NASA Satellites Reveal Major Shifts in Global Freshwater

In a
first-of-its-kind study, scientists have combined an
array of NASA satellite
observations of Earth with data on
human activities to map locations where
freshwater is changing
around the globe and why.

The study,
published today in the journal Nature, finds that
Earth’s wet land areas are
getting wetter and dry areas are
getting drier due to a variety of factors,
including human
water management, climate change and natural cycles.

Between 2002 and 2016, the Gravity Recovery and Climate
Experiment (GRACE) tracked the movement of freshwater around
the planet. Credit: NASA’s Goddard Space Flight Center/Katy
Mersmann

A
team led by Matt Rodell of NASA’s Goddard Space Flight
Center in Greenbelt,
Maryland, used 14 years of observations
from the U.S./German-led Gravity
Recovery and Climate
Experiment (GRACE)
spacecraft mission to
track global trends in freshwater in 34
regions around the world. To understand
why these trends
emerged, they needed to pull in satellite precipitation data
from the Global Precipitation Climatology Project, NASA/U.S.
Geological Survey Landsat
imagery, irrigation
maps, and published reports of human
activities related to agriculture, mining
and reservoir
operations. Only through analysis of the combined data sets
were
the scientists able to get a full understanding of the
reasons for Earth’s
freshwater changes, as well as the sizes
of those trends.

“This
is the first time that we’ve used observations from
multiple satellites in a
thorough assessment of how freshwater
availability is changing everywhere on
Earth,” said Rodell. “A
key
goal was to distinguish shifts in terrestrial water
storage caused by natural
variability — wet periods and dry
periods associated with El Niño and La Niña,
for example —
from trends related to climate change or human impacts, like
pumping groundwater out of an aquifer faster than it is
replenished.”

Freshwater is
found in lakes, rivers, soil, snow, groundwater
and ice. Freshwater loss from
the ice sheets at the poles —
attributed to climate change — has implications
for sea level
rise. On land, freshwater is one of the most essential of
Earth’s
resources, for drinking water and agriculture. While
some regions’ water
supplies are relatively stable, others
experienced increases or decreases.

“What
we are witnessing is major hydrologic change,” said
co-author Jay Famiglietti of NASA’s Jet Propulsion
Laboratory
in Pasadena, California, which also managed the GRACE mission
for
NASA’s Science Mission Directorate in Washington. “We see
a distinctive pattern of the wet land
areas of the world
getting wetter — those are the high latitudes and the
tropics
— and the dry areas in between getting dryer. Embedded within
the dry
areas we see multiple hotspots resulting from
groundwater depletion.”

Famiglietti noted that while water loss in some
regions, like
the melting ice sheets and alpine glaciers, is clearly driven
by
warming climate, it will require more time and data to
determine the driving
forces behind other patterns of
freshwater change.

“The pattern of wet-getting-wetter,
dry-getting-drier during
the rest of the 21st century is predicted by the
Intergovernmental Panel on Climate Change models, but we’ll
need a much longer
dataset to be able to definitively say
whether climate change is responsible
for the emergence of any
similar pattern in the GRACE data,” Famiglietti
said.

The
twin GRACE satellites, launched in 2002 as a joint mission
with the German
Aerospace Center (DLR), precisely measured the
distance between the two spacecraft
to detect changes in
Earth’s gravity field caused by movements of mass on the
planet below. Using this method, GRACE tracked monthly
variations in
terrestrial water storage until its science
mission ended in October 2017.

However, the GRACE satellite observations alone
couldn’t tell
Rodell, Famiglietti and their colleagues what was causing the
apparent trends.

“We examined information on precipitation, agriculture and
groundwater
pumping to find a possible explanation for the
trends estimated from
GRACE,” said co-author Hiroko Beaudoing
of Goddard and the University of
Maryland in College Park.

For
instance, although pumping groundwater for agricultural
uses is a significant
contributor to freshwater depletion
throughout the world, groundwater levels
are also sensitive to
cycles of persistent drought or rainy conditions.
Famiglietti
noted that such a combination was likely the cause of the
significant groundwater depletion observed in California’s
Central Valley from
2007 to 2015, when decreased groundwater
replenishment from rain and snowfall
combined with increased
pumping for agriculture.

Southwestern
California lost 4 gigatons of freshwater per year
during the period. A gigaton
of water would fill 400,000
Olympic swimming pools. A majority of California’s
freshwater
comes in the form of rainfall and snow that collect in the
Sierra
Nevada snowpack and then is managed as it melts into
surface waters through a
series of reservoirs. When natural
cycles led to less precipitation and caused
diminished
snowpack and surface waters, people relied on groundwater more
heavily.

Downward
trends in freshwater seen in Saudi Arabia also
reflect agricultural pressures. From 2002 to 2016, the
region
lost 6.1 gigatons per year of stored groundwater. Imagery from
Landsat
satellites shows an explosive growth of irrigated
farmland in the arid
landscape from 1987 to the present, which
may explain the increased drawdown.

The team’s
analyses also identified large, decade-long trends
in terrestrial freshwater
storage that do not appear to be
directly related to human activities. Natural
cycles of high
or low rainfall can cause a trend that is unlikely to persist,
Rodell said. An example is Africa’s western Zambezi basin and
Okavango Delta, a
vital watering hole for wildlife in northern
Botswana. In this region, water
storage increased at an
average rate of 29 gigatons per year from 2002 to 2016.
This
wet period during the GRACE mission followed at least two
decades of
dryness. Rodell believes it is a case of natural
variability that occurs over
decades in this region of Africa.

The researchers
found that a combination of natural and human
pressures can lead to complex
scenarios in some regions.
Xinjiang province in northwestern China, about the
size of
Kansas, is bordered by Kazakhstan to the west and the
Taklamakan desert
to the south and encompasses the central
portion of the Tien Shan Mountains.
During the first decades
of this century, previously undocumented water
declines
occurred in Xinjiang.

Rodell and his
colleagues pieced together multiple factors to
explain the loss of 5.5 gigatons
of terrestrial water storage
per year in Xinjiang province. Less rainfall was
not the
culprit. Additions to surface water were also occurring from
climate
change-induced glacier melt, and the pumping of
groundwater out of coal mines.
But these additions were more
than offset by depletions caused by an increase
in water
consumption by irrigated cropland and evaporation of river
water from
the desert floor.

The
successor to GRACE, called GRACE Follow-On, a
joint mission with
the German Research Centre for Geosciences
(GFZ), currently is at Vandenberg
Air Force Base in California
undergoing final preparations for launch no earlier
than May
22.

For more
information on how NASA studies Earth, visit:

https://www.nasa.gov/earth

For more information on GRACE and GRACE-FO,
visit:

http://www.csr.utexas.edu/grace

https://grace.jpl.nasa.gov

and

https://www.nasa.gov/gracefo

News Media Contact

Alan Buis
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-0474
Alan.Buis@jpl.nasa.gov

Steve Cole
NASA Headquarters, Washington
202-358-0918
Stephen.e.cole@nasa.gov

2018-102

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