Mapping Earth's hidden water

A new NASA satellite will measure the planet's soil moisture.


On Jan. 29, at California's Vandenberg airforce base, a rocket will shoot a satellite into space where it will unfurl an antenna shaped like a giant circular fence. From its orbit, 426 miles above Earth, the Soil Moisture Active Passive instrument, or SMAP, will collect observations of the hidden water in the dirt beneath our feet.

A new NASA satellite will measure the moisture lodged in the earth's soil and help scientists understand how ecosystems are responding to global warming.

It’s NASA’s fifth ever “mission to planet Earth,” a fitting tagline, says Dara Entekhabi, a professor at Massachusetts Institute of Technology and the project’s lead scientist. “There’s some stuff that we know more about on other planets than on our own.”

Now scientists are turning to a space-bound observatory to fill in the gaps of their understanding.

Soil moisture is one of those gaps.

It’s also a key barometer for everything from plant growth to daily weather, to where floods are likely to occur. Variations in soil moisture affect weather by adding or limiting moisture to the atmosphere, enabling cloud formation or intensifying dry spells during periods of drought. Understanding the availability of that water is crucial if we are to predict how landscapes are responding to a warming climate, says Alejandro "Lejo" Flores, a geoscience professor at Boise State University.

For decades, scientists studied the land and atmosphere separately, but the advent of super computers in the late 1980s helped reveal that hydrological and carbon cycles are actually connected. The lynchpin of that connection is soil moisture.

The trouble was that existing ground-based measurements were too sparse to show detailed variations, and in many parts of the world they were non-existent. What researchers needed was a way of taking a snapshot of moisture levels around the entire globe across time and space. 

Five years ago, the European Space Agency launched a soil moisture satellite that provided data at a spatial resolution of about 25 miles – not nearly small enough to capture the true variability in places like the Western U.S., where moisture levels can differ radically across small areas.

Scientists anticipate that a more accurate account of soil moisture will have a variety of applications – from improving weather forecasting and natural disaster predictions to bolstering climate models.

More than two decades after the idea for SMAP took hold, and after an initial attempt failed in 2005, their efforts are about to take off. The final price tag: $915 million.

Once in orbit, SMAP will fly between the poles and peer – via microwave radiation – into the layer of soil that covers the earth everywhere it’s not frozen or under water. Using the data NASA will construct two maps for scientists to use: a soil moisture map with a resolution of 6.2 miles and a map tracking the freeze-thaw state of the soil with a resolution of 1.9 miles.

To understand why such high resolutions matter, take the flash floods that hit Fort Collins, Colorado, in 1996.  Since no one anticipated the severity of the flooding, scientists have used the event as a case study to see what would it would have taken to make better predictions – a process know as “hindcasting.”

Researchers knew that they needed better measurements to capture the true variability of soil moisture of the surrounding land: a patchwork of wet and dry ground, from irrigated fields to forest burns. They hope SMAP will provide the kind of resolution they need to better predict future floods.

And as temperatures rise, these kinds of measurements will prove even more crucial – particularly in places prone to extreme weather. To understand drought, for instance, scientists need to understand how certain soils dry out during times of water stress. SMAP’s data could also help probe some longstanding climate-related mysteries, like whether there’s a correlation between rainy Aprils in the Pacific Northwest and dry Julys in the Great Plains.

“It’s being able to flick a switch and see a whole new part of the landscape,” Lejos says. 

Sarah Tory is an editorial intern at High Country News.

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