Overexploitation of groundwater causes widespread urban subsidence in China

There is a phenomenon that the Chinese government is facing which is now well known to researchers and which has been studied for several decades: the subsidence of their country's urban centers. Indeed, studies elsewhere of the Houston, Jakarta, Mexico City and New Delhi conurbations have already demonstrated that these areas are settling in on themselves, each with its own dynamics depending on urban development and soil type.

In April, a study published in the journal Science summarised research that was carried out on a large scale in China to clarify the  situation and produce scientific data. Between 2015 and 2022, a team of researchers from several Chinese universities examined a total of 82 urban areas with populations of over 2 million. They used a combination of data from the InSAR (Interferometric Syntheric Aperture Radar) system on the European Space Agency's Sentinel-1 satellite, which measures vertical ground movements, and ground level readings from over 1,600 wells in the areas surveyed.

The findings are stark: 44.7% of China's major cities are sinking at a rate of 3 millimetres per year, affecting the lives of 270 million city dwellers, and 15.8% are experiencing an even more rapid rate of subsidence over 10 millimetres per year, affecting 70 million people (a syndrome scientists describe as “rapid descent”). Some areas are reaching a rate of 2.2 centimetres per year, the Tianjin region with its 15 million inhabitants being the worst affected. Although the annual declines are small, over time they accumulate and have significant influence: Shanghai, for example, has fallen by 3 meters since 1920. Five China's regions have been defined as being at risk: the Harbin-Changchun, Beijing-Tianjin, Zhengzhou-Pingdingshan, Wenzhou-Tuzhou and Kunming-Nanning areas.  

This phenomenon is all the more dangerous when combined with other criteria. More than half of the 82 cities surveyed are located on or near the sea. In 2030, 6% of China's territory will be below sea level. At this rate, by 2120, 26% of the country will be below sea level. Researchers have found that the earth is sinking faster than sea level is rising due to global warming; however, taken together, these two phenomena would ultimately put 100 million people at risk of severe flooding.

A number of explanations have been put forward to explain these subsidence events, the main ones being the rapid urbanisation and sprawl, and the construction of monumental buildings, which in a very short space of time – by geological standards - have led to the overexploitation of groundwater by the local population. When water is extracted from aquifers faster than it can be replenished, voids form in the subsoil, causing land subsidence due to the weight of buildings.

Faced with this situation, the government has already taken drastic measures, introducing strict laws to control the pumping of groundwater. The first effects are already being felt. Scientists have observed a slowdown in the rate of subsidence in the Shanghai area. At the same time, the government has set up a system to capture water from the Yangtze River in the south and channel it northwards to Beijing, with the intention to limit the need for local water pumping.