The Sahelian Paradoxon

The papers "Rain-Use-Efficiency: What it Tells us about the Conflicting SahelGreening and Sahelian Paradox" by Dardel et al. and "Spatio-temporalvariability of hydrological regimes around the boundaries between Sahelian andSudanian areas of West Africa: A synthesis" by Descroix et al. deal with the "Sahelian Paradoxon" and its evolution throughout the last centuries.

The Sudanian and Sahelian region is characterized by continuous human made land use changes, thereby, influencing the local hydrology (e.g. river discharge) and the soil up to desertification. Varying rainfall pattern provide different hydrological environments in Sudan and the Sahel region. The first region suffers of reduced stream flows; the second one is characterized by an increased run-off coefficient (run-off divided by rainfall) up to increased discharges independent from lowering rainfall from rising run-off fed water tables.

The Sahelian Paradoxon refers to the increased run-off during 1968-1995 droughts throughout the region. During this time the region suffered from low rainfall periods forcing its people to change their land use techniques and crops, cut forests to gain new agricultural usable land and over-gaze areas by their cattle leading to desertification. 
Observations on the other hand show a re-greening trend in the Sahel zone since the 1980s which is not yet fully understood. Explanations range from varying climate situations up to theories about different farming techniques. Evidence shows that this re-greening is made from herbage rather than productive green plants.

The so called "second Sahelian Paradoxon" regards the issue of increased vegetation productivity along with an increased run-off coefficient. In the south-west of Niger increased run-off coefficient comes along with a reduced land cover vegetation and thereby with an increase of bare surface.