Water Shortage
German oak trees in drought stress

The drier it is, the more acorns there are. Whereas the trees used to undergo mast seeding only every few years, this is now the case almost every year.
The drier it is, the more acorns there are. Whereas the trees used to undergo mast seeding only every few years, this is now the case almost every year. | Photo (detail): ©picture alliance/Zoonar/Marlene Cleven

There are areas in Germany that up to now have had little cause for complaint when it came to water shortages, but now they are also increasingly having to deal with droughts, while others suffer from the consequences of floods. Innovative ideas are needed to preserve Germany's water supply.
 

Wherever you go for a walk on the island of Usedom in the early autumn of 2021, you will be pelted by acorns falling from the trees – either you have to duck your head or put up your hood. They are all over the place, covering sidewalks, streets, parks. One might think that it was a particularly fruitful year. It is, however, not quite that simple, as was explained by Chief Forester, Felix Adolphi, who is responsible for the 12,000 hectares of forest on the sunny island: “The groundwater level has dropped considerably because of the drought. This, of course, causes problems for the trees. A clear sign that the trees are weakened is that the oaks here bore a lot of acorns last year.” Adolphi speaks of mast seeding when, like this year, most of the trees are full of fruit, which is extremely exhausting for the trees. “Our oaks usually only reproduce every seven to ten years with a so-called mast seeding, but because the climatic conditions were so bad, the trees are stressed and are now trying to produce offspring as quickly as possible.” In other words, the persistent drought is giving the oaks a hard time.

The rainmaker

Germany is actually a country rich in water resources. According to the Umweltbundesamt (Federal Environment Agency) the population uses only 12.8 percent of the annual water supply of 188 billion cubic metres. However, evaluations by the German Weather Service in 2021 show that the three spring months of March, April and May did not reach the typical average rainfall level eight times in a row. According to the Helmholtz Centre for Environmental Research, almost every region in Germany was affected by an “unusual drought” in the summers of 2018, 2019 and 2020, most of all the regions with light, sandy soil – such as the holiday paradise of Usedom.

Dry soils and falling groundwater levels also endanger the drinking water supply in the long term, which is why scientists and researchers nationwide are working on Germany's water reserves. One of them is Clemens Jauch. The professor for wind energy technology at Flensburg University of Applied Sciences wants to produce precipitation. He has developed a system with which water is transported through the atmosphere to where it is needed and then falls as precipitation: on dried up meadows, on withered fields or in dry forests. Jauch uses wind turbines and wind for this purpose. “Atmospheric irrigation with wind turbines” is what he calls his idea of transporting water into the atmosphere via the rotor blades of wind turbines, which is then distributed by the wind in the form of water droplets or water vapour. “We are able to use a technical component that we already have, the rotor of wind turbines, and the wind that is also already there,” says Jauch. This could be useful for agriculture and forestry or against the threat of forest fires.

A blue-green urban jungle

It is not only drought, however, that is taking its toll on Germany, but also heavy rainstorms are hitting the country more and more frequently. This is a risk, especially for cities, whose water drainage systems are quickly overwhelmed. Resilience researchers at the Fraunhofer-Institut für System- und Innovationsforschung ISI (Fraunhofer Institute for Systems and Innovation Research ISI) in Karlsruhe are investigating how cities can prepare for more extreme weather events. “If we focus on drought and heavy rain phenomena, the fundamental question is – how do we shape the urban water cycle?”, Says Susanne Bieker, Head of the Transformation and Innovation Systems in Urban Areas project. Urban areas are sealed in such a way that rainwater cannot be absorbed by roofs or seep through streets and squares. It flows into the sewerage system, which is mainly designed for slow sewage flow and is quickly overwhelmed by heavy rain. The result – flooded streets and flooded basements. However, if the rainwater is kept where it falls – for example over green spaces, green roofs or facades – it can evaporate and produce a cooling effect. According to studies, a green roof covered with moss and grass can retain 30 to 70 percent of the annual precipitation. A small roof forest with trees, bushes and perennials almost 100 percent. Ponds, lakes, canals and large meadows, where water can collect after a downpour, are also helpful. Vegetation cover on roofs like here in the Essen district of Kettwig can be an effective protection against too much precipitation. Vegetation cover on roofs like here in the Essen district of Kettwig can be an effective protection against too much precipitation. | Photo (detail): © picture alliance/ZB/euroluftbild.de/Hans Blossey There are practical examples of the implementation of these blue-green concepts, such as the “Leipziger BlauGrün” project, which is funded by the Bundesministerium für Bildung und Forschung (Federal Ministry of Education and Research). At the former open-air freight station in Eutritzsch in the centre of Leipzig, over two thousand new apartments, a school campus and commercial buildings are to be built by 2022 – with roof gardens, green areas, cisterns. “Our intelligent control system combines data such as the water levels of cisterns, the water quality or data from soil moisture sensors with externally available data such as the weather forecast,” explains Marius Mohr, innovation field manager for water technologies and resource recovery at the Fraunhofer-Institut für Grenzflächen- und Bioverfahrenstechnik IGB (Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB). In other words – if the weather forecast announces heavy rain for the region, cisterns are automatically emptied in order to be able to absorb the new rain.

“Hose The Hood!”

It is not only science and research contributing something to climate resilience, all individuals can do their bit, too. Take as an example the trees in our cities and on our steets. They regulate the microclimate, provide shade, filter emissions from the air and soil and are the habitat of typical city bird and insect species. With the “Mein Baum – Meine Stadt” (My Tree – My City) campaign, the street tree registry office in Hamburg enables residents to donate a street tree. In addition, however, such a tree needs at least ten litres of water per day, preferably in one or two larger amounts of water per week. City dwellers are therefore called upon in a similar campaign in Berlin called “Gieß den Kiez!” (Hose The Hood!) – to help with the watering of street trees. Perhaps a solution for the Usedom oaks, too. If it is hot and dry, only watering will help. If there is currently no gardener available, as is the case here in Mainz, any help is welcome – this is why in Berlin residents have been asked to "Hose the Hood!" If it is hot and dry, only watering will help. If there is currently no gardener available, as is the case here in Mainz, any help is welcome – this is why in Berlin residents have been asked to "Hose the Hood!" | Photo (detail): © picture alliance/dpa/Sebastian Gollnow