More than half of the world's largest lakes are losing water. This is reported by an international research team after evaluating satellite data in the journal "Science". According to the scientists, the dehydration is largely due to warming of the climate and human consumption. But the analysis does not only contain bad news: The authors also provide hints on how water decline could be reduced.

Natural lakes and reservoirs store about 87 percent of Earth's fresh water, even though they cover only 3 percent of the land area, the group writes. But in many places, these water reservoirs are threatened: For example, northeastern Spain recently reported that the reservoirs in Catalonia are only 26 percent full on average after months of drought – a year ago it was 58 percent. In Italy, an unusually low water level was recently recorded for Lake Garda: Compared to the same period last year, it was said to have halved.

In recent years, studies have shown that the volume of lakes worldwide is shrinking, with climate change repeatedly cited as a factor in development. However, it is difficult to determine exactly what influence short- and long-term climate fluctuations globally have on the water stored in lakes, as human activities such as the management of reservoirs, water abstraction and land use change also play a role.

The study by the team led by hydrologist Fangfang Yao from the University of Colorado at Boulder now paints a more accurate picture. The researchers developed a technique for measuring changes in water levels in nearly 2000,90 of the world's largest lakes and reservoirs, which together contain roughly <> percent of the freshwater stored in lakes.

To record changes in water levels, the team used 250,000 satellite images from 1992 to 2020. The result: 53 percent of the world's lakes recorded water losses, some of them considerable. On average, this amounted to a total of about 22 gigatons per year. This corresponds to almost half of the water volume of Lake Constance, which itself is listed as a shrinking body of water on an interactive map published for the study.

To explain this development, the scientists used climate and hydrology models. According to the study, climate change and human consumption are primarily responsible for the decline in the volume of natural lakes. Contrary to earlier studies, water loss was detectable not only in dry, but also in humid regions of the world such as the tropics.

With regard to reservoirs, the research team found significant water losses for two-thirds of these waters. This was mainly due to deposits. This is because dams block the natural removal of sediments in rivers such as sand, gravel or boulders. Over time, these deposits accumulate in reservoirs, thus reducing their volume.

Only recently, a UN study in the journal "Sustainability" warned that the world's reservoirs are in danger of losing around a quarter of their original storage capacity by 2050 due to the input of sediments. For Germany, a volume loss of 35 percent was predicted.

While the majority of lakes worldwide are shrinking, according to the current work, there has been a significant increase in water volume at 24 percent. These include bodies of water in sparsely populated areas of the inner Tibetan Plateau, in the Great Plains of the United States, as well as regions with new reservoirs such as the Yangtze, Mekong and Nile river basins. The Müritz in Mecklenburg-Vorpommern is also listed on the interactive map belonging to the study as a lake with growing volume.

The authors stress that their analysis is not only an inventory, but also contains indications of possible solutions. "If human consumption is an important factor in the decline of water reservoirs in the lakes, we can adapt and explore new strategies to reduce the decline on a large scale," says co-author Ben Livneh. As an example, he cites Lake Sevan in Armenia, where a regulation of water withdrawal has ensured that the volume has increased.

In a commentary on the study, geophysicist Sarah Cooley of the University of Oregon emphasizes how important such laws would be worldwide. She points to the finding that it is estimated that almost a quarter of the world's population lives in a catchment area with a large, drying lake: "Given the importance of these lakes for ecosystems, water supply, irrigation and/or hydropower, the potential consequences of lake drying up are important both locally and globally."