National Park: The war for Doñana's resources has no truce
Report: A quarter of the Earth will be very dry if the temperature rises two degrees
With the increase in the world population and the decrease in rainfall in some regions due to changes in the climate, humanity needs more and more water to drink and irrigate crops.
In addition to desalination, another way to obtain it is to remove it from underground aquifers.
But like other interventions by man, this technique breaks the balance of nature.
One of the consequences of extracting water from aquifers is land subsidence,
a phenomenon that occurs slowly and gradually and is called subsidence in geology
.
It has been known and studied for more than a century, but normally both research and actions to mitigate it have been carried out locally.
Now, an international team led by Spanish scientists publishes in the journal
Science
the first world map that identifies the areas most at risk of land subsidence due to the extraction of water from aquifers.
More than 1,200 million people live in the identified risk areas
, the majority in Asian countries, especially in China and India.
Among the cities most prone to suffering are Jakarta, Venice, Tokyo or Mexico City.
In Spain, the results of this work indicate
high probabilities in areas of Andalusia (Almería, Granada, Seville and Doñana), in Lorca (Murcia), the Community of Madrid, Zaragoza or Barcelona
.
Geologists believe that population growth and economic activity, added to the more frequent periods of drought, will probably increase land subsidence and with it, the damage generated.
The models foresee that in 2040, 19% of the world population will suffer subsidence, which is why they consider it important to identify the most vulnerable areas in order to take measures to help mitigate this phenomenon.
Material damage
"It is a silent danger, it occurs slowly and its impact is seen slowly. It damages houses and infrastructures, opening cracks in the ground. The ground is gradually sinking and flooding increases," Gerardo explains in a telephone conversation Herrera-García, researcher at the Geological and Mining Institute of Spain (IGME) and leader of this work.
"
We see subsidences ranging from a few millimeters to tens of centimeters throughout a year
. In Mexico or Iran, they have subsidence of 30 centimeters per year. However, an earthquake in a large area can instantly cause deformations of 10 to 30 cm a year. That
is
why it
is a more difficult problem to perceive from a social point of view
.
"
As it is a slow process, it does not usually cause victims but does cause significant economic damage, he adds.
As Herrera-García explains, subsidence, that is, the deformation, displacement or sinking of the surface,
can be produced both by natural causes and by the action of man
.
The main natural causes are tectonic plate movement and volcanism.
"In Spain, it is typical in areas where water runs underground, especially in the Ebro Basin, in Zaragoza. Karst or carbonate rocks dissolve when they come into contact with water, and subsidence occurs due to collapse, which it can cause a lot of damage. Sometimes the process is very fast and holes appear in the ground, or it can be gradual, "he says.
Among the anthropogenic causes, the extraction of minerals, the drilling of tunnels and the exploitation of underground aquifers stand out in which this work is focused: "When you extract water, you remove volume, empty the pores of the soil, soft soils are compressed and collapsed" explains this geologist
A risk map
Herrera-García, who is the Spanish representative in the UNESCO International Subsidence Initiative, proposed to this group of experts to make a global map of the subsidence of the land that occurs due to the extraction of water from underground aquifers, for which which
analyzed data collected in 200 regions of 34 countries in which this phenomenon has been described.
Not all groundwater withdrawals have the same risk of subsidence.
It occurs especially in soils with a high clay content.
"It is not the same to extract water from an aquifer with soft materials than in an area of limestone rock
,
" he
says.
"The most susceptible places are those that are located in the river basins because they drag material -clays, silts-, which accumulated on a geological scale causes that in an aquifer such as that of Lorca (Murcia) there is 150 meters thick of soft soil When the water is removed, it compresses and deforms ".
On the other hand, areas with an arid or temperate climate but with long dry periods are more vulnerable "because there is scarce water and if there is a high population or crop density, it is necessary to pump water from the subsoil", he adds.
The risk map has been prepared combining all these factors and shows the probability of subsidence due to the extraction of water from aquifers: in the areas marked in green the probability is very low;
it is rising in the regions marked in yellow and orange and is very high in those marked in red.
In Spain, "the entire area of Doñana is made of soft sediments and due to climatic conditions and land use there is a lot of exploitation of underground water," explains the geologist.
Calculation of deformations
The map indicates the areas in which the phenomenon is likely to occur, but not necessarily that it is occurring or will occur, or its magnitude.
Therefore, another complementary study they are carrying out is to use radar satellite data to measure the deformations of the ground and determine how much the ground is sinking.
In Spain, the Lorca basin (Murcia) stands out, with the highest rate of subsidence in Europe, 15 centimeters a year
, in a very large and agricultural area.
"In Murcia, the maximum we have measured was two or four centimeters a year during the periods of maximum drought, from 1992 to 1995, and from 2004 to 2008. They are rates much lower than those of Lorca but comparatively they produced much more damage because they occurred in more populated areas. The same happens in Mexico or Iran, where there are rates of deformation in cities whose buildings are suffering very significant damage. "
In Madrid, subsidence occurs in the detrital tertiary aquifer: "It ranges from Colmenar Viejo to the Pozuelo, Aravaca and El Pardo area, it is a sparsely populated area and does not affect the capital. The peculiarity is that the subsidence measured with radar satellites is similar to that of Murcia, four centimeters a year, but the deformation occurs only when the wells of the Canal de Isabel II are used for periods of drought. aquifer recovered. It has a more elastic behavior because its materials are less plastic, "he explains.
In the Region of Murcia, the IGME has been advising governments for decades and working to measure subsidence and to be able to make responsible exploitation of aquifers that causes minimal impact.
Internationally,
Japan and the Netherlands are the countries that have managed to better manage this problem
: "They have adapted and are resilient. The Dutch have seven million people living below sea level due to subsidence but they have a good management system, dams, and have adapted. In Tokyo they had a serious problem in the 50s and 60s because due to the great urbanization and industrialization of the country they extracted a lot of water. They began to rationally exploit the aquifers, they stopped extracting it from the city's aquifer and they bring it from outside, "he says.
At the other extreme is Indonesia: "They draw a lot of water from the subsoil due to the great demographic expansion and urbanization.
Jakarta is increasingly sinking
, about 25 centimeters a year, and
suffers more floods
. It is an extreme case because the Government is considering move the capital to the island of Borneo for this reason, although other measures could be adopted. With the right policies this problem could be alleviated, "he says.
China and Mexico are taking measures to alleviate the serious problem they have with land subsidence.
On the other hand, its risk map identifies areas in Latin America and Africa in which this phenomenon is not occurring but are at risk of suffering it if they urbanize: "If cities grow, they will need more water and they could exploit underground aquifers."
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