The object of this paper is to investigate the sustainability of water use in human settlements. The water system of a small municipality in northern Italy is discussed as a case study. By measuring water exchanges between different sectors of activity within the municipal borders, and considering underground and surface waters as natural components of this system, a network of flows is built and successively investigated by network analysis, a tool that is comprised in the apparatus of ecosystem ecology. By calculating the reciprocal dependence of compartments, the amount of resource that is involved in cycling, the length of exchange pathways, and the organisation of flows, network analysis shows that the system does not use water in a sustainable manner and provides general criteria for improving sustainability. In this respect, options to improve sustainability are also discussed. The results of this study support the idea that the ecosystem approach can provide an interesting conceptual perspective in which sustainability issues can be framed, and that network analysis is a promising tool to handle these issues in practice.