A couple of years ago, a friend of ours published an outstanding book on the escalating water crisis in India. In “Watershed: How we destroyed India’s water….” , Mridula Ramesh describes amongst other things how cities like Delhi, Bangalore and several other high density urban centres in northern India, groundwater is being used both for washing, cleaning and for industrial use with the result that India’s underground aquifers are being rapidly depleted. Now, news coming in from China suggests that the depletion of underground water has another unexpected impact: subsidence i.e. the ground beneath the Chinese cities is sinking. As this BBC article explains:
“Nearly half of China’s major cities are sinking because of water extraction and the increasing weight of their rapid expansion, researchers say.
Some cities are subsiding rapidly, with one in six exceeding 10mm per year.
China’s rapid urbanisation in recent decades means far more water is now being drawn up to meet people’s needs, scientists say.
In coastal cities, this subsidence threatens millions of people with flooding as sea levels rise.
China has a long history of dealing with subsiding land, with both Shanghai and Tianjin showing evidence of sinking back in the 1920s. Shanghai has sunk more than 3m over the past century.
In more modern times, the country is seeing widespread evidence of subsidence in many of the cities that have expanded rapidly in recent decades.
To understand the scale of the problem, a team of researchers from several Chinese universities have examined 82 cities, including all with a population over 2 million.
They’ve used data from the Sentinel-1 satellites to measure vertical land motions across the country.
Looking across the period from 2015 to 2022, the team was able to work out that 45% of urban areas are subsiding by more than 3mm per year.
Around 16% of urban land is going down faster than 10mm a year, which the scientists describe as a rapid descent.
Put another way, this means 67 million people are living in rapidly sinking areas.
The researchers say that the cities facing the worst problems are concentrated in the five regions highlighted on the map shown.
The scale of decline is influenced by a number of factors, including geology and the weight of buildings. But a major element, according to the authors, is groundwater loss.
This essentially means the extraction of water underneath or near cities for use by the local population…
In China, the research team were able to associate the extraction of water from over 1,600 monitoring wells with increasing levels of subsidence.
“I think the water extraction is, to my mind, probably the dominant reason,” said Prof Robert Nicholls, from the University of East Anglia, who was not involved in the research.
“In China there are lots of people living in areas that have been fairly recently sedimented, geologically speaking. So when you take out groundwater or you drain the soils, they tend to subside.””
Coming soon to on an OTT platform near you, that new blockbuster “Subsidence: That Sinking Feeling” starring actors from all parts of India’s vibrant film industry.
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