October 5th, 2005
Is the Earth elastic?
Scientists from Brazil and the U.S. think so. In a surprising discovery, they’ve found that a GPS station in Manaus, near the center of the Amazon River basin, showed that the Earth level was going up and down by almost 3 inches (75 mm) every year with the seasonal floods of the big river. In other words, Earth sinks three inches under weight of flooded Amazon. So far, this study has only used GPS dat, but a new mission using satellites is under preparation. Proving that the Earth is elastic is only one aspect of this project. The researchers also want to estimate more precisely what is the total amount of water on Earth, which to my surprise, is today completely unknown. But it would "greatly improve our ability to predict drought, flooding and climate change," according to the scientists.
According to the Ohio State University news release mentioned above, two of its scientists, Doug Alsdorf and Michael Bevis used a simple model for simulating the water flow through the Amazon River basin.
They used a very simple approach colloquially called a "bathtub model," which assumed that the water level rose and fell uniformly across the Amazon, like running water in a bathtub.
They used a simple model because scientists know 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.
This kind of data tends to make me feel humble: Texas is about 30% greater than France! Imagine a flood covering France!
Anyway, let’s go back to what the researchers found.
With colleagues in the United States and Brazil, Bevis and Alsdorf merged the water model and the GPS data to show that between 1995 and 2003 the bedrock around Manaus rose and fell in a regular pattern that coincided with the basin’s annual flood. The bedrock sank slowly as the floodwaters gathered, then rose back up as the waters receded. The average change in height was about three inches, [three times more than they expected.]
Below is an image of an "overlay of two radar mosaics constructed by the Global Rain Forest Mapping Project shows the patterns of flooding in a section of the central Amazon River Basin. The image is nearly centered on the GPS station in Manaus, Brazil. One mosaic was acquired during the low-water period of late1995, and the second during peak stage in 1996. Dark blue indicates channels that always contain water, while white depicts floodplains that seasonally flood and drain, and green represents non-flooded areas." (Credit: OSU).
Here is a link to a larger version of this image.
But does this study prove that Earth is more elastic than previously thought? According to the researchers, they need to improve both the computer model they used and replace GPS stations by satellite observations.
This is the goal of the proposed WatER satellite mission (WatER meaning "Water Elevation Recovery"), which "would use radar to measure global water levels every eight days."
Will this proposal be accepted? I’ve no idea, but at least it was presented to the National Research Council Decadal Survey in this response to their request for information on May 13, 2005 (PDF format, 30 pages, 3.41 MB).
For more information, this study about an elastic Earth has been published by Geophysical Research Letters under the name "Seasonal fluctuations in the mass of the Amazon River system and Earth’s elastic response" (Vol. 32, L16308, August 24, 2005). Here is a link to the abstract.
In addition, Geophysical Research Letters published two other papers this year about the same subject: "Improved estimation of terrestrial water storage changes from GRACE" (Vol. 32, L07302, April 6, 2005) (link to the abstract) and "Water slope and discharge in the Amazon River estimated using the shuttle radar topography mission digital elevation model" (Vol. 32, L17404, September 8, 2005) (link to the abstract).
Sources: Ohio State University news release, via EurekAlert!, October 4, 2005; and various web sites
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