March 3, 2021
To study the effects of humans on freshwater resources, scientists have now carried out the first global survey of fluctuating water levels in the Earth’s lakes and reservoirs – including those that were previously too small to be measured from space.
The study, published March 3 in the journal Nature, was based on NASA’s Elevation and Cloud Height Satellite 2 (ICESat-2) launched in September 2018.
ICESat-2 sends 10,000 pulses of laser light to earth every second. When these pulses are reflected back to the satellite, they provide high-precision measurements of surface height along the satellite’s orbit every two feet. With these trillions of data points, scientists can distinguish more features of the earth’s surface, such as small lakes and ponds, and track them over time.
Scientists used these elevation measurements to survey 227,386 bodies of water over 22 months and found that the water level in the Earth’s lakes and ponds fluctuates an average of about 0.22 m from season to season. At the same time, the water level of man-made reservoirs fluctuates on average by almost four times this amount – about 0.86 m.
While natural lakes and ponds are more than 24 to 1 more than man-made reservoirs in their study, scientists calculated that reservoirs account for 57% of the total global variability in water storage.
To study the effects of humans on freshwater resources, scientists have now carried out the first global survey of fluctuating water levels in the Earth’s lakes and reservoirs – including those that were previously too small to be measured from space. Credits: Goddard Space Flight Center / NASA Scientific Visualization Studio
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“Understanding that variability and finding patterns in water management really shows how much we’re transforming the global water cycle,” said Sarah Cooley, a remote sensing hydrologist at Stanford University in California who led the research. “The human impact on water storage is much higher than expected.”
In natural lakes and ponds, the water level usually varies with the seasons, filling up when it rains, and filling up when it is hot and dry. In reservoirs, however, managers influence that variation – they often store more water during the rainy season and divert it when it’s dry, which can exaggerate natural seasonal variations, Cooley said.
Cooley and her colleagues also found regional patterns – reservoirs vary most in the Middle East, southern Africa, and the western United States, while the natural differences in lakes and ponds are more pronounced in tropical areas.
The results form the basis for future studies on how the relationship between human activity and climate changes the availability of freshwater. As populations grow more demanding on freshwater and climate change changes the way water moves through the hydrological cycle, studies like this can shed light on how water is managed, Cooley said.
“This type of dataset will be so valuable to see how human water management will change in the future and in which areas the greatest changes are occurring or what threats exist to their water storage,” said Cooley. “This study gives us a really valuable foundation on how humans are modulating the water cycle on a global scale.”
The researchers’ methods also relied on a second satellite mission – Landsat, the decade-long mission jointly monitored by NASA and the US Geological Survey. The team used Landsat-derived two-dimensional maps of bodies of water and their size and provided them with a comprehensive database of the world’s lakes, ponds and reservoirs. Then ICESat-2 added the third dimension – the height of the water table with an uncertainty of about 10 cm. When these measurements are averaged over thousands of lakes and reservoirs, the uncertainty decreases even more.
Although ICESat-2’s mission focuses on the frozen water of the Earth’s cryosphere, creating data products using unfrozen water was also part of the original plan, according to Tom Neumann, ICESat-2 project scientist at NASA’s Goddard Space Flight Center in Greenbelt. Maryland. Now that the satellite is in orbit, scientists are discovering more smaller lakes and reservoirs than previously thought – in this study they discovered ponds that are half the size of the Lincoln Memorial Reflecting Pool.
“We are now able to measure all these lakes and reservoirs again and again with the same” ruler “,” said Neumann. “It’s a great example of another scientific application that these altitude measurements make possible. It’s incredibly exciting to see the questions people can explore with these data sets. “
For more information on ICESat-2, please visit www.nasa.gov/icesat-2
Header image: Lake Mead, along the Colorado River. Credit: National Park Service
By Kate Ramsayer
NASA Goddard Space Flight Center, Greenbelt, Md.
Last updated: March 3, 2021 Editor: Kate Ramsayer