area, predominantly an agricultural area in 1988, has nearly 48.7% of built-up area. The decadal changes in land use and urban sprawl has led to warmer temperature and increasing urban heat island effect spreading to peri-urban landscape.
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This was one of the key findings of a study titled “Monitoring spatio-temporal dynamics of urban and peri-urban land transitions - A case study of Chennai Metropolitan Area” by the Centre for Water Resources, Anna University.
According to the study that covered an area of 1,189 sq. km., including parts of Kancheepuram and Tiruvallur districts, the CMA had a vegetation cover of nearly 17,770 hectares, which was 14.9% of the area in 1988. It has drastically shrunk to 7,288 hectares, which was only 6.1% of the area in 2017.
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Similarly, waterbodies that occupied nearly 8,023 hectares, which was 6.7% of the area in 1988, has decreased to an extent of 5,569 hectares, 4.6% of the CMA.
While the extent of agricultural lands had reduced from 42.2% in 1988 to 19.2% in 2017, it has been replaced by urban built-up area. The rapid urbanisation, particularly along East Coast Road and Rajiv Gandhi Salai and GST Road, led to increase in built-up area to 48.7% in 2017 from just 17.7% in 1988. The urban settlements has become denser from just 21,122 hectares in 1988 to 57,839 hectares now, according to the study.
Heat island effect
As peri-urban areas are developed denser urban sprawl, particularly in the last three decades, they are experiencing a warmer temperature due to urban heat island effect, said M. Krishnaveni, Professor (Water Resources) and Director of Institute for Ocean Management, who co-authored the study.
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The loss of waterbodies and wetlands and greenery that have been replaced by built-up area has increased the land surface temperature (LST) since 1988. It is the radiative temperature of the earth surface or how hot the earth would be after solar radiation as measured by remote sensor.
For instance, the extent of peri-urban areas that experienced an LST of 31 degree Celsius to 34 degree Celsius has gone from 29.77%in 1988 to nearly 68.18% in 2017. The impact of urban sprawl is felt on the city’s suburbs too with the decreasing tree cover, agricultural lands and shrinking waterbodies. While the space experiencing LST of above 34 degree Celsius was hardly 0.01% in 1988, it has now increased to 3.21%, said M. Mathan of the Centre for Climate Change and Disaster Management and the study’s author.
Remote sensing spectral indices and GIS technologies were used for assessing the land use dynamics over the decades. More areas in the city now experienced warmer temperatures due to urban heat island effect, he said.
Efforts need to be taken to increase green cover and protect wetlands in peri-urban areas to sustain a LST of below 30 degree Celsius. Creating an inventory of waterbodies and ecologically sensitive zones would help curtail construction activities and reduce negative impact of urbanisation, said Ms. Krishnaveni.