No abstract

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 OPEN ACCESSWater 2010, 2 171 trend is evidenced using experimental runoff plots and discharge data from the local to the regional scales.

In Central Yakutia, frozen river banks are affected by a combination of thermal and mechanical erosion. Exceptional bank retreat of up to 40 m per year is observed. This results from ground thawing produced by heat transfer from the flow of water through the frozen ground, followed by mechanical transport of the thawed sediments. A one-dimensional model is proposed to estimate the thermal erosion efficiency. A test of this model is a comparison of results obtained from experiments carried out in a cold room. A hydraulic channel allows measurements of the thaw front propagation, as well as the thermal erosion rate, in simulated ground ice that is subjected to warm water flow. Various laboratory simulations demonstrate the validity of the mathematical model for the range of laboratory conditions. A hierarchy of parameters (Reynolds number, water and ground ice temperatures) is proposed to explain the present efficiency of thermal erosion along the Siberian rivers. From the characteristics of the Lena River (geometry, temperature and discharge) during the flood season, the erosion of banks with different ice content predicted by the model is in agreement with field observations.