More than 50% of the largest lakes in the world are losing water and, with this, environmental problems such as the supply of drinking water to nearby populations are increasing. This is the main conclusion of a data review study in thousands of freshwater bodies around the planet whose results are published this week in the journal Science. The main causes of the problem are known: climate change and the unsustainable use of water by humans.
On the positive side of this study, its lead author, Professor Fangfang Yao, visiting CIRES member and cla expert from the University of Virginia (United States), points out that the new analytical method used to track storage trends of Water and the Reasons for the Problem now enables scientists to provide water managers and communities with information on how to best protect critical water sources and important regional ecosystems.
"This is the first comprehensive assessment of the trends and drivers of global variability of lake water storage based on a series of satellites and models," Yao said.
He was motivated to do research by environmental crises in some of the largest bodies of water on Earth, such as the drying up of the Aral Sea between Kazakhstan and Uzbekistan.
So he and his colleagues at the University of Colorado Boulder, Kansas State University, France, and Saudi Arabia created a technique to measure changes in water levels in nearly 2,000 of the world's largest lakes and reservoirs, which they represent 95% of the total water storage of the lake. on earth.
The team combined three decades of observations from a series of satellites with modeling to quantify and attribute trends in lake water storage globally.
Globally, freshwater lakes and reservoirs store 87% of the planet's water, making them a valuable resource for human and terrestrial ecosystems. Unlike rivers, lakes are not well monitored, but they provide water for a large part of humanity, even more than rivers.
But despite its value, long-term trends and changes in water levels have been largely unknown, until now.
"We have pretty good information on iconic lakes like the Caspian Sea, the Aral Sea and the Salton Sea, but if you want to say anything on a global scale, you need reliable estimates of lake levels and volume," said Balaji Rajagopalan, member de CIRES, professor of engineering at the University of California at Boulder and co-author. "With this novel method we can provide information on global lake level changes with a broader perspective."
For the new paper, the team used 250,000 snapshots of the lake area captured by satellites between 1992 and 2020 to study the 1972 area of the largest lakes on Earth. They collected water levels from nine satellite altimeters and used long-term water levels to reduce any uncertainty. For lakes without a long-term level record, they used recent water measurements made by newer instruments on satellites. Combining recent level measurements with longer-term area measurements allowed scientists to reconstruct the volume of the lakes going back decades.
The results were staggering: 53% of lakes globally experienced a decline in water storage. The authors compare this loss to the magnitude of Lake Meads, the largest reservoir in the United States.
To explain trends in natural lakes, the team took advantage of recent advances in water use and climate modelling. Climate change and human consumption of water dominated the global net decline in the volume of natural lakes and water losses in some 100 large lakes, Yao said. "And many of the human and climate change footprints in lake water losses were previously unknown, such as the drying up of Lake Good-e-Zareh in Afghanistan and Lake Mar Chiquita in Argentina."
Yao and his colleagues also evaluated storage trends in reservoirs. They found that almost two-thirds of the large reservoirs on Earth experienced significant water losses.
"Sedimentation dominated the decline in global storage in existing reservoirs," said Ben Livneh, also a co-author, CIRES member and associate professor of engineering at CU Boulder. In long-established reservoirs, those that were filled before 1992, sedimentation was more important than droughts and years of heavy rains.
While most of the world's lakes are shrinking, 24% experienced significant increases in water storage. The growing lakes tend to be in unpopulated areas in the interior of the Tibetan Plateau and the great plains of northern North America and in areas with new reservoirs, such as the Yangtze, Mekong, and Nile river basins.
The authors estimate that about a quarter of the world's population, 2 billion people, reside in a drying lake basin, indicating an urgent need to incorporate human consumption, climate change, and the impacts of sedimentation in the sustainable management of water resources.
And their research offers insight into possible solutions, Livneh said. "If human consumption is a major factor in declining lake water storage, then we can adapt and explore new policies to reduce large-scale declines."
This happened in one of the lakes the team studied, Lake Sevan in Armenia. Lake Sevan has seen an increase in water storage over the past 20 years, which the authors linked to the enforcement of conservation laws on water withdrawal since the early 2000s.