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March 3, 2021
from Brown University
Humans have made a remarkable impact on the planet, from clearing forests for agriculture and urbanization to changing the chemistry of the atmosphere with fossil fuels. Now, for the first time, a new study in the journal Nature reveals the extent of human impact on the global water cycle.
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The study used NASA’s ice, cloud and land elevation satellite (ICESat-2) to compile the world’s largest data set for seasonal water levels in more than 227,000 lakes, ponds and reservoirs worldwide. The data shows that while human-managed reservoirs make up only a small percentage of all water bodies, they account for 57% of all seasonal water storage changes worldwide.
« We tend to view the water cycle as a purely natural system: rain and snowmelt flow into rivers that flow to the ocean where evaporation starts the entire cycle again, « said Sarah Cooley, a postdoctoral fellow at Stanford University who started the research project while a graduate student at Brown University. « But humans actually intervene significantly in this cycle. Our work shows that humans are responsible for much of the seasonal variability in surface water storage on earth. »
Cooley led the work with Laurence Smith, professor of environmental science Brown, and Johnny Ryan, postdoctoral fellow at the Institute for Environment and Society at Brown.
The researchers say the study is a critical foundation in tracking the global hydrological cycle as climate change and population growth are re-stressing freshwater resources.
ICESat-2 was launched into orbit in 2018 and its primary role is to track changes in the thickness and height of ice sheets around the world. This is done with a laser altimeter, which uses light pulses to measure the height with an accuracy of 25 millimeters. Cooley, who has experience with satellites to study water levels in arctic lakes, was interested in applying the satellite’s accurate measuring capacity to lake levels worldwide.
According to Cooley, the ICESat-2 laser altimeter has a much higher resolution than instruments that with which the water level was measured in the past. This made it possible to collect a large, precise data set with small ponds and reservoirs.
« With older satellites, the results have to be averaged over a large area, which limits the observations to only the largest lakes in the world, » said Cooley. « ICESat has a small footprint so we can get levels for small lakes that we couldn’t reach before. This was important in understanding global water dynamics as most lakes and reservoirs are quite small. »
By October From 2018 to July 2020, the satellite measured water levels in 227,386 bodies of water ranging in size from the American Great Lakes to ponds less than a tenth of a mile in area. Each body of water was observed at different times of the year to track changes in water levels. The researchers compared the observed waters to a database of reservoirs around the world to determine which waters were man-made and which were natural.
While countries like the US and Canada measure reservoir levels and make this information publicly available, many countries publish such data do not. And only very few lakes and ponds without a reservoir are measured at all. Without the precise satellite observations, this analysis would not be possible, the researchers said.
The study found that natural lakes and ponds varied by an average of 0.22 meters seasonally, while man-made reservoirs varied by 0.86 meters. Taken together, the much greater variation in reservoirs compared to natural lakes means that reservoirs account for 57% of the total variation. However, in some places the human influence was even stronger. In arid regions like the Middle East, the American West, India, and southern Africa, the variability attributed to human control increases to 90% and more.
« Of all volume changes in freshwater bodies around the world – all the floods, droughts, and thaws that are driving lake levels up and down – humans commanded nearly 60% of that variability, « Smith said. « That’s a huge impact on the water cycle. In terms of human impact on the planet, this is right up there with impact on land cover and atmospheric chemistry. »
As the first global quantification of human impact on the water cycle, the results will form a critical basis for future research into how the effects are affecting ecosystems around the world, the researchers say.
In a separate study recently published in Geophysical Research Letters, this was possible Use ICESat-2 data to provide research team with insight into how the reservoir water is being used. The study showed that in countries like the Middle East, reservoir levels tend to be lower in summer and higher in winter. This suggests that water is released for irrigation and drinking water during the dry season. In contrast, the trend was reversed in countries like Scandinavia. There, water is released in winter to generate hydropower for heating.
« This was an exploratory analysis to see if we could use remote sensing to understand how reservoirs are used around the world, » said Ryan.
Smith believes that satellites will play an increasing role in studying the Earth’s hydrological cycle. For the past several years he has worked with NASA on the Surface Water and Ocean Topography mission, which will be entirely dedicated to this type of research.
« I think over the next three years we will see an explosion of high quality satellite hydrology data and we’ll have a much better idea of what’s going on with water around the world, « said Smith. « That will have implications for security, cross-border water treaties, future crop forecasting and more. We are on the verge of a new understanding of the hydrology of our planet. »
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Ref: https://phys.org
