A multi-scale approach of runoff generation in a Sahelian gully catchment: a case study in Niger

Runoff production conditions in a small gully catchment are studied at four different scales: the point scale (0.001 m²), the local scale (1 m²), the field scale (of the order of 100 m²) and the catchment scale (0.2 km²). At the point scale, infiltration measurements were conducted using a tension infiltrometer. At the local and the field scale, runoff plots were setup on typical soil surface conditions of the catchment (plateau bare soil, hillslope bare soil and fallow grassland). At the catchment scale, stream discharges were measured at two gauging stations.The overland flow yield is significantly nonuniform in space, due to the high spatial variability of infiltration capacities and the depressional storage of the soil surface. The runoff and the infiltration data collected confirmed the major role played by soil crusting on runoff generation in that part of Sahel. At the point scale, hydraulic conductivity measurements have shown that infiltration and runoff were driven by the hydraulic properties of the crust. At the field scale, microtopography and heterogeneity in the soil surface crusting decreased discharge volumes. The influence of vegetation growth on runoff yield was evident in the case of the fallow sites. Analysis of discharge data at the catchment scale highlights that infiltration through the bottom of the gully between two gauging stations leads to considerable runoff water transmission losses.