2023-05-18 20:15:13
MMore than half of the world’s largest lakes are losing water. This is what an international research team reports after evaluating satellite data in the journal Science. According to the scientists, the dehydration is largely due to global warming and human consumption. However, the analysis does not only contain bad news: the authors also provide information on how the decline in water could be reduced.
Natural lakes and reservoirs store about 87 percent of the world’s fresh water, despite covering just 3 percent of the land area, the group writes. But in many places these water reservoirs are under threat: the north-east of Spain recently reported that the reservoirs in Catalonia are only 26 percent full after months of drought – a year ago it was 58 percent. In Italy, an unusually low water level was recently recorded for Lake Garda: it was said to have halved compared to the same period last year.
Studies in recent years have shown that the volume of lakes is shrinking worldwide, with climate change repeatedly cited as a factor in this development. However, it is difficult to determine the exact global impact of short- and long-term climate fluctuations on the water stored in lakes, since human activities such as the management of reservoirs, water abstraction and land use changes also play a role.
View of the Llosa del Cavall reservoir in Lleida in Catalonia. The amount of water that had accumulated in the reservoirs of the Catalan inland basins at the time did not even reach 30 percent of their actual capacity.
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Image: dpa
The study by the team led by hydrologist Fangfang Yao from the University of Colorado in Boulder now paints a more accurate picture. Researchers developed a technique to measure changes in water levels in nearly 2,000 of the world’s largest lakes and reservoirs, which together contain roughly 90 percent of all 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 which were considerable. On average, this totaled around 22 gigatonnes per year. This corresponds to almost half the water volume of Lake Constance, which is itself 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. Accordingly, climate change and human consumption are primarily responsible for the decline in volume of natural lakes. Contrary to earlier studies, a loss of water was detectable not only in dry but also in humid regions of the world such as the tropics.
A pier in the Sau reservoir has run dry after the water level in the lake, some 100 km north of Barcelona, fell.
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Image: dpa
Looking at reservoirs, the research team found significant water losses for two-thirds of these bodies of water. This was mainly due to deposits. This happens because dam walls block the natural transport of sediments in rivers such as sand, gravel or debris. Over time, these deposits accumulate in reservoirs, 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 ingress of sediment. A volume loss of 35 percent was predicted for Germany.
Lake Garda threatens to sink this low more often in the future.
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Image: dpa
While the majority of the world’s lakes are shrinking, according to the current work, 24 percent have seen a significant increase in water volume. These include waters in sparsely populated areas of the inner Tibetan Plateau, the Great Plains of the United States, and regions with new reservoirs such as the Yangtze, Mekong, and Nile river basins. The Müritz in Mecklenburg-Western Pomerania is also listed as a lake with increasing volume on the interactive map belonging to the study.
The authors emphasize that their analysis is not only an inventory, but also contains indications of possible solutions. “If human consumption is an important factor behind the decline in lake water storage, we can adapt and explore new strategies to mitigate the decline on a large scale,” says co-author Ben Livneh. As an example, he cites Lake Sevan in Armenia, where the regulation of water withdrawal ensured that the volume increased.
Geophysicist Sarah Cooley from the University of Oregon emphasizes how important such laws would be worldwide in a comment on the study. 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 up lake: “Considering the importance of these lakes for ecosystems, water supply, irrigation and/or hydropower, the potential consequences of drying out of lakes are both relevant locally as well as globally.”
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