3.1 Urban water supply system
The volume reported in the water accounting statements (7,340 ML) refers to the volume of water within the distribution pipe network and service reservoirs of the urban water system at 30 June 2012.
The storage volume of the urban water supply system is comprised of two components: the distribution pipe network (5,750 ML) and the service reservoirs (1,590 ML).
The volume of water within the urban distribution pipe network was calculated using a geographic information system (GIS) layer of the pipe network. The attributes of the GIS layer were used to select trunk mains and pipes used for distribution and reticulation. Only those features that are actively used for the potable network were selected.
The total volume of water within the system was calculated by summing the volume of each section of pipe within the system. The volume of a section of pipe is the product of the length of the pipe section (km) by the cross-sectional area of the pipe (m2).
The volume of water within the service reservoirs of the urban supply system is measured on a daily basis at each reservoir. The total storage volume in these service reservoirs at the end of the 2011–12 year was that measured on 30 June 2012.
Assumptions, limitations, caveats and approximations
This value is an estimate of the volume of water within the supply system for the Perth metropolitan area, i.e. it does not include water stored in the supply system for the southwest area (e.g. Mandurah).
It is assumed that all the pipes in the urban potable water supply system are at full capacity and have a uniform diameter. Unless the pipe network is augmented or partly decommissioned, the volume within the pipe network will always be reported as 5,750 ML.