When you look at just about any visual representation of the Earth’s water cycle, there are many common elements – water bodies, precipitation clouds, trees, and sun.
However, with all of them, there is one distinct entity missing from the picture. It’s us, humans, and our activities.
Although humans can’t influence the overall amount of water on Earth, they can and do influence the hydrologic cycle and water distribution – especially when it comes to fresh water, our most precious resource.
The fresh water is drawn from various types of water bodies – mostly artificial reservoirs. It is captured and used for food production and distribution of drinking water across the world.
Due to climate and precipitation patterns, some regions’ freshwater sources face more pressure than others, causing their levels to fluctuate drastically. Water levels on land also fluctuate naturally, dictated by the local climate.
Fluctuations were studied before individually and regionally, through gathering field data and other types of measurements; however, a comprehensive global survey of human-caused water fluctuations in exploited water reservoirs was missing – up until now.
Thanks to state-of-art LiDAR technology on NASA’s ICESat-2 (Ice, Cloud and land Elevation Satellite 2, launched in September 2018) with its satellite laser altimeter, the Agency managed to perform the first global survey of freshwater fluctuations. The results were published in Nature this March.
The Role of Satellite Remote-Sensing in Tracking Freshwater
The satellite-mounted laser altimeter enables unprecedentedly precise measuring. It sends 10,000 laser light pulses down to Earth every single second. The pulses hit the surfaces and are reflected back to the satellite, where the system’s sensors capture them. The distance measured between the surface and the sensor enables scientists to calculate exact surface height measurements, and the density of the data is stunning. ICESat-2 delivers these measurements for every 28 inches (70 cm) of Earth’s surface along its orbit.
The result is a trillion data points, which are then further analyzed to distinguish the surfaces’ features – including lakes and ponds – with the possibility to track if and how they change over time.
The researchers also relied on a second satellite mission – Landsat. The team used Landsat-derived, two-dimensional maps of water bodies and their sizes, providing them with a comprehensive database of the world’s lakes, ponds, and reservoirs. Then, ICESat-2 added the third dimension – the water level’s height, with an uncertainty of roughly 4 inches (10 cm). When those measurements are averaged over thousands of lakes and reservoirs, the uncertainty drops even more.
The survey took place from October 2018 to July 2020 and resulted in an extensive water level data set for 227,386 water bodies.
The Study’s Important Findings About Water Levels
The most peculiar finding of the study is the extent of human influence on the seasonal water level variability. In natural water bodies, the average seasonal variability was 0.22 meters. However, in human-managed reservoirs, the average was significantly higher – 0.86 meters.
Watch the NASA video: NASA Satellite Measures Human Impact in Water Storage
That means that human-managed reservoirs account for 57 percent of the Earth’s seasonal surface water storage variability. The difference is even more impressive if we consider that natural lakes and ponds in the study outnumber human-managed reservoirs by more than 24 to 1.
Sarah Cooley, a remote sensing hydrologist at Stanford University in California and the research’s leader, explained that “Understanding that variability and finding patterns in water management really shows how much we are altering the global hydrological cycle – the impact of humans on water storage is much higher than we were anticipating.”
“In natural lakes and ponds, water levels typically vary with the seasons, filling up during rainy periods and draining when it’s hot and dry. In reservoirs, however, managers influence that variation – often storing more water during rainy seasons and diverting it when it’s dry, which can exaggerate the natural seasonal variation,” Cooley said.
The variations were, expectedly, not equal across the globe and have shown strong regional patterns.
Natural variability in surface water bodies was found to be the most significant in tropical basins. As for human-induced variability, it was predictably the greatest in the arid regions – the Middle East, southern Africa, and the western United States.
As for the percentage of variability, human activity influences “67 per cent of surface water storage variability south of 45 degrees north and nearly 100 per cent in certain arid and semi-arid regions,” as the study says.
The new insights into freshwater fluctuations will be of big help in understanding the anthropogenic influence on water resources on Earth. That is of particular importance since the human population is still experiencing growth – and in a warming world. Significant pressure on the global freshwater reserves – and the related ecosystems – is inevitable. Gathering data on dynamics of the use of our most important shared resource is essential to secure sustainable water management in the future.
“The results set the stage for future investigations into how the relationship between human activity and climate alters the availability of fresh water. As growing populations place more demands on freshwater, and climate change alters the way water moves through the hydrological cycle, studies like this can illuminate how water is being managed,” Cooley concludes.
Cooley, S.W., Ryan, J.C. & Smith, L.C. 2021. Human alteration of global surface water storage variability. Nature 591, 78–81. https://doi.org/10.1038/s41586-021-03262-3
ICESat-2 Mission. NASA https://icesat-2.gsfc.nasa.gov/mission
Lakes and reservoirs, USGS Water Science School: https://www.usgs.gov/special-topic/water-science-school/science/lakes-and-reservoirs
Landsat Overview. NASA. https://www.nasa.gov/mission_pages/landsat/overview/index.html
NASA Scientists Complete 1st Global Survey of Freshwater Fluctuation. NASA. 3 March 2021. https://www.nasa.gov/feature/goddard/2021/nasa-scientists-complete-first-global-survey-of-freshwater-fluctuation/
What Is Lidar? NOAA https://oceanservice.noaa.gov/facts/lidar.html