Germany dries out - but not as much as thought

In Germany, 760 million tons of water are lost every year. Individual years with high precipitation do not compensate for the loss.

© picture alliance / Zoonar | Alexander Ludwig

In recent drought summers, the water level in rivers and lakes dropped alarmingly in some places - as here in the Rhine near Cologne.

Germany has lost an average of 760 million tons of water each year over the past 20 years. This is the conclusion reached by a team from the German Research Centre for Geosciences (GFZ) together with researchers from the University of Bonn and Forschungszentrum Jülich. According to the study, the reasons for the water loss are decreasing soil moisture as a result of massive summer droughts, dwindling groundwater, melting glaciers and the lowered water level in rivers and lakes. However, it is still unclear whether a trend for the future can be derived from this. The results have been published in the journal "Hydrology & Water Management".

Since 2002, a total of 15.2 billion tons of water have been lost, the analysis says. By way of comparison, all sectors in Germany, from industry to agriculture to private households, consume around 20 billion tons of water a year.

The research team relies on data from the GRACE (2002 to mission end 2017) and GRACE-Follow On (active since 2018) satellite missions. The dual-satellite systems measure the Earth's gravitational pull, known as the gravity field, and its changes globally on a monthly basis. From these data, mass shifts can be identified, which in turn allow conclusions to be drawn about changes in the water cycle. In 2022, it had been said that Germany's total water reservoir was losing almost 2.5 billion tons of water per year, with the southwest particularly affected. That would be roughly equivalent to the volume of Lake Constance. To verify these values, the scientists compared four different evaluation methods. They arrived at a significantly lower water loss.

A disadvantage of the measurement method, as the researchers explain, is that the spatial resolution of the gravity field data is comparatively coarse and is only about 300 times 300 kilometers. Reliable statements could therefore only be made for areas of about 100 000 square kilometers in size, which roughly corresponds to the area of the eastern German states. In addition, some other disturbing effects would have to be factored out. For example, the earth's gravitational field changes even without water masses fluctuating acutely, for example because in some regions the earth's crust is still rising today after the disappearance of the ice-age glaciers.

Over large parts of the observation period, especially in the years between 2004 and 2015, the results of all four evaluation methods agree well. There were differences mainly at the beginning and end of the time series. Slightly different analysis periods therefore result in significantly different trend values. Hydrologist at GFZ and lead author of the study, Andreas Güntner, therefore cautions, "The observations from all data sets show that a year with higher precipitation, such as 2021, is not sufficient to make up for the water storage deficits that have accumulated over the longer period." The observation series urgently needs to be continued to capture and forecast the long-term behavior of storage dynamics and water supply in Germany, he said.

spektrum.de published at 04.04.2023