While considering the process by which the drying concept emerged, we propose a critical analysis of how expert knowledge of water in Sahelian Africa has been constructed, in relation to the development of colonial policies. This leitmotif of the expert discourse fully illustrates the power relations within the colonial scientific community before asserting itself in irrigation schemes and forestry policies at stake in the colonies. More broadly speaking, its instrumentalization in these space and resource management policies illustrates ensures its coherence with twentieth-century colonial and post-colonial environmental logic. Key-words: Drought, desiccation, narratives, expertise, Sahel.