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Based on the emergence of the notion of desiccation, we propose a critical analysis of how expert knowledge of waterin Sahelian Africa was constructed in relation to the development of colonial policies. The leitmotiv of the expert discourse fully illustrates the power relations within the colonial scientific community, before asserting itself in the issues related to irrigation schemes and forestry policies stakes in the colonies. More broadly, its use in these space and resource management policies make it part of the colonial and post-colonial xxth century environmental logics.