Genèse des écoulements sur deux petits bassins versants cristallins de l’Ouest du Niger : approche multi-échelles du fonctionnement hydrodynamique ; Origin of runoffs on two small crystalline water catchment areas in the west of Niger: multi-scale approach to hydrodynamic operation

The observed increase of the catchments flow values in the Sahel during the drought years has exacerbated the interest for the studies on the superficial hydrodynamic properties of soils and their impact onto runoff volumes. This work focuses on the processes responsible for the flow production within two experimental catchments situated in the granitic basement of Western Niger. It aims at characterizing the hydrodynamical functioning at three nested spatial scales. At point scale (50 cm²), soil surface infiltrability (0-10 cm) is determined for different surface features through hydraulic conductivity (Ks) measurements carried out in situ. At the plot scale, runoff is measured from 10 m² experimental plots while at catchment scale (5 ha), stream flow is controlled by stream gauges at the exutories. Measurements show low values of hydraulic conductivity ranging from 10 mm.h-1 (minimal value measured on erosion crusts ERO) to 40 mm.h-1 (maximal value measured on cultivated sites in seasonal average). Runoff coefficients are inversely ranked, from 0.60 on ERO to 0.25 on cultivated surface. Catchment scale runoff coefficients are 0.41 and 0.28 for the not cultivated and the cultivated basins, respectively. Results obtained at these three scales are consistent and reveal the strong runoff production ability of these granitic catchments resulting from both low point scale infiltrability and small re-infiltration of runoff water within the stream network. This last component represents only 4% of annual rainfall while it may reach more than 50% for sedimentary catchments of similar size. Within these catchments, thus, a 2-4 mm rainfall is sufficient to generate a flow at the outlet due to rather small (less than 20 cm) sand deposits covering the stream beds. Finally, both at the plot and at the catchment scales, hydrodynamical functioning was found quasi independent on the initial water content. ; La hausse des écoulements observée au Sahel pendant les années de sécheresse a amené de nombreux chercheurs à ...