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Gravity probe shows groundwater reserves slipping away


September 25, 2012

Drought bites deeply – this year's corn crop in Missouri Valley, Iowa (Photo: USDA and Dav...

Drought bites deeply – this year's corn crop in Missouri Valley, Iowa (Photo: USDA and Dave Kosling)

Image Gallery (7 images)

Recently, drought seems to be a fact of life. As the lead photograph poignantly illustrates, most of the U.S. has been struggling with serious levels of drought for the past several years. Worldwide, drought affected areas include Europe, India and Pakistan, Russia, much of Africa, South America – the list goes on. But when the rains start again, everyone expresses great relief, not realizing that long-term depletion of groundwater reserves is part of the price for surviving drought. It was with this in mind that GRACE (Gravity Recovery and Climate Experiment), a joint U.S. and German space project, was designed a decade ago.

The GRACE mission consists of a pair of satellites designed to accurately map variations in the Earth's gravitational field. The two identical satellites are in a polar orbit some 500 km (310 miles) above the Earth, spaced a distance of 220 km (137 miles) apart along the same orbit. The distance between the satellites is continually measured using GPS and radar.

GRACE's plot of gravity over the Americas (Image: GRACE/NASA)
GRACE's plot of gravity over the Americas (Image: GRACE/NASA)

To people on living on Earth, gravity appears to be constant. However, it varies slightly from place to place. If there is a massive flood in India, the GRACE satellites are pulled toward the flooded plains a tiny bit more than normal. The first satellite to approach the flooded region will feel a slightly larger gravitational pull than before because of the mass of all that water. The speed of the satellite doesn't change, so it goes to a slightly smaller altitude. This changes the distance between the two satellites, which is recorded to within 10 microns (0.0004 inches), or about one part in twenty billion. By tracking trends in how the satellites’ orbits change, scientists and engineers can calculate how gravity is changing on Earth.

What can we learn from accurate measurements of Earth's gravity? The thickness of ice sheets and glaciers, the temperature of large-scale ocean currents, motions of magma, better profiles of the atmosphere, and also the amount of water stored in the ground.

Groundwater is the source of artesian springs throughout the world. It is also the main source of water for drinking and farming for much of the world. In the U.S., 37 percent of water from growing crops and 51 percent of drinking water comes from groundwater. The water table in India's wheat, rice, and barley-growing northern regions has been dropping over a foot (30 cm) per year. The water table under Moscow fell 300 feet (91 m) over the last century. Groundwater is not only a precious resource, but an overused one.

During a drought, groundwater reserves take multiple injuries. Clearly, farmers use more groundwater for irrigation when rainfall has been insufficient for growing healthy crops. This removes large amounts of water from the aquifers. A second blow is that water which would have penetrated the aquifers to refill them never fell, and much of what did fall evaporated.

It also takes time for surface water to make its way into the aquifers. The Ogallala aquifer in the central states is the largest aquifer in the U.S. It originally held some 865 cubic miles (3600 cubic km.) of water – most of it preserved from the last Ice Age. The rate of refill from surface waters is exceedingly slow, and some predictions suggest that the Ogallala will effectively be depleted within the current century.

A different sort of injury associated with overuse of groundwater is pollution. If a well is near the shore, excessive use is likely to draw the water table locally below sea level, in which case salt water will seep into the aquifer. If a source of pollution lies between the main parts of an aquifer and a particular well, overuse of the well will draw that pollution into the aquifer, but even more so into the water being drawn from the well.

The largest problem met in trying to manage groundwater resources is that the reduction in groundwater has been largely invisible. Scientists could get a feeling for how the aquifers are evolving by measuring water level in wells, but that gives rather crude results, with effects often showing up years after their causes.

The condition of the soil and groundwater over the United States on September 17, 2012 (Im...
The condition of the soil and groundwater over the United States on September 17, 2012 (Image: GRACE/NASA)

All this is now being changed by GRACE. As shown in the image above, GRACE can measure surface soil moisture, root zone soil moisture, and groundwater. Blue areas are high in groundwater, while red areas are dry. The top image is moisture in the upper 2 cm (0.8 inches) of soil, the center image shows moisture at root level (roughly the top meter (40 inches) of soil), and the lower image shows groundwater in aquifers. It is clear that the pattern of the drought is more solidly etched in the groundwater depletion. Surface waters can recover from a few good rains, but it takes more than that to restore water removed from aquifers.

The video below presents a time-lapse view of the groundwater situation for the U.S. over the past ten years. Fresh water is clearly another nonrenewable resource which requires active management in real-time. The next generation of GRACE satellites will help guide the way.

Source: GRACE

About the Author
Brian Dodson From an early age Brian wanted to become a scientist. He did, earning a Ph.D. in physics and embarking on an R&D career which has recently broken the 40th anniversary. What he didn't expect was that along the way he would become a patent agent, a rocket scientist, a gourmet cook, a biotech entrepreneur, an opera tenor and a science writer.   All articles by Brian Dodson

So these satellites are are whizzing by ~500 km above earth and they can tell a 10 micron difference in altitude? That is astounding technology - hard for the mind to perceive.

