Scientists conduct the first global study of freshwater variability

To study the impact of humans on freshwater resources, scientists have now conducted the first global accounting of fluctuating water levels in the Earth’s lakes and reservoirs – including those that previously too small to measure from space.

The research, published March 3 in the journal Nature, responsible for NASA’s Ice, Cloud and Elevation Satellite 2 (ICESat-2) land, launched in September 2018.

ICESat-2 sends 10,000 beats of laser light per second down to Earth. When returned to the satellite, these pulses deliver high surface area measurements every 28 inches (70 centimeters) across the orbit of the satellite. With these trillions of data points, scientists can differentiate more features of the Earth’s surface, such as small lakes and ponds, and monitor them over time.

Scientists used these altitude measurements to study 227,386 water bodies over 22 months and found that, from season to season, the water level in Earth’s lakes and ponds fluctuates on average around 8.6 inches (0.22 m). At the same time, the water level of human-controlled reservoirs varies on average almost a quadruple of that amount – about 34 inches (0.86 m).

Although natural lakes and ponds higher than reservoirs are managed by people with more than 24 to 1 in their study, scientists found that reservoirs made up 57% of total global variability water storage.

“Understanding the variability and detection of patterns in water management certainly demonstrates how we are changing the global irrigation cycle,” said Sarah Cooley, a remote irrigation expert at Stanford University in California, who led the research. “Human impact on water storage is much higher than we expected.”

In lakes and natural ponds, water levels usually change with the seasons, filling at wet times and draining when hot and dry. In reservoirs, however, this difference is overlooked by managers – often storing more water in wet seasons and disposing of it when it is dry, which can lead to dehydration. reversing the natural seasonal change, Cooley said.

Cooley and her colleagues also discovered regional patterns – reservoirs vary mainly in the Middle East, southern Africa, and the western United States, while the natural difference exists. the lochs and ponds more prominent in tropical areas.

The results set the platform for future studies on how the relationship between human activity and climate is changing the availability of freshwater. As growing numbers put more demands on freshwater, and climate change changes the way water moves through the irrigation cycle, studies like this can clarify how water is managed, said Cooley.

“This type of data will be invaluable in seeing how human water management changes in the future, and which areas are experiencing the greatest change, or threats to water storage. Cooley said. “This study provides us with an invaluable basis on how people are changing the water cycle at the global scale.”

The researchers ’methods relied on a second satellite mission, in addition to – Landsat, the decades-long mission co-led by NASA and the U.S. Geological Survey. The team used two-dimensional Landsat-based maps of water bodies and their sizes, giving them a complete database of the world’s lakes, ponds and reservoirs. ICESat-2 then added the third dimension – the height of the water level, with an uncertainty of about 4 inches (10 cm). When these measurements average over thousands of lakes and reservoirs, the uncertainty drops even more.

While the ICESat-2 mission focuses on the Earth’s cryosphere frozen water, the creation of data results of unfrozen water altitude was also part of the original plan, according to Tom Neumann, ICESat-2 project scientist at NASA’s Goddard Space Flight in Greenbelt, Maryland. Now, with the satellite in orbit, scientists are discovering more lakes and reservoirs than previously thought – in this study they discovered ponds half the size of a Lincoln memorial meditation pool.

“We can now measure all these lakes and reservoirs with the same ‘ruler,’ over and over again,” said Neumann. “It is a good example of another scientific application that the measurements that height enables. It is very interesting to see what questions people can study with these databases. ”