The SMOS soil moisture map will soon be in use by meteorologists and climate modellers round the world. Credit: CESBIO/ESA

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The first satellite-derived map of global soil moisture has been unveiled today at the Living Planet Symposium, an Earth-observation conference being held this week in Bergen, Norway.

The Soil Moisture and Ocean Salinity (SMOS) mission — part of the European Space Agency's (ESA) extended 'living planet' programme on Earth observation — was launched in November 2009. After a six-month commissioning and calibration period, the instrument became operational on 21 May.

Data from SMOS have now enabled researchers to assemble a comprehensive map of global soil moisture that covers all land areas of the world, except for frozen soils at high latitudes and in some mountain regions.

The map, generated on 24 June, depicts features such as the unusual current soil dryness in the southern United Kingdom, and the relative soil wetness in parts of western Africa caused by recent abundant convective rainfall.

The most surprising features on the map, however, are the relatively high soil-moisture values throughout the central United States, where scientists would have expected much drier soil at this time of year.

The SMOS satellite carries an interferometric radiometer that captures images of 'brightness temperature', a measure of the microwave radiation emitted from Earth's surface. Complex algorithms are used to process the raw data and turn them into global soil-moisture maps every three days. Maps of ocean salinity, not yet released, will be produced every 30 days.

"The instrument is behaving better than expected, and the first results have arrived earlier than we would have hoped," says Yann Kerr, director of the Center for the Study of the Biosphere from Space in Toulouse, France, and principal investigator for SMOS.

Heat trigger

Soil moisture is thought to be an important variable in projections of weather and climate. Researchers suspect, for example, that the record-breaking European summer heat wave in 2003 resulted, at least in part, from unusual dryness in the preceding spring1.

But soil-moisture measurements have so far been few and patchy. The SMOS data could therefore make seasonal weather projections more reliable in the future. They will also be used for hydrological studies and for agricultural and water-management purposes, says Kerr.

Scientists at the European Centre for Medium-Range Weather Forecasts in Reading, UK, will be the first to inject real-time SMOS data into their projections. National meteorological offices in France, the Netherlands and Australia will soon follow suit. And meteorologists, climate modellers and water-cycle researchers from numerous countries are lining up, waiting for ESA to release the first validated data sets.

'Ground truthing' of satellite-derived data is a crucial aspect of any Earth-observation mission. The SMOS data will be routinely validated against harmonized in situ measurements taken on the ground, such as those collected under the recently established International Soil Moisture Network. This validation process is under way, but will require some time to complete, says Kerr.

Meanwhile, radio-frequency interference in some parts of Europe and Asia remains a hindrance to the mission's global success. The radiometer instrument operates in a frequency band (1400–1427 megahertz) that is protected by the International Telecommunication Union, an agency of the United Nations. Yet some television and radio stations, WiFi providers and military radars in China, India and Europe seem to illegally emit frequencies in the protected wavelength range — thus interfering with the SMOS instrument.

"We will need to identify and isolate the sources to make sure our data don't get damaged," says Kerr. "We will then report their locations to national entities so that they can take legal action."

This comprehensive satellite map of soil moisture covers most land areas of the world, and was produced from data gathered by the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) mission. Its most surprising finding is the unusual wetness in western African and central US soils. (Dryness is indicated by blue, wetness by red.) Credit: CESBIO/ESA