It will take about 11 trillion gallons of water (42 cubic
kilometers)
−
around 1.5 times the maximum volume of the largest U.S. reservoir
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to recover from California's continuing drought, according to a new
analysis of NASA satellite data.
The finding
was part of a sobering update on the state's drought made possible
by space and airborne measurements and presented by NASA scientists
Dec. 16 at the American Geophysical Union meeting in San Francisco.
Such data are giving scientists an unprecedented ability to identify
key features of droughts, and can be used to inform water management
decisions.
A team of scientists led by Jay Famiglietti of NASA's Jet
Propulsion Laboratory in Pasadena, California, used data from NASA's
Gravity Recovery and Climate Experiment (GRACE) satellites to
develop the first-ever calculation of this kind
− the volume of
water required to end an episode of drought.
Earlier this year, at the peak of California's current three-year
drought, the team found that water storage in the state's Sacramento
and San Joaquin river basins was 11 trillion gallons below normal
seasonal levels. Data collected since the launch of GRACE in 2002
show this deficit has increased steadily.
"Spaceborne and airborne measurements of Earth's changing shape,
surface height and gravity field now allow us to measure and analyze
key features of droughts better than ever before, including
determining precisely when they begin and end, and what their
magnitude is at any moment in time", Famiglietti said. "That's an
incredible advance and something that would be impossible using only
ground-based observations".
GRACE data reveal that, since 2011, the Sacramento and San
Joaquin river basins decreased in volume by four trillion gallons of
water each year (15 cubic kilometers). That's more water than
California's 38 million residents use each year for domestic and
municipal purposes. About two-thirds of the loss is due to depletion
of groundwater beneath California's Central Valley.
In related results, early 2014 data from NASA's Airborne Snow
Observatory indicate that snowpack in California's Sierra Nevada
range was only half of previous estimates. The observatory is
providing the first-ever high-resolution observations of the water
volume of snow in the Tuolumne River, Merced, Kings and Lakes basins
of the Sierra Nevada and the Uncompahgre watershed in the Upper
Colorado River Basin.
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To develop these calculations, the observatory measures how much
water is in the snowpack and how much sunlight the snow absorbs,
which influences how fast the snow melts. These data enable accurate
estimates of how much water will flow out of a basin when the snow
melts, which helps guide decisions about reservoir filling and water
allocation.
"The 2014 snowpack was one of the three lowest on record and the
worst since 1977, when California's population was half what it is
now", said Airborne Snow Observatory Principal Investigator Tom
Painter of JPL. "Besides resulting in less snow water, the dramatic
reduction in snow extent contributes to warming our climate by
allowing the ground to absorb more sunlight. This reduces soil
moisture, which makes it harder to get water from the snow into
reservoirs once it does start snowing again".
New drought maps show groundwater levels across the U.S.
Southwest are in the lowest 2 to 10 percent since 1949. The maps,
developed at NASA's Goddard Space Flight Center in Greenbelt,
Maryland, combine GRACE data with other satellite observations.
"Integrating GRACE data with other satellite measurements
provides a more holistic view of the impact of drought on water
availability, including on groundwater resources, which are
typically ignored in standard drought indices", said Matt Rodell,
chief of the Hydrological Sciences Laboratory at Goddard.
The scientists cautioned that while the recent California storms
have been helpful in replenishing water resources, they aren't
nearly enough to end the multi-year drought.
"It takes years to get into a drought of this severity, and it
will likely take many more big storms, and years, to crawl out of
it", said Famiglietti.
NASA monitors Earth's vital signs from land, air and space with a
fleet of satellites and ambitious airborne and ground-based
observation campaigns. The agency develops new ways to observe and
study Earth's interconnected natural systems with long-term data
records and computer analysis tools to better see how our planet is
changing. The agency shares this unique knowledge with the global
community and works with institutions in the United States and
around the world that contribute to understanding and protecting our
home planet.
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