Fires and rapidly growing burn scars

By Jochen Kerkmann and HansPeter Roesli (EUMETSAT)

In Northern Africa, the Sahara Desert transitions into a semi-arid zone of grass and scattered shrubby vegetation known as the Sahel. Farther south, the Sahel turns into more thickly vegetated savannah. For thousands of years, people living in West Africa have burned extensive areas of the savannah to manage natural resources, for example, to clear brush, to stimulate new growth of pasture grasses, or to drive game.

The fires begin the moment the annual rains stop falling (October, November) and they continue throughout the long dry season (up to March/April). Burning ends only after new rains have fallen; by this point thousands of square kilometres of vegetation are affected. In many areas of the savannah over half of the landscape is burned by fire on an annual basis.

Multi-channel imagers like SEVIRI on MSG and MODIS on Aqua/Terra continuously monitor the fire and vegetation situation over Africa. They also monitor smoke plumes from fires (see case study Extended period of dense smoke over Argentina) and the effects of fires on vegetation (burn scars). On MSG images, burn scars can be well seen in VIS0.8 images (which is very sensitive to vegetation greenness) or in vegetation products, including the Natural Colours RGB product that combines the three solar channels VIS0.6, VIS0.8 and NIR1.6.

The MSG images below show one example of rapidly growing burn scars over southern Mali in October 2009, at the very beginning of the dry season when the Niger Inland Delta was still flooded from waters of the Niger and Bani rivers. Initially, the rapid growth of the dark areas in the RGB product seemed to indicate flooding, but after inspection of the IR channels (see IR3.9 loop, 17 Oct 2009, 12:00–17:00 UTC) it became clear that multiple fires driven by strong easterly winds had produced the long, narrow burn scars indicated with white arrows in the images below.

The two-day close-up loop of the Natural Colours RGB, 17–18 Oct 2009, 09:00–17:00 UTC) shows the rapid westward extension of the burned area on 18 October. It is not known whether this development was planned or if the fires simply got out of control (pushed by strong easterly winds).

As expected, MODIS images/products confirm the outbreak of the fires in southern Mali on 17 October which led to the large burned area on 18–19 October (see MODIS True Colour RGB images with fire pixels, source: NASA/GSFC, MODIS Rapid Response). Typically for fires in the Sahel area, the smoke plumes are faintly visible on satellite imagery. As shown on the last MODIS image, by 28 October, most of the fires were extinguished and the reflectance of the solar channels had already recovered (burn scars are less dark).

Note: the monitoring of fires in Africa is getting a boost through the AMESD project at the Agrhymet Regional Centre in Niamey.

Burn scars in the Sahel area can also be observed in the High Resolution Visible (HRV) images of MSG SEVIRI. However, between 00:00 and 14:00 UTC the African (lower) HRV window is east justified (see example), i.e. the HRV channel only covers the area east of 0 degrees longitudes (to optimise the coverage of the Indian Ocean region). Therefore, the case described above was not well observed by the HRV channel (only after 15:00 UTC).

To demonstrate the use of the HRV channel for daily burn scar monitoring, a fire case from southeastern Burkina Faso, not far away from the Mali case described above, was chosen (see active fire monitoring (FIR) product, 26 Nov 2009, 10:00 UTC). The fires started around 19 November 2009 and lasted for several days. The Meteosat-9 HRV animation (19-24 Nov 2009) shows the hourly and day-to-day increase of the burned areas (dark patches) and the faint smoke plumes. At the end of the period, on 25 November 2009, a large area in southeastern Burkina Faso and northern Benin was burned (Figure 5).

The yearly cycle of rain season (growing vegetation, floods) and dry season (fires, burn scars) can be well observed in long-term Natural Colour RGB animations composed of daily 12:00 UTC images. Two examples are given below: one for the area around Lake Chad (Cameroon, Nigeria, Niger, Chad), and the other one for the area around the Etosha Pan and the Okavango Delta (Namibia, Botswana).

Unfortunately, clouds are contaminating/disturbing the scene, but if one concentrates on the land features, one can clearly see the vegetation cycle, the annual floodings (Chad Lake, Etosha Pan, Okavango Delta, Zambezi River) and the annual burnings (sudden dark patches in the images). In addition, the long-term 'Chad' loop shows a large number of dust clouds in the area of the Bodele Depression.

Long-term, (2 years) animations

Short-term, (1-2 months) animations

Additional content

Burning the Seasonal Mosaic: Preventative Burning Strategies in the Wooded Savanna of Southern Mali
(Paul Laris, 2002, Human Ecology, Vol.30)
Wild fires threaten the city of Athens (23 August 2009)
Catastrophic fires over Portugal and Spain (3 August 2003)
Large smoke plume over the Mozambique Channel (1 September 2008)