Around 87 percent of the liquid fresh water on earth is stored in lakes. But climate change and human use have ensured that more than half of the great lakes have shrunk in the last 20 years, sometimes to the point of drying up. This is shown by a study based on satellite data and level measurements. The data can help to develop measures for a more careful use of the resource water and thus protect the lakes as ecosystems and drinking water reservoirs.
Lakes cover around three percent of the Earth's land area, but store 87 percent of our planet's fresh liquid water. They play a crucial role in human drinking water supply and also represent important ecosystems for a large number of species. However, local observations have shown that the water level in numerous lakes in different parts of the world has been falling for years. For example, the Aral Sea between Uzbekistan and Kazakhstan, which was once one of the largest inland bodies of water in the world, has largely dried up. However, global data on the water level of the lakes has not been available so far.
Serious water losses
A team led by Fangfang Yao from the University of Colorado in Boulder has now recorded the development of the lakes on a global scale for the first time. "This is the first comprehensive assessment of the trends and causes of variability in global lake water storage based on a range of satellites and models," says Yao. For 1972 large lakes worldwide, which together comprise around 90 percent of the water stored in lakes, the researchers combined satellite data and level measurements to determine the development of the water level between 1992 and 2020. "With this novel method, we are able to provide insights into global changes in lake levels from a broader perspective," says Yao's colleague Balaji Rajagopalan.
The result: 53 percent of the lakes have lost water in the last 20 years. Losses average around 22 gigatonnes of water per year. This corresponds to almost half the volume of Lake Constance. This affects not only lakes in dry regions, but also those in humid tropical and arctic regions. The researchers found the greatest water losses in the Aral Sea and the Caspian Sea, but numerous lakes in Europe are also affected by the drying up, including Lake Garda, Lake Maggiore and Lake Constance.
Sustainable management has an effect
With the help of modeling of climate change and human water use, Yao and his team have also got to the bottom of the causes of water loss. Both factors – climate change and water use – therefore have a decisive influence on the decline in water reserves in lakes. Another factor comes into play with reservoirs: since dams and dam walls not only hold back water, but also sediment particles such as sand and small stones, these are deposited and gradually ensure that the volume of the reservoirs decreases. "Sedimentation dominated global storage decline in existing reservoirs," explains Yao's colleague Ben Livneh. Almost two-thirds of the world's reservoirs have suffered significant water losses since 1992.
In contrast, the researchers found an increase in the amount of water in around 24 percent of the lakes. These primarily include lakes in sparsely populated regions on the Tibetan plateau and in the northern Great Plains in North America, but also, for example, the Müritz in Mecklenburg-Western Pomerania. In some of these regions, Yao and his team found evidence that measures to protect the lakes are having an effect: they were able to link the increase in water in Lake Sevan in Armenia to nature conservation laws that have been regulating water withdrawal from the lake since the early 2000s.
"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," Livneh says. From the point of view of the researchers, such measures are urgently needed. "Around a quarter of the world's population lives in areas with drying lakes," they write. Sustainable management and the modernization of dams could help to secure the water supply in the long term.
Source: Fangfang Yao (University of Colorado Boulder) et al., Science, doi: 10.1126/science.abo2812