A group of scientists, led by Spanish researchers, has created the first world map of subsidence of the land caused by the extraction of underground water. According to the study, 10% of the earth’s surface is gradually sinking and in the areas of the Earth most likely to suffer subsidence 1.2 billion people live and 21% of the world’s most important cities are located. In Asia, the most affected continent, 86% of the population is exposed. In terms of economic impact, up to 12% of world GDP is in danger.
The determining factors that increase the probability of subsidence are several: lithology (type of soil), topography, land use or climate. “The greatest probability occurs in arid or temperate zones with periods of drought”, comments the author Herrera-García. He Trigger for subsidence is excessive water withdrawal, either for agricultural, industrial or urban use. They are gradual processes that affect large areas, over long periods of time, and tend to end fissures in the earth, damage buildings and civil infrastructure, and increase the susceptibility and risk of flooding.
“In overexploited aquifers, the natural recharge is less than the volume that is extracted,” explains Pablo Ezquerro, also from IGME and co-author of the study. The pores of these soils are left empty, compacting themselves due to the lack of water that contributed to sustaining the infrastructure.
During the next decades, world population and economic growth will continue to increase demand for water and the depletion of groundwater. In addition to the acceleration of the processes of urbanization and agrarian intensification in some of the most exposed regions, such as the Ganges and Brahmaputra basins in India or the plains of northeast China, climate change also has a role. Increased temperatures and recurrence of droughts further weaken the natural recharge of aquifers, increasing the occurrence of land subsidence and related damage or impacts.
The study also shows that by the year 2040 some 635 million people could suffer the consequences of this process silent. The results of the study were presented in an article titled “Global threats of land subsidence due to groundwater depletion” in Science magazine: “To raise awareness and inform decision making, we assess potential global subsidence due to groundwater depletion, a key first step toward formulating effective land subsidence policies that are lacking in most countries of the world ”, explain the authors.
Centimeter by centimeter
To carry out the study, the team led by Herrera García reviewed a large amount of scientific literature and found that, during the last century, there were up to 200 sinks by subsidence in 34 different countries motivated by the depletion of groundwater.
Several well-known examples of subsidence are represented in the Italian city of Venice, whose subsidence due to the extraction of underground water has been accentuated since the 40s of the last century. Another example is Mexico City, which due to being planned on clay soils some areas sank up to 8 meters in the last 250 years. Another example, perhaps less known, can be found in the city of Berenizkí, in Russia, a city whose inhabitants have been forced to leave the area.
But not all is lost. The scientific and technical knowledge available from this type of study allows us to understand the processes and take steps to slow down and even reverse subsidence, as is the case in Tokyo. The Japanese capital had been sinking since the end of the 19th century, lowering its ground to four meters. In the decade of 1969, the management of the aquifers and the sustainable policies for the exploitation of groundwater managed to stop it.