Groundwater reserves are declining at an ever-increasing rate around the world. This is confirmed by a study that analyzed the development of water levels in almost 1,700 aquifers in 40 countries. According to this, 71 percent of reservoirs have lost water over the last few decades, with the decline in many cases accelerating over time. However, the team also notes some positive examples in which political measures have led to groundwater reserves recovering.
The largest freshwater reserves in the world are underground. In many regions, groundwater is the most important source from which we obtain water for our daily supplies, agriculture and industry. In Germany, too, around 74 percent of drinking water comes from groundwater. However, climate changes and excessive water use can lead to groundwater reserves decreasing and no longer being usable.
Accelerated decline
“However, the global extent and pace of local groundwater decline are poorly understood because locally recorded groundwater levels have not been summarized on a global scale,” writes a team led by Scott Jasechko from the University of California, Santa Barbara. Although satellite measurements provide information about large-scale trends, they are not fine enough to capture local changes. To close this research gap, Jasechko and his team evaluated data from 170,000 monitoring wells in more than 40 countries. They recorded 1,693 groundwater aquifers, so-called aquifer systems, which account for around 75 percent of the world’s groundwater extraction.
“Our data show that rapid declines in groundwater levels of more than half a meter per year are widespread in the 21st century, particularly in arid regions with extensive agricultural land,” the team reports. “Critically, decline has accelerated over the past four decades in 30 percent of aquifers. This underlines the urgent need for more effective measures to combat groundwater depletion.”
More consumption than regeneration
According to the researchers, the most important cause is that we humans exploit groundwater reserves faster than they can regenerate. The problem is exacerbated by climate change, which not only leads to less rainfall in many regions, but also causes water to evaporate more quickly through higher temperatures before it can reach the groundwater. Heavy rainfall cannot compensate for the losses. The soil cannot absorb too much water at once. The excess then flows off the surface instead of seeping away gradually.
According to Jasechko and his team, dwindling groundwater reserves can cause numerous problems. On the one hand, the decline endangers the drinking water supply for many people and can mean that agricultural areas can no longer be irrigated. If the groundwater level falls below sea level, there is also a risk of seawater penetrating the aquifers in coastal regions, causing the groundwater to become salinized and unusable. Where there is a lack of groundwater in the ground, ground subsidence can also occur. Rivers and lakes can also dry up.
Political measures required
But the study also gives hope. “Our evaluation shows that people can bring about a change with concentrated efforts,” says Jasechko. In 16 percent of the aquifer systems examined, a previously existing negative trend has been reversed – mostly because people have taken targeted measures to reduce groundwater consumption or promote its recharge.
One of the positive examples is the Geneva aquifer in the border region between France and Switzerland. Between 1960 and 1970 its level fell drastically because water was pumped out in an uncoordinated manner in both Switzerland and France. Some wells dried up and had to be closed. In order to preserve the common water supplies, politicians and authorities in both countries agreed on an artificial supply of water from the Arve river. This enabled them to stabilize the level and raise it again. “This aquifer has no longer reached its original level, but this example still shows that it doesn’t have to be the case that groundwater levels just fall,” says co-author Hansjörg Seybold from ETH Zurich.
The researchers also see their results as a call to action for politicians: “Our analysis illustrates the potential for the recovery of exhausted aquifers and at the same time shows how much work still lies ahead of us to protect groundwater resources.”
Source: Scott Jasechko (University of California, Santa Barbara) et al., Nature, doi: 10.1038/s41586-023-06879-8