Soils of UK and Europe drying out

SMOS satellite images showing europe drying out between March and April 2011

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As much as we all enjoy the warm weather, some rain would be welcome.

The scale of just how dry the start of 2011 has been is evident in some fascinating data from one of Europe's latest Earth observation satellites.

Smos senses the moisture in the top layers of soil, and it is very clear in these maps that the ground across the UK and much of Europe is now gasping for water.

Last month was the warmest April on record in Britain.

It was also the 11th driest month (since 1910), with on average just half the usual rainfall. And in parts of south-east England, there was less than 10% of normal precipitation.

Smos artist's impression (Cesbio) "Space chopper": The Smos radiometer makes the satellite look like a helicopter

Smos is an experimental mission of the European Space Agency (Esa), and is providing some novel information to meteorologists, hydrologists and other scientists interested in how water moves around the globe.

The 760km-high satellite carries an 8m-wide interferometric radiometer that senses the natural emission of microwaves coming up off the planet's surface.

Variations in the water content of soils will modify this signal.

The maps at the very top of this page - which were specially prepared for the BBC - are monthly averages. Blue colours denote wetter earth; yellow colours show drier conditions.

The comparison between the two maps illustrates neatly the process of drying out through March and April.

Data from Smos agrees well with recorded rainfall patterns.

Smos data This figure shows the averaged difference between the first and the last week of April. The yellow-reddish areas indicate that soils over the UK and northern France were significantly drier in the last week of April than in the first one

Traditionally, meteorologists have used air temperature readings taken just above the ground to infer soil moisture content. Smos is providing a new means to get at this information.

Rainfall anomaly (Met Office)

Understanding how much water is held in the earth under our feet is a key variable if you want to make weather forecasts.

If soils are wet then much of the energy coming from the Sun will be used to evaporate water from the ground, keeping temperatures cooler than they would otherwise be. Generally speaking, more precipitation would be initiated as well.

But if soils are dry, then the water available for evaporation is limited. As a result, the incoming solar energy will warm the surface, raising the temperatures above it. Over a sustained period, this can lead to drought conditions.

At the European Centre for Medium-Range Weather Forecasts (ECMWF), researchers are experimenting with Smos data.

The ECMWF, which is based at Reading, UK, produces forecasts that look 10 days ahead and more, including making seasonal forecasts.

"We already have soil moisture data in our forecast system which does not use Smos, and our soil moisture data is already quite good. But there is potential to improve it with Smos," said the centre's Dr Patricia de Rosnay. "At the moment we are not sure at what range into the future, Smos data will be most useful to us in making forecasts."

UK MARCH & APRIL COMBINED

  • 1929 - 57.8mm - Driest (since 1910)
  • 1947 - 258.2mm - Wettest (since 1910)
  • 2011 - 86.5mm - 4th driest (since 1910)

April alone was the 11th driest month (since 1910 for the UK) at 36.7mm

Some studies that were completed after the great European heatwave of summer 2003 showed that if more had been known about the relative dryness of soils in the springtime, meteorologists might have been in a much better position to predict the extreme conditions that followed a few months later. It is an area for ongoing research.

Smos was launched in October 2009. Its radiometer instrument will also measure variations in the saltiness of seawater, a capability which is reflected in the mission's full name: Soil Moisture and Ocean Salinity (Smos) satellite.

The investigation of soil moisture data is led from the Centre for Spatial Studies of the Biosphere (Cesbio) in Toulouse, France.

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