National Geographic News
Photo of the San Joaquin River in California.

The San Joaquin River rounds a bend north of Kerman, California. Just downstream, the river runs dry.

Photograph by Peter Essick, National Geographic

Michelle Nijhuis

for National Geographic

Published August 21, 2014

Throughout the western United States, a network of Global Positioning System (GPS) stations has been monitoring tiny movements in the Earth's crust, collecting data that can warn of developing earthquakes.

To their surprise, researchers have discovered that the GPS network has also been recording an entirely different phenomenon: the massive drying of the landscape caused by the drought that has intensified over much of the region since last year.

Geophysicist Adrian Borsa of the Scripps Institution of Oceanography and his colleagues report in this week's Science that, based on the GPS measurements, the loss of water from lakes, streams, snowpack, and groundwater totals some 240 billion metric tons—equivalent, they say, to a four-inch-deep layer of water covering the entire western U.S. from the Rockies to the Pacific. (Related: "Water's Hidden Crisis"

The principle behind the new measurements is simple. The weight of surface water and groundwater deforms Earth's elastic crust, much as a sleeper's body deforms a mattress. Remove the water, and the crust rebounds.

As the amount of water varies cyclically with the seasons, the crust moves up and down imperceptibly, by fractions of an inch—but GPS can measure such small shifts. (Related: "California Snowpack Measure Shows No End in Sight for Drought")

Borsa knew all this when he started to study the GPS data. He wasn't interested in the water cycle at first, and for him the seasonal fluctuations it produced in the data were just noise: They obscured the much longer-term geological changes he wanted to study, such as the rise of mountain ranges.

When he removed that noise from some recent station data, however, he noticed what he describes as a "tremendous uplift signal"—a distinct rise in the crust—since the beginning of 2013. He showed his findings to his Scripps colleague Duncan Agnew.

"I told him, 'I think we're looking at the effect of drought,'" Borsa remembers. "He didn't believe me."

The Dry Land Rebounds

But Borsa was right. As he, Agnew, and Daniel Cayan of Scripps report in Science, the recent uplift spike is consistent across the U.S. West, and consistent with recent declines in precipitation, streamflow, and groundwater levels. With a great weight of water removed, the crust is rebounding elastically across the whole region.

The median rise across all the western GPS stations has been four millimeters, just under a sixth of an inch. But the Sierra Nevada mountains, which have lost most of their snowpack, have risen 15 millimeters—nearly six-tenths of an inch.

Four maps showing a time series of uplift and subsidence as it relates to drought.

The GPS data complements satellite observations from NASA's ongoing Gravity Recovery and Climate Experiment, or GRACE. The GRACE satellites measure small changes in the Earth's gravity field caused by the movement of water on and under the Earth's surface, allowing researchers to estimate groundwater and soil moisture conditions around the world. GRACE can operate where GPS networks don't exist—much of Africa and South America, for instance.

But where it's available, as in the western U.S., GPS data can provide a more rapid and detailed picture of drought and its causes.

"We only see the big picture," says Stephanie Castle, a water resources specialist at the University of California, Irvine, and the lead author of a recent study that used GRACE data to quantify groundwater loss in the Colorado River Basin. "The uplift data can point out more specifically where the depletion is happening."

Where the Water Goes

This new precision has big political implications: With more than 99 percent of California still in a severe drought, and rights to its surface water severely overallocated even in a good year, many of the state's farmers are supplementing their water supplies by pumping more water from underground aquifers.

In the Central Valley so much groundwater has been extracted that the ground has subsided more than 30 feet in some places—swamping the much smaller regional uplift caused by the elastic rebound of the underlying crust.

Photo of a farm worker in California.
A farmworker shifts a pipe near Huron, California. The drought has already cost field-work jobs and will likely cost more.
Photograph by Peter Essick, National Geographic

California has some of the weakest groundwater regulations in the nation, and access to its well-drilling records is highly restricted. The GPS data isn't detailed enough to point fingers at individual farmers, but its 125-mile resolution is good enough to identify especially profligate regions.

As climate change worsens water stress throughout the American West and beyond, such knowledge may well be vital. Borsa and his colleagues started out trying to filter the noise of the water cycle out of the GPS data; they ended up showing that the GPS network could help reveal what's really going on with water.

"All of a sudden we've turned the whole thing around," Borsa says. "It's a huge change, and it makes the network useful to whole new branches of scientists and managers."

