Source: https://earthshots.usgs.gov/earthshots/node/112
Timestamp: 2019-04-24 00:17:39+00:00

Document:
In 1978, the Pearl River Delta (PRD) region in southern China had a population of just under 10 million. The population was scattered between several medium-sized cities on an interlacing network of rivers and streams.
Some of those cites have become megacities, and the entire region has merged into one continuous, if scattered, megalopolis with a combined population greater than the six U.S. New England states plus New York, New Jersey, and Pennsylvania.
The PRD now has a population higher than Tokyo, making it the world’s largest urban area.
In 1978, China began new economic policies that loosened regulation for migrating within the country, and people either moved permanently to cities to find work in industry or moved there temporarily for seasonal work. In-migration has been the largest contributor to the rapid population rise. The new model also opened up the country to foreign investment and less government control on private businesses.
Landsat’s long record and spatial resolution make it ideal for mapping change in areas that are urbanizing rapidly, such as the Pearl River Delta. Urban area growth dominates the later images in this Landsat time series.
The PRD was mostly rural before the 1978 reforms began. The major industries were farming and aquaculture. The major crops were rice, sugar cane, peanuts, soybeans, bananas, and oranges. Fish were raised in a huge system of artificial ponds.
The region’s economic growth is now outpacing the rest of the country. The relatively small farming and fishing villages have become large metropolitan areas. The PRD is a major center for telecommunications, biomedicine, robotics, genomics, and manufacturing of electronics, household appliances, computer equipment, toys, garments, footwear, plastic products, and ceramics.
The dark green areas in between cities seen in these images are the region’s dike-pond system. These artificial wetlands have been there since the Tang Dynasty, over a thousand years ago. Grass carp are grown for food in the ponds. Vegetables, sugar cane, and other plants are grown on the dikes. Mulberry trees are also grown on the dikes, and the leaves are used to feed silkworms.
In this area south of Foshan, the dike-ponds are diminishing. The vast increase in manufacturing puts pollutants into the delta, reducing the amount of fish produced. Additionally, infrastructure is built on land that once had fertile soils and fish ponds. However, the dike-ponds are also extending toward the south and east, replacing farmland. Their extent now covers a wider range and is increasingly fragmented.
Cities that are growing this rapidly must also show accompanying infrastructure changes. The additions of bridges, highways, railroads, and airports are intended to merge the cities of the delta into a single megalopolis.
One ambitious project is the Hong Kong-Zhuhai-Macau Bridge, set to open in 2018, to connect the three cities and cut the travel time from Hong Kong to Zhuhai from 3 hours to 30 minutes. Visible as a long, thin line across the bay, the bridge-tunnel system includes 42 kilometers of bridges over water and a 7-kilometer tunnel. The tunnel allows ships to enter and exit the bay and runs between two artificial islands. The entire system is about 15 times longer than the Golden Gate Bridge.
Built to withstand an 8.0 earthquake, the bridge will make it easier for goods from the entire PRD to be transported to Hong Kong’s international airport for export. The total cost of bridge was $7.56 billion.
The natural color image from the European Space Agency (ESA) Sentinel satellite shows the bridge at 15-meter resolution. Ships are visible in the water, including the wakes behind the ships.
The urban growth in the PRD is vast. There are no longer any clear urban centers—a different growth trajectory from other large cities in China such as Beijing and Shanghai that have grown around defined historical urban centers.
Buildings and paved surfaces replaced vegetation at a rapid pace over the past three decades. Housing, factories, and the highway system all increased mostly at the expense of farmland.
In the early images of this series, cities are pink areas, separate from one another, among the green forested and agricultural areas. The largest city in this set of images is Dongguan. A noticeable change by the 1994 image is a road network beginning to connect those cities. The urban area soon begins filling in the former agricultural areas and surrounds the hilly forested areas.
In the southern part of the images, blue rectangles are aquaculture, and they are gradually pushed toward the coast by the expanding urban areas.
As an example of how the cities of the region have merged after the 1978 reforms, these images are a closer look at Guangzhou and Foshan. Even in 2000, they were clearly separate cities. The land between them then fills in with urbanization.
The 2017 image shows the appearance of what appear to be several black dots between the two cities. These are high-rise residential buildings. They look a little similar to the fish ponds, but these are a bit smaller, and they are dark because of the shadows they cast on the ground.
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