The Colorado River supplies water to more than 40
million people as it snakes through seven U.S. states, including the part of
southeastern Utah seen in this photo snapped by an astronaut aboard the
International Space Station. The Colorado basin was identified in a NASA-led
study as a region experiencing intense human water use. NASA
The novel approach to estimating river water storage and discharge also
identifies regions marked by ‘fingerprints’ of intense water use.
A study led by NASA researchers
provides new estimates of how much water courses through Earth’s rivers, the
rates at which it’s flowing into the ocean, and how much both of those figures
have fluctuated over time — crucial information for understanding the planet’s
water cycle and managing its freshwater supplies. The results also highlight
regions depleted by heavy water use, including the Colorado River basin in the
United States, the Amazon basin in South America, and the Orange River basin in
southern Africa.
For the study, which was recently
published in Nature
Geoscience, researchers at NASA’s Jet Propulsion Laboratory in Southern California
used a novel methodology that combines stream-gauge measurements with computer
models of about 3 million river segments around the world.
A NASA-led study combined stream-gauge measurements
with computer models of 3 million river segments to create a global picture of
how much water Earth’s rivers hold. It estimated that the Amazon basin contains
about 38% of the world’s river water, the most of any hydrological region
evaluated. NASA
The scientists estimate that the total volume of water in Earth’s rivers on
average from 1980 to 2009 was 539 cubic miles (2,246 cubic kilometers). That’s
equivalent to half of Lake Michigan’s water and about 0.006% of all fresh
water, which itself is 2.5% of the global volume. Despite their small
proportion of all the planet’s water, rivers have been vital to humans since
the earliest civilizations.
Although researchers have made
numerous estimates over the years of how much water flows from rivers into the
ocean, estimates of the volume of water rivers collectively hold — known as
storage — have been few and more uncertain, said JPL’s Cédric David, a
co-author of the study.
He likened the situation to
spending from a checking account without knowing the balance. “We don’t know
how much water is in the account, and population growth and climate change are
further complicating matters,” David said. “There are many things we can do to
manage how we’re using it and make sure there is enough water for everyone, but
the first question is: How much water is there? That’s fundamental to everything else.”
The NASA-led study estimated flow through 3 million
river segments, identifying locations around the world marked by intense human
water use, including parts of the Colorado, Amazon, Orange, and Murray-Darling
river basins, shown as gray here. NASA
Estimates in the paper could eventually be compared with data from the
international Surface Water and Ocean Topography (SWOT) satellite to improve measurements of human impacts
on Earth’s water cycle. Launched in December 2022, SWOT is mapping the
elevation of water around the globe, and changes in river height offer a way to
quantify storage and discharge.
‘Fingerprints’
of Water Use
The study identified the Amazon
basin as the region with the most river storage, holding about 204 cubic miles
(850 cubic kilometers) of water — roughly 38% of the global estimate. The same
basin also discharges the most water to the ocean: 1,629 cubic miles (6,789
cubic kilometers) per year. That’s 18% of the global discharge to the ocean,
which averaged 8,975 cubic miles (37,411 cubic kilometers) per year from 1980
to 2009.
Although it’s not possible for a
river to have negative discharge — the study’s approach doesn’t allow for
upstream flow — for the sake of accounting, it is possible for less water to
come out of some river segments than went in. That’s what the researchers found
for parts of the Colorado, Amazon, and Orange river basins, as well as the
Murray-Darling basin in southeastern Australia. These negative flows mostly
indicate intense human water use.
“These are locations where we’re
seeing fingerprints of water management,” said lead author Elyssa Collins, who
conducted the analysis as a JPL intern and doctoral student at North Carolina
State University in Raleigh.
A New Way to
Quantify Rivers
For decades, most estimates of
Earth’s total river water were refinements of a 1974 United Nations figure, and
no study has illustrated how the amount has varied with time. Better estimates
have been hard to come by, David said, due to a lack of observations of the
world’s rivers, particularly those far from human populations.
Another issue has been that there
are many more stream gauges monitoring the levels and flow of large rivers than
there are of small ones. There’s also broad uncertainty in estimates of land
runoff — the rainwater and snowmelt that flow into rivers.
The new study started from the
premise that runoff flowing into and through a river system should roughly
equal the amount that gauges measure downstream. Where the researchers found
inconsistencies between simulated runoff from three land surface models and
gauge measurements taken from approximately 1,000 locations, they used the
gauge measurements to correct the simulated runoff numbers.
Then they modeled the runoff
through rivers on a high-resolution global map developed using land-elevation
data and imagery from space, including from NASA’s Shuttle Radar Topography Mission. This approach yielded discharge rates, which were
used to estimate average and monthly storage for individual rivers and the
planet’s rivers in total.
Using a consistent methodology
enables comparisons in flow and human drawdown between different regions.
“That way we can see where in the
world the most amount of river water is stored, or where the most amount of
water is being emptied into oceans from rivers,” said Collins, now a
postdoctoral researcher at the University of North Carolina at Chapel Hill.
Source: NASA-Led
Study Provides New Global Accounting of Earth’s Rivers - NASA
