19.3 Leakage to groundwater
a. Urban water supply system overview
The urban water supply system is one of three sub-components of Perth's urban water system. The three sub-components are the:
- Urban water supply system
- Wastewater system
- Recycled water system.
There are three main sources of water for Perth's urban water supply system:
Urban water consumption (part of 19.4) was 232,602 ML during the 2011–12 year, representing approximately 73% of all urban water supply system outflows. Approximately 20% of the urban water was transferred back to surface water storages (part of 19.5). The majority of this water was transferred for the storage of desalinated water produced during winter and to balance the storages in preparation for peak summer supply. The remaining urban water was lost through leakage to groundwater (19.3) and operational losses (19.20).
The following figure shows all the inflows and outflows associated with the urban water supply system. For more detail on a particular flow associated with the urban water supply system, refer to the line item notes.
Schematic diagram of water inflows and outflows during the 2011–12 year for Perth's urban water supply system. Line item numbers are provided in brackets.
b. Supporting information
This line item volume (22,207 ML) represents the total leakage from the urban water system into groundwater aquifers within the Perth region during the 2011–12 year.
Leakage from the urban water system (defined as real losses) was calculated using the following equation:
Real losses = non-revenue water – (apparent losses + unmetered authorised consumption)
Non-revenue water is water that is lost before it reaches the customer. The volume of non-revenue water is calculated by subtracting the metered volume of water consumed (revenue water) from the metered volume of water supplied. The revenue water was first adjusted to take into account the difference between total water delivered bulk meter readings (done daily) and the customer meter readings (done twice per year for each customer). This is known as the meter lag adjustment.
The volume of apparent losses comprises two components: unauthorised consumption (e.g. water theft); and customer meter under-registration (e.g. meter inaccuracies). The unauthorised consumption is assumed to be 0.1% of the metered volume of water supplied. Customer meter under-registration is assumed to be 2% of revenue water.The volume of unmetered authorised consumption is calculated as 0.5% of the metered volume of water supplied.
Assumptions, limitations, caveats and approximations
There is no differentiation between leakage to the landscape and leakage to the groundwater store, it is simply assumed, in this case, that all losses go to the groundwater store.
The volumes of apparent losses and unmetered authorised consumption are not measured and are estimated based on the infrastructure leakage index calculations for Perth and Mandurah.