This paper critically explores the politics that mediate the use of environmental science assessments as the basis of resource management policy. Drawing on recent literature in the political ecology tradition that has emphasised the politicised nature of the production and use of scientific knowledge in environmental management, the paper analyses a hydrological assessment in a small river basin in Chile, undertaken in response to concerns over the possible overexploitation of groundwater resources. The case study illustrates the limitations of an approach based predominantly on hydrogeological modelling to ascertain the effects of increased groundwater abstraction. In particular, it identifies the subjective ways in which the assessment was interpreted and used by the state water resources agency to underpin water allocation decisions in accordance with its own interests, and the role that a desocialised assessment played in reproducing unequal patterns of resource use and configuring uneven waterscapes. Nevertheless, as Chile’s ‘neoliberal’ political-economic framework privileges the role of science and technocracy, producing other forms of environmental knowledge to complement environmental science is likely to be contentious. In conclusion, the paper considers the potential of mobilising the concept of the hydrosocial cycle to further critically engage with environmental science.