Follow Michelle Nijhuis on Twitter.

julie Keeney
julie Keeney

I live close to the river pictured above.  There are a lot of different things going on I think that have put us where we are now.  Historically, this valley, before it was settled, before any wells were drilled, it would flood every spring and make huge wetlands covered in shallow water, and it gave everything a thorough watering and flowed down to the water table, then slowly drained back into its unaltered river banks and went back to being a river, that had salmon and every kind of wildlife.  It also had a huge land locked fresh water lake called the Tulare Lake Basin.  Then we came along, we dried up the lake, we dewatered almost all of the wetlands, we stuck our straws into the water table and started sucking, and then later we put a lot of our canal system underground into pipe lines and covered up vast areas with blacktop and pavement.

Water attracts water.  We have no lake in the middle of the valley anymore, we have no wetlands anymore, except for the saline and alkaline laden ones along the delta.  We have no surface water sufficient to encourage the clouds to rain here anymore.  Our spongy mountains have had all the moisture drained out of them, and when we do have a rainy year, we can only hold so much before they have to start releasing for fear of dam failure.  Then you have the good intentioned people trying to restore salmon to the river, which demands a certain amount of water to keep going, even though I think they have been gone for too long, the water's too shallow and too hot for them now, and I don't think there is enough in that river for them to eat.  We'd be better off trying to keep the salmon runs we still have healthy and maybe aim for a steelhead or bass population in the san joaquin.

And lastly, when they did all the hydro mining in our mountains here, they blew so much sand, silt and rock down into our rivers that they became silted up and are now unable to hold much in the way of storage water.

In this area, our leaders I feel are playing games trying to turn water into the next big commodity, or else they would have done something about this by now.  We need to have in reservoir storage about 5 years back-up supply in water.  We need to dredge the accessible river beds out so they can hold more water, thereby keeping it colder and holding larger quantities which is good for the fish.  They need to refill the Tulare Lake Bed.  JG Boswell can just go into water storage biz instead of cotton.  They need to keep water in all of our storm drain basins until it's clear that they are going to have more rain than they can hold.  We need to get over the whole green lawn thing.  And lastly, our farmers, who have cried the loudest, because after all they cannot grow food without water, they need to lay off so much pumping.  Even with record cuts to their water supplies, they have still managed to produce record amounts of food.  Don't plant the most water intensive things you can plant, and instead, maybe we could not throw so much of it away through the culling process.  No one cares if all the apples are the same size or not.  We also need to invest in more water storage, whether it's through deepening the river channels, digging more hole or whatever, we need more storage.

Guerry McClellan
Guerry McClellan

Epiorogenesis, non-tectonic uplift, described in Florida 40 years ago by Opdyke et al. That case was the result of karstification and spring discharge and resulted in vertical displacements measured in tens of feet. 

I was impressed by this report until I got to the amount of uplift.  Six tenths of an inch is a very small uplift, when measured by GPS.  Even with the very best GPS and a local reference beacon, the quoted resolution if often only about 0.5 inches. So one has to be concerned about the measurements.  Are these measurements accurate or precise?  With the absence of hundreds of local reference points, these results may be very precise, only showing the reproducibility of errors when making a measurement.  That does not mean they are accurate.

The most persuasive observation in the implied "uplift" over such a large area.  Again, who made these measurements and how unbiased is the data collection?  Until the data analysis is complete, this remains only a very interesting potential interpretation.

Gwendolyn Mugliston
Gwendolyn Mugliston

Disproportionate distribution of water and drought, forest fires, and economic troubles associated with water have been going on for years and years in California.  What is interesting to me is the people keep wanting and electing men and women who don't seem to understand the long term implications of drought, using the ground waters until there is no more and huge populations.  

One of the most interesting implications bearing fruit will be the mass migration of peoples from CA, NM and AZ, maybe NV and UT.  Where do they think they are going to go and what people will they try to displace?  

I look forward to people talking about this because in my opinion desalinization plants, astro turf and composting toilets will be far too expensive to erect or buy in the amount of time left. Maybe CA should try to combine all those wonderful things with birth control?  Wowzie, I can imagine the bruhaha that would cause.  

It's just my opinion.  

Thomas Meixner
Thomas Meixner

Michelle,  Thanks for the chat on Twitter.  My concern about interpretations of this particular study is the pattern I have observed in the past of people taking hydrologic science results out of context.  My guess is in the next few days there will be headlines declaring western ground waters over tapped because of this study.    Now in many places there is certainly unsustainable groundwater pumping particularly in California's Central Valley.  

