A fast-growing city in India is a study area for urban impervious surface and the urban heat island effect. Located in western India about 120 kilometers southeast of the coastal city Mumbai, Pune is known as an educational and industrial center. Its population has grown from 450,000 in 1950 to nearly 6 million in 2017. People from rural areas are migrating to the city for better employment opportunities. The education and industrial sectors are also attracting people to Pune from other countries.
False-color Landsat images show the steady expansion of the Pune metropolitan area. Forests cover higher elevation land, shown in bright green mostly to the west. Dull colors are shrubland and grassland, which occupy the lower elevations. However, urban land—the lavender and purple hues—has been expanding over grasslands, barren, and agricultural land.
Pune’s proximity to Mumbai is also leading to its growth. The Mumbai–Pune expressway has reduced travel time between the two cities, so Pune has become a destination for those looking for housing away from Mumbai.
West of Pune is the Western Ghats mountain range, a relatively low range of forested mountains. One of the peaks west of Pune reaches just over 1,220 meters (4,000 feet) of elevation. Rivers flow out of the mountain range toward the east—reservoirs along the rivers store water for the populations downstream. The Mula River and the Mutha River meet in Pune to form the Mula-Mutha River as it flows toward the east.
In such a large city as Pune where the population is growing and those built-up surfaces are expanding so rapidly, the associated effects of streamflow changes and the urban heat island effect need to be measured and monitored.
Scientists studying the effects of urban growth are concerned with surface imperviousness—the amount of land covered by human-made features such as asphalt, concrete, and rooftops. These surfaces are impervious to rainwater. On these surfaces, rainfall does not soak into the land; instead, it runs down the streets or across parking lots, often pooling up in low-lying areas, increasing the risk of local flooding.
During heavy rainfall, areas of increased impervious surface can be more vulnerable to flash floods. The excess water flows directly to streams, often by way of storm water drains. This directly increases streamflow and reduces the amount of water that infiltrates into the ground.
In these images, Pune itself is at the bottom right at the sharp bend of the Mula-Mutha River. Toward the top center of these images is Pimpri-Chinchwad. The pink, maroon, and lavender tones are the colors associated with impervious surfaces. In many areas, urban areas are expanding over what was once either agriculture or bare ground, visibly increasing impervious surface.
Even at Landsat’s 30-m resolution, roads and structures can be seen to increase across the landscape. Some individual buildings can be seen, too—the bright rectangles to the north. Green rectangles are farm fields in the center of the images. Over time, many of them convert to built-up impervious surface.
Scientists use hydrological models to quantify how streamflow changes in areas of increased urban growth. They can better understand how the entire watershed is changing due to urbanization. In the Mula-Mutha subwatershed where Pune is, average streamflow increased from 179.14 m3/s to 185.23 m3/s between 1980 and 2009.
Landsat data can be used to map impervious surfaces and quantify the effect of the increased runoff it causes. Landsat can also be used with the European Space Agency’s Sentinel-2 satellites to get more frequent coverage of study areas.
Those constructed surfaces have another measurable effect. Construction materials like concrete and brick absorb heat. They then release the heat at night, increasing temperatures and creating an urban heat island.
One study found that over 2001–2016, the land surface temperature rose in Pune in areas where there is reduced green cover. The outskirts of Pune especially experienced this increase in temperatures. An increase of 4–5 degrees Celsius is possible between the city and rural areas. The urban heat island effect can also mean warmer than normal nighttime temperatures.
Besides the urban heat island effect, urbanization can also affect regional climate. Studies have observed that the change in temperatures can cause a change in local wind and precipitation patterns.
The urbanization appears in the images as pink and lavender hues. Pink lines are roads that spider-web across the landscape.
With the world population becoming increasingly urban, the need for assessment and monitoring of urban areas becomes more important. Decades of consistent data from the Landsat satellites make them effective for this kind of environmental monitoring.
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