NASA GRACE – Gravity Recovery And Climate Experiment – Water weighs a heavy 8.34lbs per gallon.  So if you have a huge aquifer of it contained beneath the earth, you can actually measure water’s own gravity as it’s volume and mass change over time.  That’s exactly what the NASA mission GRACE did from space…but how?  The twin GRACE satellites, launched in 2002, made detailed measurements of Earth’s gravity field changes revolutionizing investigations about Earth’s water reservoirs.  The mission uses a microwave ranging system to measure changes in the speed and distance between two identical spacecraft flying in a polar orbit 140 miles apart, and 310 miles above Earth.  The satellites circle the globe 15 times a day, creating global coverage every 30 days.  While they fly the sensors feel minute variations in Earth’s gravitational pull. When the first satellite passes over a region of slightly stronger gravity, a gravity anomaly, it is pulled slightly ahead of the trailing satellite. This causes the distance between the satellites to increase. The ranging system is sensitive enough to detect separation changes as small as 10 micrometers (approximately one-tenth the width of a human hair.)  The first spacecraft then passes the anomaly, and slows down again as the extra gravity tugs at it from behind; meanwhile the following spacecraft accelerates, then decelerates over the same point. By measuring the constantly changing distance between the two satellites and combining that data with precise positioning measurements from Global Positioning System (GPS) instruments, scientists can construct a detailed map of Earth’s gravity anomalies.

Changes in Aquifer Water Levels

So what does all this space data on water’s gravity tell us?  Researchers armed with a decade of satellite information, have reported 21 of the world’s 37 largest aquifers have passed their sustainability tipping points, also know as the term #PeakWater.  Meaning more water was removed than replaced during the decade-long study period from 2003-2013.  For example, the Californian Central Valley Aquifer System’s water level was estimated to drop -8.9mm/yr during the study.  GRACE is the first tool that is able to estimate from space how much water is moving in and out of our aquifers.  In the United Nations Intergovernmental Panel on Climate Change (IPCC report AR5 from 2014), groundwater pumping was estimated to contribute about 15-25% of the rate of global sea level rise.  The data unfortunately doesn’t accurately indicate how much groundwater is left in our aquifers.  Best estimates might be off by several orders of magnitude, so we really don’t know…

Thirteen aquifers are considered in the “most troubled” category, indicated a long-term problem likely to worsen.  Aquifer water levels are often challenged to keep pace with the pumping demands of fast growing populations and their subsequent consumption.  Just as strenuous is the explosion of populations rapidly entering the middle class.  These new consumers’ dollars require a massive volume of #VirtualWater.  This virtual hidden water debt comprises water utilized in agriculture and industrial production, but invisible to the consumer.  This invisible water is often then exported around the world as food, goods and services. Consumed but no longer local.  Unreplenished.

GRACE’s successor, GRACE-FO (Follow On), was launched in 2018 with the same mission, to continue to give us new insights into planet Earth and how we can sustainably maintain our health and livelihood.

Changes in Water Storage vs. Aquifer Stress

 

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References for further reading:

NASA GRACE Groundwater Applications

NASA GRACE Tellus

NASA GRACE-FO (Follow On)

GRACE Data Analysis Portal

NASA Mission Page

Jet Propulsion Laboratory Mission Page

Water Resources Research: Quantifying Renewable Groundwater Stress with GRACE

Water Resources Research: Uncertainty in global groundwater storage estimates in a Total Groundwater Stress framework

Twitter: #PeakWater

Wikipedia: Peak Water

World Economic Forum: Discovering the hidden cost of #VirtualWater

United Nations Intergovernmental Panel on Climate Change (IPCC report AR5 from 2014)

Tedx Talks: Invisible water, the hidden virtual water market | Seth Darling | TEDxNaperville

Circle of Blue: NASA Satellites Reveal Global Groundwater Depletion

Circle of Blue: Groundwater Depletion Stresses Majority of World’s Largest Aquifers

The Washington Post: NASA data show how the world is running out of water

Wikipedia – GRACE

Wikipedia – GRACE-FO (Follow On)

 

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