How full are the reservoirs?

Germany five years ago: withered meadows, parched fields, a plague of bark beetles in the forests. And suddenly wells drying up, a completely new experience. In 2019, the country was in the middle of a multi-year drought. Researchers reported falling groundwater levels, and suddenly there was a discussion about whether water could also become scarce in Central Europe.

Germany 2024: Record rainfall, repeated storms, flooding in Saarland, Baden-Württemberg and Bavaria. Now there are reports of groundwater levels that are too high in some places, of groundwater pressing into basements from below.

So it's time to take a look underground: How have groundwater levels developed, what has been the effect of recent rainfall - and what role does climate change play?

Germany obtains two thirds of its drinking water from groundwater, the importance of this resource can hardly be overestimated. Nevertheless, the data situation is rather modest: Each federal state monitors quantity and quality separately, sometimes using different methods.

In order to obtain a comprehensive picture, Süddeutsche Zeitung has evaluated data from a new nationwide portal of the Federal Institute for Geosciences and Natural Resources (BGR). It is based on more than one hundred measuring points throughout Germany, which have been selected so that their groundwater levels are representative of developments in many places.

“Groundwater levels have recovered noticeably in many places,” says Sabine Attinger, hydrologist at the Helmholtz Center for Environmental Research (UFZ) in Halle. The researcher attributes this to the considerable rainfall in recent months. According to the German Weather Service (DWD), the twelve-month period since last July has been the wettest since measurements began in 1881.

However, the rainfall was distributed quite differently, as a team led by Attinger recently determined.

For example, there was significantly more precipitation in the west of Germany, in the Lüneburg Heath or in the Allgäu region on an annual average. However, the picture changes the further east you look. In the Uckermark, the Ore Mountains and the Swabian-Franconian stepped landscape, only an average amount of water fell. “The precipitation deficit of the past (dry) years has therefore not been reduced uniformly across Germany as a whole and in some regions not at all,” the researchers conclude.

This is also reflected in the groundwater, as can be seen at individual measuring points. In Holthausen in Münsterland, for example, the levels have recovered.

“The groundwater has not yet been completely replenished here,” says Sabine Attinger about the situation in Eastern Germany. Low levels can be observed above all in Saxony and in the south of Brandenburg.

When it comes to groundwater, however, hydrologists think less in terms of national borders and more in terms of “large hydrogeological areas”, each with comparable underground conditions. Germany is divided into nine such large areas:

How quickly groundwater recharges depends not only on precipitation, but also on the composition of the soil and the groundwater-bearing layers. “Sometimes the groundwater can fill up within hours, sometimes it can take a decade,” says Sascha Oswald, Professor of Water and Mass Transport in Landscapes at the University of Potsdam. Some groundwater levels could possibly still rise even in a long-term dry phase “because they benefit from recharge from ten years ago”.

In any case, precipitation is a decisive factor, says Oswald. Patterns similar to those at the Holthausen, Heggelbach and Mülsen measuring points can also be seen in the larger areas.

“In the eastern German states, the recovery is not yet as advanced,” says Andreas Güntner from the German Research Center for Geosciences in Potsdam (GFZ). Overall, however, the hydrologist believes that we can be happy that the groundwater levels have recovered so quickly. So was Germany lucky? Andreas Güntner speaks more of “normal climate variability”: very dry years were followed sooner or later by wet ones.

Nevertheless, the pace of recovery is astonishing. In 2022, US geologist Jay Famiglietti said in an ARD report that Germany had lost 2.4 cubic kilometers of water every year since 2000. The estimate is based on data from the GRACE satellite mission, which records groundwater as well as soil moisture and surface water. It would be as if the entire Lake Constance had dried up in 20 years. 

Last year, a team led by Andreas Güntner already corrected the value significantly downwards, to around a third of Famiglietti's estimate. Güntner also points out that the period since 2000, when the GRACE mission began, is too short to recognize long-term trends.

Normal climate variability or historic lows

A team led by Friedrich Boeing from the UFZ recently shed light on this gap. In the journal Environmental Research Letters, the researchers looked at water deficits in Germany over the past 250 years. According to the study, there have always been periods in which the total amount of stored water has decreased. Sooner or later, the deficit has always tipped back into positive territory. However, the researchers point out that it has never taken as long to compensate for a water deficit as it did after the drought from 2018 to 2021.

The big question now is how climate change influences this rhythm. What is certain is that global warming will cause the air temperature to rise in the long term, and with it the evaporation rate. Water that evaporates from the upper layers of the soil will later be missing in the groundwater-bearing layers. “This means we need more precipitation to compensate for the losses,” says Sascha Oswald.

However, it is unclear whether Germany will get them. “In the Mediterranean region, all climate models agree that it will get drier. And in Northern Europe, everyone agrees again that it will get wetter there,” says Sabine Attinger. In the region in between, the models do not show a uniform trend: some predict more precipitation in Central Europe in the future, some less.

More extremes ahead

However, most experts agree that extremes will also increase in Germany as a result of climate change: More periods of drought with high temperatures, but in between very wet years with heavy rainfall again. “We have to adapt to this high variability and therefore also to more uncertainty,” says Sabine Attinger.

Not an easy task, the country must invest in heavy rainfall prevention, for example in higher dykes and natural water retention basins. But it also needs to arm itself against drought, for example by planting heat-tolerant crops and trees or storing more water, for example in underground cisterns.

Sascha Oswald compares the current high groundwater levels to a stop at the gas station on a long journey. After a long dry spell, the tank has now been filled up. “But we don't know when the next filling station will come.”