Soil H2O from space

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Soil moisture around the Amazon, gauged from space

A plot of microwave “brightness temperatures” measures the water content of the soil around the lower Amazon. This is an early result from Europe's Soil Moisture and Ocean Salinity satellite, SMOS, launched in November 2009 and formally operational since late May 2010. It's a sample of what is now becoming available globally every 3 days, and SMOS should help to fill a huge gap in human knowledge of the water cycle. Credit: ESA/SMOS.

Despite repeated claims that the weather system is known well enough for making multi-decade forecasts of the changing climate, the ignorance can be truly profound. The global water cycle is a key theme where understanding is poor and reliable data are notoriously scanty. The Intergovernmental Panel on Climate Change avoids forthright language that might dismay those who want to treat its findings as gospel, yet in relevant parts of Working Group I’s 2007 report there are hints of desperation. See Section 3.3: “Changes in Surface Climate: Precipitation, Drought and Surface Hydrology”, available here:

Soil moisture is crucial

  • as a climate factor in its own right, vital for natural and cultivated plant growth, and for the management of water resources
  • as a strong influence on the global carbon cycle by its contribution to terrestrial photosynthesis
  • as a way station in the hydrological system’s non-stop traffic between rainfall, evaporation, transpiration from plants, and run-off into streams and rivers.

If you want ever to model the water cycle accurately, you really must know the soil moisture content. The European Space Agency claims for SMOS an accuracy of 4%, “comparable to detecting one teaspoon of water mixed into a handful of soil” from an altitude of 760 km. A calculation gives the water content in soil to a depth of 1-2 metres – the root zone.

How’s it done? The presence of water affects the electromagnetic behaviour of soil, and hence the intensity of radio microwaves that it radiates. But the long wavelength, about 20 cm, of the most useful microwaves implies that you need a large radio telescope in space to observe them well.

SMOS, pictured by ESA

The L-band radiometer on SMOS, called MIRAS, uses the radio astronomers’ technique of aperture synthesis. It simulates a much bigger antenna with 69 small antennas strung out along three arms arranged in a Y, and so resolves areas 50 km wide within a hexagon on the Earth’s surface about 1000 km across.

The same instrument on SMOS gauges the saltiness of sea water, using the electromagnetic contrast between pure water and salty water – hence “Ocean Salinity” in the mission’s name. But that’s another story.

Good luck to mission manager Susanne Mecklenburg, mission scientist Matthias Drusch, and all the SMOS team.

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