The objective of this paper is to point out existing risks of current design and management strategies in water supply systems and to identify possible ways of designing and operating schemes which minimize these risks. This paper is motivated by the observation that existing design principles and engineering rules (best practice) seem to cope insufficiently or even conflict with current trends of declining water demand. In order to evaluate this situation, an agent-based model comprising the current rules of best practice was developed in a participatory process. Once the model was validated with data sets from a real utility, multiple-scenario testing was used to explore different design strategies, thus allowing ideas for developing alternative management and design schemes to be generated. The simulations show that the traditional risk of insufficient supply security must be supplemented by considering the opposite risk of excessive security (over-capacity). The introduction of demand-side measures may help to calibrate existing best practice with the trends of the current operating environment. Ideas are brought forward on how to shape incentive systems for stakeholders in order to facilitate such a shift.