Humans have had a remarkable impact on the planet, from clearing forests for agriculture and urbanization, to changing the chemistry of the atmosphere with fossil fuels. Today, a new study from the journal Nature reveals for the first time the scale of human impact on the global water cycle.
The study used NASA’s Ice, Cloud and Land Elevation Satellite (ICESat-2) to pull together the largest ever dataset on seasonal water levels in more than 227,000 lakes, ponds and reservoirs around the world. Data reveals that although human-managed reservoirs represent only a small percentage of all water bodies, they account for 57% of the total seasonal changes in water storage around the world.
“We tend to think of the water cycle as a purely natural system: rain and snowmelt flow into rivers, which flow into the ocean where evaporation starts all over again,” said Sarah Cooley, postdoctoral researcher at Stanford University, who initiated the research. project while a graduate student at Brown University. “But humans are in fact substantially involved in this cycle. Our work shows that humans are responsible for most of the seasonal variability in surface water storage on Earth.”
Cooley led the work with Laurence Smith, professor of environmental science at Brown, and Johnny Ryan, postdoctoral researcher at the Brown Institute for Environment and Society.
The researchers say the study provides an essential baseline for tracking the global hydrological cycle as climate change and population growth place new pressures on freshwater resources.
An extraordinary dataset
Launched into orbit in 2018, ICESat-2’s primary mission is to track changes in the thickness and elevation of ice caps around the world. It does this with a laser altimeter, which uses pulses of light to measure elevation with an accuracy of 25 millimeters. Cooley, who has experience using satellites to study water levels in arctic lakes, wanted the satellite’s precise measurement capability to cover lake levels around the world.
Cooley says the ICESat-2 laser altimeter has a much greater resolution than instruments used to measure water levels in the past. This brought together a large, accurate data set including small ponds and reservoirs.
“With older satellites, you have to average the results over a large area, which limits observations to only the largest lakes in the world,” Cooley said. “ICESat has a small footprint, so we can get levels for small lakes that we couldn’t get near before. This was important for understanding the overall water dynamics, as most lakes and reservoirs are quite small. . “
From October 2018 to July 2020, the satellite measured water levels in 227,386 bodies of water, ranging from the American Great Lakes to ponds smaller than a tenth of a square mile. Each body of water was observed at different times of the year to track changes in water levels. The researchers crossed the water bodies they observed with a database of reservoirs around the world to identify which water bodies were controlled by humans and which were natural.
While countries like the United States and Canada measure reservoir levels and make this information publicly available, many countries do not publish this data. And very few lakes and ponds without reservoirs are gauged. So there was no way to do this analysis without the precise satellite observations, the researchers said.
Water cycle control
The study found that while natural lakes and ponds varied seasonally by 0.22 meters on average, human-managed reservoirs varied by 0.86 meters. Together, the much greater variation in reservoirs compared to natural lakes means that reservoirs account for 57% of the total variation. In some places, however, the human influence was even stronger than that. For example, in arid regions such as the Middle East, the American West, India and southern Africa, the variability attributed to human control reaches 90% and more.
“Of all the changes in volume in freshwater bodies around the planet – all the floods, droughts and snowmelt that cause lake levels to rise and fall – humans have requisitioned almost 60% of this. variability, ”Smith said. “It’s a huge influence on the water cycle. In terms of human impact on the planet, it’s right up there with impacts on land cover and atmospheric chemistry.”
As the world’s first quantification of human impacts on the water cycle, the results will provide a crucial baseline for future research into how impacts affect ecosystems around the world, the researchers say.
In a separate study published recently in Geophysical Research Letters, the research team was able to use ICESat-2 data to shed light on how the reservoir water is used. The study showed that in places like the Middle East, reservoir levels tend to be lower in summer and higher in winter. This suggests that water is released during the dry season for irrigation and drinking water. In contrast, the trend in countries like Scandinavia was in the opposite direction. There, water is released in winter to produce hydroelectric power for heating.
“This was an exploratory analysis to see if we can use remote sensing to understand how reservoirs are being used globally,” Ryan said.
Smith says he expects satellites to play an increasing role in studying the Earth’s water cycle. For several years now, he has been working with NASA on the Topography of surface waters and oceans mission, which will be entirely devoted to this type of research.
“I think over the next three years we are going to see an explosion of high quality hydrological satellite data, and we will have a much better idea of what is going on with water all over the planet,” said Smith. “This will have implications for security, transboundary water agreements, predicting the future of crops and more. We are at the forefront of a new understanding of the hydrology of our planet.
The research was supported by NASA Studies with the ICESat-2 program (80NSSC20K0963) and the NASA Surface Water and Ocean Topography Mission (80NSSC20K1144S).