10.6 Leakage from urban water system
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.