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Land use changes and their hydro-geomorphologic effects in a Sahelian endorheic catchment basin Volume 22, issue 1, Janvier-Mars 2011

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Université Abdou Moumouni de Niamey Faculté des lettres et sciences humaines Département de géographie BP 418 Niamey Niger, LTHE /IRD Laboratoire d’étude des transferts en hydrologie et environnement BP 53 38041 Grenoble France, Université Paris 8 Département de géographie CNRS UMR 8591 Laboratoire de géographie physisque 1, place Aristide Briand 92195 Meudon cedex France, Université Abdou Moumouni de Niamey Faculté d’Agronomie Département de génie rural, eaux et forêts BP 11011 Niamey Niger

Rural catchment basins in the Sahel have for several decades been subject to strong pressure on their resources, which is leading to a radical change in their hydro-geomorphologic behavior. We here take as a case study the endorheic catchment area of Dantiandou valley, located at approximately seventy kilometers east of Niamey (Niger). We particularly focus on one of its sub-areas, the basin of Sama Dey (28,2 km 2). From 1990 to 1999 this catchment basin underwent strong changes, including a degradation of the Tiger bush (-4%) on the plateaus and a reduction of fallows (-3%) on the agricultural glacis grounds. The decrease in ground vegetation is leading to the formation of encrusted surfaces (+12%), which generates run-off and erosion on very fragile wind sand soils. On the terrains, the mean annual streaming ratio amounts to 50.4% of the total rainfall on encrusted surfaces and 2.8% on the millet fields, with loss of soil of 56.6 t/ha and 2 t/ha respectively. The mean annual streaming ratio amounts to 46% at the upstream station and 26% at the downstream station. The aim of the study is to highlight current hydro-geomorphological processes, where the determining factor is soil crusting due to its sensitivity to rain battering following the clearing or the cultivation of the land. The method is based on the observation and follow-up of streaming and erosion on small plots (2006-2008), on flow measurements (2004-2008), and the charting of the evolution of ground occupation and morpho-pedological units. As significant results, one can retain that over the selected period, the factors that brought about streaming and erosion were, by order of importance: soil properties - in particular their sensitivity to rain battering, soil occupation, and the degree of the slope.