Water utilities have started to install water quality sensors to protect their network against intentional and accidental contamination events. In this context, for taking appropriate action, not only are early-warning detection systems important, but so is the identification of the contamination source, and knowledge of the present and future contamination extent. For the latter there is a need for reliable and updated network models. The main objective of this paper is to specify which performance criteria should be considered to place water quality and water quantity sensors for both early detection and model calibration. Firstly, a brief bibliographical review is given for optimal sensor location design. Next, formulations and objectives for early-warning detection are proposed. Problem formulations that aim to minimize the estimator variance for calibration are then specified. Finally, the method is applied on the real WDN system of the CUS water service (1,000 km, 25,000 nodes and 45,000 customers).