Around the world, water
is constantly moving. From 2002 to
2016. a pair of NASA satellites witnessed massive shifts
in the freshwater stored on land related to water management, climate change
and natural cycles. The Gravity Recovery and Climate Experiment,
or GRACE mission, used precise measurements of the motions of two
spacecraft in Earth’s orbit to track the movement of water through the oceans, land
and atmosphere. NASA scientists combined GRACE
data with satellite-based observations of precipitation and crop irrigation,
climate model predictions, and other information in order to identify
the causes of regional trends in freshwater storage.
In this visualization, blues indicate areas with more stored freshwater
than the average, and oranges and reds denote areas with less.
The science team classified the major trends observed by GRACE as driven by:
Natural variability, human activity or climate change.
For instance, the steady decrease
in freshwater storage in Greenland is caused by the melting of glaciers,
which drain water into the oceans.
In the western United States, a long drought reduced mountain snowpack and river flows,
causing heavy reliance on aquifers for crop irrigation and
severe depletion of freshwater resources.
Precipitation measurements together with GRACE data show how natural variations
in the weather and unsustainable rates of water use conspired to deplete
groundwater in California.
In southern Africa, the Okavango Delta region experienced a huge
increase in stored freshwater during the period of the GRACE mission.
The science team analyzed precipitation data for the area and found that it was caused
by a pronounced increase in rainfall.
Between 2004 and 2012, the region saw about 15% more
annual rainfall than during the previous twenty-five years.
The rain ended a regional drought, and replenished water storage in the area.
In northern Saudi Arabia, GRACE detected a dramatic
decrease in fresh water stored in aquifers. Images taken by
NASA’s Landsat program show a rapid increase in irrigated cropland,
supported by water pumped from those aquifers.
Most of that water is non-renewable on human timescales, but in 2014
the Saudi government ended a domestic wheat farming program, and GRACE data
suggest that aquifer levels may be stabilizing.
In northwest China, GRACE revealed a rapid
decrease in freshwater storage without an obvious cause.
Scientists knew that mountain glaciers were melting, but the melt-water did not leave the region,
so they looked for another explanation.
As it turns out, much of the region’s surface water is redirected to
agricultural areas and the desert to the south, where it evaporates,
leaving the region with a net loss of water.
The original GRACE satellites stopped operating in 2017,
but NASA and the German Research Center for Geosciences are partnering to launch a new
satellite pair, GRACE Follow-On, in late spring 2018
to continue providing data about freshwater trends around the world.