Student - September 14, 2006

State of vegetation in Sahel is worsening

Studies using satellite images have showed for years that agricultural areas in Sahelian countries are not declining in quality. Nevertheless there has been deterioration, say Dr Lars Hein of the Environmental Systems Analysis group and Dr Nico de Ridder of the Plant Production Systems group.

The Sahel is vital for the eighty to one hundred million pastoralists in the area from Senegal to Somalia, but the area has experienced many droughts. To determine whether the savannah is degrading, researchers look at how efficiently rainwater is converted into vegetation. Rain use efficiency is the most important indicator of deterioration. Satellite images show that this factor remained fairly constant between 1980 and 2000 in the Sahel countries, from which one would conclude that the savannah is not degrading.

However, this is not the whole story, according to Hein and De Ridder. Analysis of the satellite images also needs to take into account the rainfall pattern in the Sahel in the last 25 years. The researchers calculated this for six areas, and their results show that that the efficiency with which rainfall is converted into vegetation is highest in years with an average amount of rainfall. In very dry or wet years the efficiency decreases. The researchers therefore think that the vegetation in the Sahel has deteriorated in the last 25 years. The annual amount of rainfall in the area during the dry years around 1980 has now become the average amount twenty years later. That would mean that the rain use efficiency would have to have increased. But that is not the story the satellite pictures tell; therefore it would seem to be the case that there has been deterioration.

‘The consequence is that the Sahel is more vulnerable to drought than we think,’ says Hein. ‘Looking back, every period of ten years has had at least three years of drought. Because of the underlying degradation, the vulnerability to new droughts in the Sahel has probably actually increased.’ Hein and De Ridder will re-interpret the satellite images using this new information together with Professor Michael Schaepman and Allard de Wit of the Centre for Geo-Information. / Martin Woestenburg

Global Change Biology (2006) 12, 751–758.