Increasing river flows in the Sahel ?

Despite the drought observed since 1968 in most of the West African Sahel, runoff and rivers discharges have been increasing in the same region. This trend is related with land use change rather than climate change. This paper aims to describe the regional extension of such a phenomenon and to demonstrate that the increase in runoff is observed from the point scale up to the regional scale. It highlights the opposition of functioning between a Sahelian zone, where the Sahel’s paradox applies, and the Sudanian and Guinean areas, where runoff has been logically decreasing with the rainfall. The current trend is evidenced using experimental runoff plots and discharge data from the local to the regional scales

Origins of streamflow in a crystalline basement catchment in a sub-humid Sudanian zone : the Donga basin (Benin, West Africa) Inter-annual variability of water budget

During the last quarter of the 20th century, West Africa underwent a particularly intense and generalized drought. During this period, the biggest drops in streamflow were observed in the Sudanian zone rather than in the Sahelian zone, but the reasons are still poorly understood. In 2000, a meso-scale hydrological observatory was set up in the sub-humid Sudanian zone of the Upper Oueme Valley (Benin). Three embedded catchments of 12-586 km(2) located on a crystalline bedrock were intensively instrumented to document the different terms of the water budget and to identify the main streamflow generating processes and base-flow mechanisms at different scales. Geophysical, hydrological and geochemical data were collected throughout the catchments from 2002 to 2006. Crossing these data helped define their hydrological functioning. The region has seasonal streamflow, and the permanent groundwater in the weathered mantle does not drain to rivers, instead, seasonal perched groundwaters are the major contributor to annual streamflow. The perched groundwaters are mainly located in seasonally waterlogged sandy layers in the headwater bottom-lands called bas-fonds in French-speaking West Africa of 1st order streams. During the period 2003-2006, regolith groundwater recharge ranged between 10% and 15% of the annual rainfall depth. Depletion of permanent groundwater during the dry season is probably explained by local evapotranspiration which was seen not to be limited to gallery forests. During the 4-year study period, a reduction of 20% in annual rainfall led to a 50% reduction in streamflow. This reduction was observed in the two components of the flow: direct runoff and drainage of perched groundwater. Thanks to the comprehensive dataset obtained, the results obtained for the Donga experimental catchment are now being extrapolated to the whole upper Oueme valley, which can be considered as representative of sub-humid Sudanian rivers flowing on a crystalline basement, for example, the upper courses of several major West African rivers (Senegal, Niger, Bani, and Volta)

Les changements d’usage des sols et leurs conséquences hydrogéomorphologiques sur un bassin-versant endoréique sahélien

International audienceDespite the strong reduction in rainfall observed after 1968, the water table of some endorheic areas in the Sahel has been found to be rising over the last several decades. It has been previously demonstrated that this is due to land use changes which have led to a severe increase in runoff and erosion. In such areas, the excess in runoff causes a strong increase in the number of ponds, their sizes and thus, their duration. Ponds have been identified as the main zones of deep infiltration of water. The aim of this study was to investigate whether other areas of the Sahelian region could also be defined as deep infiltration ones as well, and then, whether they were contributing to aquifer recharge. Soil water content was surveyed for five consecutive years (2004-2008) by implementing a set of measurement devices at different depths. The hydrologic water balance was monitored at stream flow gauge stations located upstream and downstream of two small endorheic catchments. The observed replacement of bush vegetation by crops and fallow areas led to the appearance of extended bare soil areas due to both aeolian and hydric erosion, triggering a strong reduction in soil infiltrability under millet fields and fallow lands as well as in the soil water holding capacity. It also resulted in the formation of a great number of gullies and sand sediment deposits in the endorheic areas. Measurements showed that sandy deposits correspond in fact to large areas of deep infiltration: tens of thousands of cubic meters of water infiltrated catchments of less than 1 km(2). Runoff decreased by up to 50% in the sandy deposit areas, while infiltration (close to 1300 mm h(-1)) was observed up to depths of 10 m. These factors would raise the water table and significantly modify the surface and sub-surface components of the water cycle

Experimental evidence of deep infiltration under sandy flats and gullies in the Sahel

Despite the strong reduction in rainfall observed after 1968, the water table of some endorheic areas in the Sahel has been found to be rising over the last several decades. It has been previously demonstrated that this is due to land use changes which have led to a severe increase in runoff and erosion. In such areas, the excess in runoff causes a strong increase in the number of ponds, their sizes and thus, their duration. Ponds have been identified as the main zones of deep infiltration of water. The aim of this study was to investigate whether other areas of the Sahelian region could also be defined as deep infiltration ones as well, and then, whether they were contributing to aquifer recharge. Soil water content was surveyed for five consecutive years (2004-2008) by implementing a set of measurement devices at different depths. The hydrologic water balance was monitored at stream flow gauge stations located upstream and downstream of two small endorheic catchments. The observed replacement of bush vegetation by crops and fallow areas led to the appearance of extended bare soil areas due to both aeolian and hydric erosion, triggering a strong reduction in soil infiltrability under millet fields and fallow lands as well as in the soil water holding capacity. It also resulted in the formation of a great number of gullies and sand sediment deposits in the endorheic areas. Measurements showed that sandy deposits correspond in fact to large areas of deep infiltration: tens of thousands of cubic meters of water infiltrated catchments of less than 1 km(2). Runoff decreased by up to 50% in the sandy deposit areas, while infiltration (close to 1300 mm h(-1)) was observed up to depths of 10 m. These factors would raise the water table and significantly modify the surface and sub-surface components of the water cycle

Evolution de l'ecoulement d'un petit bassin-versant sahelien en lien avec les changements d'usage des sols

Land-use changes have been significant these last decades in West Africa, particularly in the Sahel region; in this area, climatic and demographic factors have led to a rise in cropped areas in recent decades causing strong changes in the water cycle and in river regimes. This study compares the rainfall-runoff relationships for two periods (1991-1994 and 2004-2011) in two small and similar neighbouring Sahelian catchments (approx 0.1 km(2) each). This allows identification of the different hydrological consequences of land-use/land-cover change, particularly the fallow shortening and the consequent degradation of topsoil. The main land surface change is a 75% increase in crusted soil area. Runoff increased by more than 20% on average between the two periods while flood duration decreased by 50% on average. However, runoff values remained largely constant in the lower part of the northern basin due to a strong increase in in-channel infiltration. Editor D. Koutsoyiannis; Associate editor T. Wagene