With nearly 30 years' experience of managing stored surface water for water supply, the Municipal Water and Sewage Company of Seville (EMASESA), has introduced a Water Supply Natural Resources Vigilance Program. It consists of four levels that are mutually complementary: early warning system; vigilance of raw water resources; research; and integration of limnological knowledge. Selecting the reservoir that should be used, intake depth, transfer strategies, etc, are some examples of active management that this Vigilance Program allows us to achieve. Guaranteeing high quality raw water resources, at all times, which therefore favours their treatment, is one of the priority objectives for EMASESA. This is because it not only has repercussions from an economic point of view but it also contributes to the efficient and sustainable use of water resources.