Abstract
Climate and environmental changes have led some Sahelian rivers watersheds in West Africa to a paradoxical behavior: runoff coefficients have increased despite rainfall deficit (1970-1990) and re-greening (1990 to present). The lack of reliable data on the monitoring of environmental dynamics in West Africa has limited application of Newtonian models to understand this paradoxical hydrological behavior. This thesis aims to better understand the hydrological behavior of the Sahelian watersheds despite the context of insufficient data. The methodology adopted consisted first of an analysis of hydro-climatic and environmental variability to observe the changes undergone by the watersheds. Secondly, Budyko type model based on Darwinian approach was employed to quantify the isolated and combined contributions of climate and environmental changes to variations in surface runoff, and to establish the climate-environment coevolution relationship at the watershed scale. Finally, this climate-environment coevolution relationship allowed coupling a Budyko type model and SWAT (Soil and Water Assessment Tool) model to take into account the environmental dynamics in the simulation of runoff. The developed methodology was applied to seven nested watersheds (38 - 21,178 km2) located in the Sahelian part of the Nakanbé River watershed (a tributary of the Volta River in Burkina Faso) over the period 1965-2018. The results of variability analysis confirmed that the Nakanbé nested watersheds at Wayen experienced both Sahelian hydrological paradoxes and the study period was divided into one baseline period (1965-1977) and three impact periods: 1978-1994, 1995-2006, and 2007-2018. The impact studies showed that environmental degradation was the main driver of the first hydrological paradox, while climate-environment interaction became dominant during the second Sahelian hydrological paradox. The coevolution study showed the existence of a reciprocal cause-effect relationship between climate and environment in the Nakanbé watersheds and indicated that Fu Budyko-type model parameter appears to be a good indicator of the spatio-temporal variability of soil water holding capacity. Considération of changes in soil water holding capacity in hydrological modeling by coupling Darwinian (Budyko of Fu) and Newtonian (SWAT) models has improved the runoff simulation in the nested watersheds of the Nakanbé River.Keywords: Climate and environmental changes, Climate-environment coevolution, Budyko conceptual framework, SWAT model, Nakanbé nested watersheds, Sahel