I certainly agree with you that this study our of Scripps is very nice technically.  IN fact I would argue the Pattern in this study supports a reinterpretation of the Castle et al. paper from GRL in July.  The strong uplift outside of irrigation districts and areas of high pumping indicate that at least part of (and maybe a big part of) the groundwater loss seen in their work is due to drainage of groundwater systems in mountains and elsewhere as opposed to just groundwater.

Admittedly my concerns here are in part mercenary, I think we know far too little about how mountain systems work in response to climate forcing and to really understand the signal in groundwater storage that Borsa et al see we need to improve this understanding.  As an Exmaple  several years ago I was talking with Graham Fogg from UC Davis about this challenge and he mentioned a story from the 70's drought in California-

The California  Department of Water Resources uses an empirical relationship to estimate runoff from snowpack observations in the Sierra Nevada.  When the drought ended the empirical formula predicted more flow than actually happened.  The best explanation was that the normally very high runoff coefficients from the Sierra were in error because a significant amount of water seeped into the bedrock.

Borsa's work gives us away to potentially quantify that effect in advance now which is awesome.



PS  The link out to the article in Science appears to lead somewhere else at Science. 

Andrew Jackson
Andrew Jackson

And how do they know the ground deformation is happening due to the water and not other natural forces?

megan mckinney
megan mckinney

Since the ground is shifting up and down so much it should be interesting to see what would happen when the drought ends. Rivers could bend toward lower land, there can be lakes and ponds that were never there before and lakes would dry off. Also I wonder if it's affecting fault and earthquake activity at all? After all some piece only move centimeters per year, would the friction increase if the piece were to suddenly move up and down and if so could there be a higher potential for earthquakes?

Courtney Pelzel
Courtney Pelzel

Grass lawns need to go.  The artificial turf looks better and the cosmetic grass many of us are used to benefits nothing and no one.  (Except the yard guy) And is a huge waste of water.

Jason Usborne
Jason Usborne

I think it is fair that California should outlaw water toilets. The entire state should retrofit every commode with composting toilets. Water desalination plants also need to be constructed along with solar co-generation plants to power them. For individual homesteaders and businesses, one may absorb water from the air and purify it.

Paulo Simoes
Paulo Simoes

We need to smarten up on this water issue quick or there's going to be a lot of problems down the road for sure.  

G. Hunt
G. Hunt

@julie Keeney I dream of a world where politicians have the balanced sensibility that you have expressed in your well-articulated post. 

George Jimenez
George Jimenez

@Andrew Jackson

There are no guarantees in science. However, correlation in time, despite not being dispositive on causation, can be quite persuasive in these matters.  This is particularly true in cases where it is not feasible to set up a physical experiment to test the hypothesis. 

In short, a strong correlation between the subsidence and the quantity of water suggests the relationship, including the cyclic nature of the data within a year, and the variation from year to year under different water conditions.  Further study in science is always justified (over time in this geographic area and also other parts of the planet).

Scientific conclusions, properly worded, are best phrased as "most likely explanation" or "most probably" or "possible" or "unlikely."  Often times, scientists eliminate these qualifying words, which recognize that the truth of any conclusion, most likely :), lies on a probability distribution whose range is (0,1), not [0,1].

craig hill
craig hill

@megan mckinney What makes you assume the drought will end? What if we are entering an arid period akin to that of the desert SW? Global heating is not stopping, it's accelerating, which means much more and much worse.

Quakes could go either way, and probably will, more for the rebound up. It doesn't have to go up AND down like a trampoline to shake a quake into being. It just needs to shift up or down, which dislodges the structure below keeping everything calm until a fault slips.  

Veronica  V.
Veronica V.

@megan mckinney I am no scientist but I have thought that very same thing.

And I have been hearing other people remark that they think we may have a sudden rash of earthquakes! People that are quite old have made these remarks that have lived in CA their whole lives.

G. Hunt
G. Hunt

@Courtney Pelzel Artificial surfaces are environmentally problematic on multiple levels. Impervious cover, the manufacture and decomposition, the opportunity cost of not using a yard as a place where native plants can still live and attract native insects and birds. I truly hope that's not a thing in So Cal, covering land with plastic. Ugh.

Deane Alban
Deane Alban

@Courtney Pelzel Especially golf courses. 

grand mesa
grand mesa

@Paulo Simoes ................sorry, but it's a bit late already. You sound like a recording 35 years ago.


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