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Abstract. Les changements climatique et environnemental ont entraîné l'augmentation exacerbée des coefficients d'écoulement dans certains hydrosystèmes du Sahel ouest africain. Ce «paradoxe hydrologique sahélien» a fait l'objet de plusieurs études, mais très peu d'entre elles ont abordé les contributions de chacun des changements climatique et environnemental. L'objectif de cette étude est de quantifier les contributions de chacun des facteurs (climat et environnement) au changement des écoulements dans la partie sahélienne du bassin du Nakanbé sur la période 1965–1994. Les tests statistiques de stationnarité ont permis de découper la période d'étude en sous-périodes de base et d'impact. La quantification des impacts a été effectuée à travers l'application des techniques d'élasticité et de décomposition aux modèles Budyko de Fu et de Choudhury. L'analyse des chroniques de données hydro-climatiques du bassin a confirmé le paradoxe hydrologique avec une rupture dans les écoulements en 1977 (test de Pettitt, p value = 0,021). Ainsi, la période 1978–1994 comparativement à 1965–1977 a été caractérisée par une diminution des précipitations de 8,2 %, une augmentation de l'évapotranspiration potentielle de 1,3 % et une augmentation des écoulements de 91,5 %. Il ressort de cette étude que les contributions à l'augmentation des écoulements de 91,5 % ont été de -29±2 % pour le changement climatique, +90±1 % pour la dégradation environnementale et +33 % pour les interactions couplées climat-environnement. Ces résultats montrent que l'impact de l'environnement est plus prépondérant et représente environ 3 fois celui du climat en intensité. L'adoption de meilleures pratiques de gestion de la dynamique environnementale pourrait donc permettre de réguler les changements dans les écoulements du bassin. Abstract. Climate and environmental changes have caused the increasing runoff coefficients of some sahelian catchments in West Africa. Many previous studies have focused on this “sahelian hydrological paradox”, but few have addressed the individual contribution of climate and the environment. This study aims to quantify the contributions of each factor (climate and environment) to the change of runoff in the sahelian part of the Nakanbé catchment over the period : 1965–1994. Based on time-series stationarity statistical tests, the study period was divided into baseline and impacted periods. The quantification of impacts was carried out by applying elasticity and decomposition techniques to the Budyko type models of Fu and Choudhury. Statistical analysis of the annual hydro-climatic data detected 1977 as the year of break in the runoff (Pettitt test, p value = 0.021) and confirmed the sahelian hydrological paradox. Thus, the period 1978–1994 compared to 1965–1977 was characterized by a decrease of 8.2 % in precipitation while increases of 1.3 % and 91.5 % have been observed respectively for potential evapotranspiration and annual runoff. The results indicated that the contributions to the runoff increase of 91.5 % were -29±2 % for climate change, +90±1 % for environmental degradation and +33 % for interactions. It appears that, environmental change is the main cause of the increase in the runoff coefficient in the sahelian catchment of Nakanbé and the intensity of its impact is approximately 3 times more than the one induced by climate change. Therefore, good management of environmental dynamics at the catchment scale could help to regulate runoff changes.

<p><strong>Abstract:</strong></p><p>Runoff simulation in highly anthropized watersheds is complex, but essential for water management, especially in poorly gauged and data-scarce hydrosystems of the West African Sahel. In this study, the physically-based and semi-distributed hydrological model SWAT (Soil and Water Assessment Tool) was applied to simulate daily runoff in the Nakanbé watershed at Wayen (21,178 km<sup>2</sup>) in Burkina Faso over the period 2006-2012. The data used concerned 9 water reservoirs, 5 hydrometric stations, 15 rain gauges, 3 synoptic stations, a digital elevation model (spatial resolution: 30m), a soil map (scale: 1:5,000,000), a land use and land cover map (scale: 1:200,000). The 5 hydrometric stations (Tougou, Dombré, Rambo, Ramsa and Wayen) gauge 5 nested watersheds ranging in size from 38 to 21,178 Km<sup>2</sup>. The added value of nested watersheds was assessed through the following three calibration/validation (C/V) schemes: one-site C/V (OS) at the watershed outlet (Wayen); multi-sites C/V considering nested sub-basins (MS1) and multi-sites C/V without considering nested sub-basins (MS2). The results at the watershed outlet (Wayen) indicated that the three C/V schemes (OS, MS1, MS2) are overall satisfactory (KGE above 0.7). However, MS2 scheme (KGE = 0.75) performed better as compared to the traditional one-site calibration scheme (OS: KGE = 0.73). On the other hand, the MS1 scheme (KGE =0.71) performed worse than the OS scheme. The water balance analysis at Wayen showed that 60 to 90% of the annual rainfall is lost as evapotranspiration whereas 5 to 8% contributes to surface runoff, groundwater recharge remaining very low. The comparison of the three calibration/validation schemes provides evidence that accounting for nested sub-basins does not necessarily improve the quality of rainfall-runoff simulations. Yet, multi-sites modeling remains the best scheme when the watersheds used are not nested. The availability of higher resolution data might allow a better understanding of spatial scale effects.</p><p><strong>Keywords: </strong></p><p>Nested watersheds, SWAT model, Nakanbé, West Africa Sahel, Burkina Faso.</p>