Though the recent landing of the Perseverance Rover on Mars has
drawn our attention, it is important to recall (or know) that NASA is also
gathering data on our own planet, Earth. The latest research was published last
week in the journal” Nature.” Using data gathered by NASA’s Ice, Cloud and
land Elevation Satellite 2 (ICESat-2), launched in September 2018, and the
older Landsat mission jointly overseen by NASA and the U.S. Geological Survey, this
new research begins the investigation of mankind’s impact on freshwater resources
and the planet’s water cycle.
NASA scientists conducted the first global accounting of
fluctuating water levels in Earth’s lakes and reservoirs. Though the
scientists had not expected it, they found that reservoirs made up the majority
of total variability of water storage despite the fact that natural lakes and
ponds outnumber human-managed reservoirs by more than 24 to 1. The variability in reservoirs only makes
sense, you build a reservoir when you need to store water for later; but the amount of fluctuation was controlled by human action is an
indication of how much of the surface fresh water is being used by mankind. It
will be interesting to see how droughts impact this number during a longer duration
study.
ICESat-2 gathers information by sending 10,000 laser light pulses down to Earth every second. When reflected back to the satellite, those pulses deliver high-precision surface height measurements for every 28 inches along the satellite’s orbit. Using the trillions of data points collected, scientists can measure volume of Earth’s lakes and ponds over time. The scientists used the Landsat two-dimensional maps of bodies of water and their sizes, providing them with a comprehensive database of the world’s lakes, ponds, and reservoirs. Then, ICESat-2 added the third dimension – height of the water level.
The scientists found from season to season, the water level
in Earth’s lakes and ponds that are natural and unmanaged fluctuate on average about
8.6 inches each year. However, the water level of human-managed reservoirs fluctuates
on average nearly four times that amount – about 34 inches each year. Still the volume in human managed reservoirs has to exceed natural ponds and lakes by more than 6 to 1 for the majority of fluctuation to be attributed to human management.
Understanding that variability and finding patterns in water
management really shows how much we are altering the global hydrological
cycle,” said Dr. Sarah Cooley, a remote sensing hydrologist at Stanford
University in California, who led the research. “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. 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.
Dr. Cooley and her colleagues found regional patterns as
well – reservoirs vary the most in the Middle East, southern Africa, and the
western United States, while the natural variation in lakes and ponds is more
pronounced in tropical areas.
In the future the scientists will investigate how human
activity and climate alters the availability of freshwater. As the ever-growing
populations place more and 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 said.
This data could eventually be used for better water
management to maximize water availability as populations continue to grow and
the climate continues to change.
For more information on ICESat-2, visit www.nasa.gov/icesat-2
To read the full report Human alteration of
global surface water storage variability | Nature
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