This project aims at collecting works and data which illustrate the Sahelian Paradox, which is more runoff with less rain, observed since the 70s in some places, and more genrally since the end of
the 80s and early 90s in the whole Sahel.
The Sahel area in Africa is located North of the 750 mm isohyet of annual rainfall amount, and South of 250 mm. In this area runoff is mainly driven by the surface conditions, evaporation is high and groundwater contribution to surface runoff is negligeable. The vegetation plays a major role in maintaining soil water available for transpiration in the subsurface. With climate change since the end of the 60s, early 70s, rainfall decreased together with the density of vegetation. Large trees and with them deep roots, decreased in the landscapes. This led to a decrease of the soil water holding capacity, which led to increasing runoff coefficient by excess of water. This runoff increase while rainfall were still below average was called the Sahelian Paradox. This situation was enhanced in some places by human activities like forest cut, agriculture extension, fallow reduction or disappearance. As a consequence extreme flood events increased in the Sahelian rivers, causing damages to dams, bridges, roads, and other infrastructures, fertile top soils were more rapidly removed, the morphology of river beds changed, becoming more like arid rivers from northwards regions, the sediment transport increased together with erosion.
The first observations of this phenomenon were made by ORSTOM researchers in Burkina Faso during the 80s, but it was thought that it was only local. By the end of the 90s it was proven on the Nakanbe river basin at the Wayen station, that this phenomenon was also visible at larger scales. Then it was recognized in many rivers in Mauritania and Niger, and then in Mali. The hydrological regime of the Niger river at Niamey also changed markedly since the 2000s, showing regularly a greater peak during the Sahelian rainy season, and thus proving also that this phenomenom can modify hydrological regimes of the largest rivers.
These changes in surface conditions also led to the huge increase of the surface or local lakes, either in Niger or in Mali. Eventually, it is also responsible for the increase of the groundwater table in the Niamey region.
Many papers have already been published about this. But there are a certainly a number of works that could be shared and would help better describe this phenomenon in many countries.
Other areas in the world where this could be observed are China and Queensland in Australia, for instance.
This site aims at collecting and sharing informations on this topic from all possible contributors. ... [more]
