In order to ensure better availability of water resource to different users, we undertook a dependability study of system for drinking water supply. A real case study was done on the water supply network in the city of El Hadjarin Algeria. We started from the site modelling, which provided a calculation of a Hydraulic Criticality Index (HCI) in order to prioritize the importance of pipes in the network. Once the indices were calculated, a modelling procedure of reliability block diagram was applied. The objective was to study and analyse the existing network to measure the water availability problems and try to provide viable solutions. An action on the structural aspect and hardware may be required in order to ensure better availability of water and make the system more reliable. Spatial modelling is also proposed to be able to monitor different parts of the network where problems may occur.