Multi-scale analysis of runoff from a statistical perspective in a small Sahelian catchment under semi-arid climate

In Sahelian and semi-arid regions, understanding the runoff processes and at different spatial scales is critical to assess sources of variation and further improve distributed modelling. In this study, through a multi-scale analysis, runoff measurements on different soil surface features and at different scales of observation were carried over 6 years of monitoring in a typical Sahelian landscape in Burkina Faso. Statistical and dimensional analyses were used to investigate significant differences in runoff response behaviour on plots of 1 m(2), 50 m(2) and 150 m(2), hydrologic units of 6 and 34 ha and the catchment of 37 km(2). Results showed that on both cultivated and bare soils, the runoff excess decreases as the area increases, under similar rainfall pattern and prior antecedent soil moisture conditions. On degraded soils, the processes of runoff generation on the plots of 50 and 150 m(2) are identical and significantly different from those observed on the unit plot (1 m(2)). A minimum plot length of 10 m was found to be sufficient to accurately estimate runoff on degraded soils. On cultivated soils, runoff is significantly different from one site to another because of the spatial variability of hydrodynamic properties of the soil. Such results show that the scale effect on runoff is related to the spatial heterogeneity of soils and is further intensified by the rainfall intensity. Such results highlight the value of quantification of runoff on homogeneous units, hence allowing an innovative approach to the problem of scale transfer.