The research focuses on a 4-million-inhabitant tropical region supplied by a network of open-water reservoirs where the dry season lasts for 8 months (Jun−Dec). We analysed the impact of four climate change scenarios on the evaporation rate and the associated availability (water yield distributed per year). The worst-case scenario shows that by the end of the century (2071−2099), the evaporation rate in the dry season could increase by 6 %, which would reduce stored water by about 80 %.