25th September, 2012 @ 06:40 am PDT

It looks like Nuclear Powered Desalination Plants should be on someone's drawing board soon.

Robert DuBois
25th September, 2012 @ 08:29 am PDT

surely a desalination process can be used to provide irrigation water from sea water and pump it inland? Or pump it down from Canada!

25th September, 2012 @ 09:37 am PDT

Watching the video though, it's clear that the water drought has only been severe in the last 2 years. This is just a common trend in the worlds natural balance. In the first 45 seconds of the video there is a mass amount of blue that circulates around the US, and it's percentage seems to out do the red until the end. I do not think that this should be to much of a concern. Here in 5 years we'll be complaining about all the floods, we cannot control the whether, but just learn to live with it. And if the water isn't in the ground, then it's somewhere, it doesn't just disappear, it just circulates.

It also appears that in the last 3 years the drought focuses around major cities. this may be caused because roads and pavement create a rise in temperatures, and keep the rain water from sinking into the soil. All the runoff goes down drainage systems and gets dumped into the nearest river or lake (flowing away from the city). I believe if we continue to monitor the water levels, we'll see a balance soon enough.

25th September, 2012 @ 10:46 am PDT

And yet at the same time elsewhere we have problems with an overabundance of freshwater. So if we suck excess water from where we have flooding and inject it into the aquifers after treatment we solve 2 problems.

25th September, 2012 @ 11:51 am PDT

One better than a previous post. Build desalination plants around the country and pump that water into the aquifers. Bringing the aquifers back to aspirate levels and distributing fresh water to farms and wells around the country cheaply and efficiently by using what nature had already made.

26th September, 2012 @ 02:27 am PDT

Using solar energy that is free to desalinate the water, around the country where it is dry and use this same free energy to power pumps and get the water around the country

Criss Marvin Dawn
26th September, 2012 @ 07:54 am PDT

Fracking destroys aquifers, doesn't it? All of the water used - pulled from below ground - is not recycled. It becomes pollution.

26th September, 2012 @ 09:37 am PDT

Some water users need to get real. Building cities in the desert then sucking up water resources from rivers and groundwater to make them "viable" is plain and simple madness and cannot continue. Most of Aridzona should not be populated, same for Nevada.

26th September, 2012 @ 09:48 am PDT

What kind of statement is “Fresh water is clearly another nonrenewable resource.” Isn’t it called a “Water Cycle” for a reason; just because we are outpacing the cycle with the increased demand for water does not make it “nonrenewable!” If I remember my Science classes from childhood; water does not disappear, it changes stages between solid; liquid and gas. It has 3 different cycles accruing at the same time; the largest cycle is ice to water to steam to rain back to ice. The intermediate cycle uses the earth’s crust as a filter and deposits fresh water into the aquifers where it’s then pumped to the surface used and then filtered back down into the aquifer. The shortest cycle is rainfall, evaporation, rainfall.

Warm weather can speed up evaporation or melt ice faster but it does not help the filtering of clean water; the aquifer cannot be replenished itself any faster than it is doing so naturally. If we have the technology to build cities in the deserts; we should be able to find a safe man-made way to help the aquifer keep up with our water demands. Understanding just how long it takes for fresh water to recycle itself naturally should be the first step in producing any kind of solution to aid our water shortage issues.

26th September, 2012 @ 11:01 am PDT

re; KMH

No fracking does not destroy or even pollute aquifers. Faulty well pipe lining have pollute ground water but not the fracking that happens several miles away from the aquifers.

re; kelvint63

Aquifers may not naturally replenish any faster but if water is injected by unnatural means such as pumping water down wells the aquifer is replenished faster.

26th September, 2012 @ 12:18 pm PDT

I think the bigger problem isn't that aquifers aren't naturally renewable, but that if you stress them too much, you can destroy their quality. Once that happens, even if the aquifers are replenished (which according to the article seems dubious even with "nuclear desalination" who pays for that?), you may never get a working aquifer back. Unfortunately it's once again the problem that conserving resources never ends up on a balance sheet. No one heralds using less.

26th September, 2012 @ 03:10 pm PDT

Desalination is only a short term answer. Look up what the bi-products are for the desalination process and you will understand why they won't work longer term on the scale required to feed the aquifers..

27th September, 2012 @ 02:38 am PDT

re; Tommo

The bi-products of desalination is brine with concentrations of industrial chemicals that can be profitably extracted.

There is also the bi-products of the energy used and if they're handled competently no cause for concern.

27th September, 2012 @ 01:51 pm PDT

Slowburn loves telling one side of the story.

Here in South-western Australia we have been running out of water for decades, possibly centuries.

Desalination plants are a reality but extracting minerals from the brine is not profitable. It is returned to the ocean.

Sewerage recycling is also on the horizon and is a reality in Queensland.

Fracking seriously disrupts farming and there is a growing backlash.

1st October, 2012 @ 03:59 am PDT
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