This annual rise and fall of earth's crust is the largestever detected, and it may one day help scientists tally the totalamount of water on Earth.
“What would you do if you knew how muchwater was on the planet?” asked Douglas Alsdorf, assistant professor ofgeological sciences at Ohio State University. “That's a really excitingquestion, because nobody knows for sure how much water there is.”
Havingan estimate of Earth's entire fresh water cache – from hiddengroundwater, to the world's rivers and wetlands, to mountaintopglaciers – would greatly improve our ability to predict drought,flooding and climate change.
The study appears in the journal Geophysical Research Letters.
Thestudy began in 2004 after Michael Bevis, now an Ohio Eminent Scholarand professor of civil and environmental engineering and geodeticscience at Ohio State, detected an up-and-down motion at a globalpositioning system (GPS) station he'd placed in the ground near a lakein the Andes. He concluded that as the water level in the lake rose andfell, the ground nearby moved in response. At the time, he was aprofessor at the University of Hawaii.
Bevis began to look forsimilar oscillations in data recorded by other GPS stations aroundSouth America. Other scientists had already reported detecting suchchanges up to half an inch in other parts of the globe, but theysuspected that the greatest motion would occur beneath the Amazon RiverBasin, the largest river system in the world. In late 2004, one groupused satellite data to predict that the bedrock beneath the Amazonwould rise and fall about one inch every year.
But when Bevislooked at the data from a GPS station in Manaus, Brazil – near thecenter of the river basin – he saw not a one-inch change, but threeinches.
He recruited Alsdorf to help him couple his data to acomputer model of water flow through the basin. They used a very simpleapproach colloquially called a “bathtub model,” which assumed that thewater level rose and fell uniformly across the Amazon, like runningwater in a bathtub.
They used a simple model because scientistsknow relatively little about the Amazon River Basin, Alsdorf explained.Its sheer size – approximately equal to the continental United States,with a flood area the size of Texas – hinders detailed study.
Likemany researchers, they suspect that the amount of water that flowsthrough the Amazon into the Atlantic Ocean every year is about tentimes larger than that carried by the Mississippi River into the Gulfof Mexico.
“The old joke is, we know the discharge of the Amazon, give or take the Mississippi,” Alsdorf said.
Withcolleagues in the United States and Brazil, Bevis and Alsdorf mergedthe water model and the GPS data to show that between 1995 and 2003 thebedrock around Manaus rose and fell in a regular pattern that coincidedwith the basin's annual flood. The bedrock sank slowly as thefloodwaters gathered, then rose back up as the waters receded. Theaverage change in height was about three inches.
Alsdorf wasquick to point out caveats of the study. The researchers have data foronly one GPS station, and the “bathtub model” is greatly simplifiedcompared to the natural variability in water level throughout theAmazon. What's more, scientists aren't exactly sure of the compositionof the bedrock beneath the basin.
Despite the uncertainties ofthe study, the three-inch oscillation is the most dramatic measured todate, and it's the first known recording of a land mass oscillating inresponse to the flow of a river.
It also raises the possibilitythat scientists could one day calculate the amount of water in theAmazon – that is, they could “weigh” the river system based on how muchit makes the earth sink.
Similar techniques could be used tocalculate the amount water on the planet, but much more data would beneeded from all over the globe, Alsdorf said.
As a first step, heand his colleagues want to install more GPS stations around Manaus andthe rest of the Amazon to see if the sinking varies by location. Hesuspects that similar effects could also be detected in the Congo Riversystem in Africa.
But to monitor water flow worldwide wouldrequire a satellite, and Alsdorf leads the American portion of aninternational team that is proposing a new satellite to do just that.The Water Elevation Recovery (WatER) mission would use radar to measureglobal water levels every eight days.
Data from WatER would givescientists a better estimate of fresh water storage and riverdischarge, and improve models of the global water cycle and climatechange, he said.
Coauthors on the Geophysical Research Letterspaper included Eric Kendrick, senior research associate in theDepartment of Civil and Environmental Engineering and Geodetic Scienceat Ohio State; Luiz Paulo Fortes of the Institute Brasilieiro deGeographia e Estatνstica in Brazil; Bruce Forsberg of the InstitutoNacional de Pesquisas da Amazonas in Brazil; Robert Smalley Jr. of theUniversity of Memphis; and Janet Becker of the University of Hawaii.
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