Drought in Europe

By Helmut Bauch and Jochen Kerkmann (EUMETSAT)

The Metop-A ASCAT soil moisture product (Figure 1) provides an estimate of water saturation of the 5 cm topsoil layer, in relative units between 0 and 100%. The animation illustrates the dry conditions persisting over large parts of Europe since February 2011.

According to a report from the WMO (issued by WMO RA VI Pilot RCC on Climate Monitoring, Lead Centre DWD, on 05 May 2011), a long-lasting dry period persisted over large parts of Europe from January to May 2011. According to data from the Global Precipitation Climatology Centre (GPCC), in particular for the months February to April 2011, there was a significant rain deficit over large parts of Europe. The three-month totals over this period ranged between 40 and 80% of the long-term mean (1951–2000) over large areas (see figure, source: GPCC), and in many parts of central Europe, even below 40%.

The United Kingdom had extremely dry conditions in March and April, especially in southeastern parts, and it experienced its driest March since 1953. Other parts of western and central Europe had a dry February, March and April. Indeed, 2011 was one of the driest 10 year periods in nearly the whole of Switzerland since 1864.

In Germany, spring 2011 (March, April, May) was the driest since the start of regular weather observations in 1893. As a consequence, water levels in rivers were very low, particularly in the Rhine, where shipping was affected (according to the German Federal Hydrological Agency). The dry conditions also caused fires, dust outbreaks and high pollen concentrations. For example, dune fires broke out in the coastal parts of The Netherlands, and in some parts of Germany the Forest Fire Danger Index reached its highest possible level. In northeastern Germany (in the area of Rostock), on 8 April 2011, strong northerly winds triggered a dust storm that caused fatal road accidents on the A19 highway.

There are several ways of monitoring rainfall with satellites:

1. The most direct way is to use cumulative rainfall products like the GPCC products shown above or the accumulated Multi-sensor Precipitation Estimate (MPE) products. As shown above, accumulated satellite rainfall products (especially if blended with ground observations) are well suited to monitor rainfall conditions over large areas like Europe.
2. An indirect way is to use vegetation products that reflect the effect of the rainfall conditions on the density and vigor of green vegetation (e.g. the Fractional Vegetation Cover (FVC) product from the Land SAF). However, while the link between rainfall and vegetation is quite strong (direct) in large parts of Africa (see e.g. the case study from Kenya), in Europe, where you have more forests, lower evapotranspiration and more artificial irrigation, vegetation reacts more slowly to low rainfall. For this reason, the Metop-A Normalized Difference Vegetation Index (NDVI) from 22 May 2011 (weekly composite, source: source: NOAA Satellite Information Service) shows relatively high NDVI values in areas that had not received substantial rainfall for months (e.g. central Germany). Note that NDVI values greater than 0.1, generally denote increasing degrees in the greenness and intensity of vegetation. Values between 0 and 0.1 are commonly characteristic of rocks and bare soil, and values less than 0 often indicate clouds or snow.
3. Another indirect way of monitoring rainfall is to look at soil moisture products, like the ASCAT soil moisture product , that provide an estimate of water saturation of the 5 cm topsoil layer, in relative units between 0 and 100%. As regards the 2011 drought in Central Europe, the image and animation below illustrate the rapid decrease in surface soil moisture during the period April-May 2011. This decrease is partly due to the persisting dry conditions but could also be caused by seasonal effects, i.e. a general decrease of soil moisture in central Europe from early to late spring. A comparison with other years (currently not available) would allow one to find an answer to this question.

The Advanced Scatterometer (ASCAT) on Metop-A is a follow-on to the wind mode of the Active Microwave Instruments (AMI) on ERS-1&2. The prime objective of the instrument is to measure the wind field at the ocean surface. Other mission objectives, which are supported by ASCAT include the measurement of ice boundaries, sea ice concentrations, sea ice type, snow cover over land and soil moisture.

The Surface Soil Moisture product is given in swath geometry. The algorithm used to derive this parameter is based upon a linear relationship of soil moisture and scatterometer backscatter and uses change detection techniques to eliminate the contributions from vegetation, land cover and surface topography, considered invariant from year to year.

Seasonal vegetation effects are modelled by exploiting the multiple viewing capabilities of ASCAT. The processor has been developed by the Institute of Photogrammetry and Remote Sensing of the Vienna University of Technology. The soil moisture product is available in HDF5 format from the EUMETSAT Data Centre, or in BUFR format via EUMETCast.

Additional content

ASCAT Soil Moisture Product Handbook (Version 1.3, source: TU Wien)
Metop Soil Moisture Product Documentation (Vienna University of Technology)
ASCAT Soil Moisture Report Series (Vienna University of Technology)
Precipitation anomaly March 2011, Germany (source: DWD)
Precipitation anomaly April 2011, Germany (source: DWD)