Water leak

Top down and bottom up water balances for understanding real losses distribution
K Goraya, M Lukin
Publication Date (Web): 9 February 2018
DOI: https://doi.org/10.21139/wej.2018.009

Unitywater is a water distribution and retail utility in South East Queensland, Australia. It operates and maintains approximately 6,000 km of water mains servicing a population of over 700,000 people in the rapidly growing area north of the city of Brisbane. There are approximately 300,000 retail customers in the supply area and customers are metered and billed quarterly. Unitywater’s water supply network encompasses two distinct regions, including the Northern Region, covering towns and cities on the Sunshine Coast, and the Southern Region covering Caboolture, Redcliffe and suburbs on the northern outskirts of Brisbane. These regions are comparable in terms of serviced population, network size, pipe materials and age, though network pressures are on average about 10 metres higher in the north. However, real losses measured in each region are significantly different – typically being about 90 L/conn/day in the Northern Region compared with 55 L/conn/day in the Southern Region. Overall, Real Losses across Unitywater’s service area averages about 70 L/conn/day.

In accordance with its Corporate Strategic Plan, Unitywater has been pursuing initiatives to reduce leakage from an overall level of 70 L/service connection/day down to a target of 50 L/service connection/day over the next 10 years. The main driver for the leakage reduction is the rising bulk water unit price, which is at $2.68 per kilolitre in 2016. However, to help direct appropriate activities and to achieve this target it is necessary to better understand both;

  • the top-down water balance calculation, in order to provide a greater level of confidence that the results, particularly at the regional level, reflect the level of real losses present in the network, and;
  • the actual distribution of these real losses across the various infrastructure components of the water supply network.

To that end, over the last 3 years a review of the assumptions used in the top-down water balance calculation has been conducted to improve the level of certainty associated with the overall leakage result. In addition, a “bottom-up” or Component Analysis has been carried out to try and quantify the relative contribution to the overall leakage result by the various elements of the water supply network (i.e. reservoirs, trunk mains, reticulation mains, service connections, etc). 

With the review of top-down water balance calculations, the real losses within Unitywater’s network reduce from about 70 L/conn/day to 52 L/conn/day. This reduced estimate is almost wholly attributable to the use of the actual 5% retail meter under-registration rather than the WSAA guideline figure of 2%.

The bottom up analysis shows that the Background Leakage in the Unitywater network is 24 L/conn/day. This figure could be higher to account for some double-counting between reported and unreported leakage.

The bottom up analysis nevertheless proved very informative. The bottom-up analysis suggests;

1. Service connections are the major contributor to leakage within the network with 56% of the total losses occurring through these
2. Real losses are worse in the North than the South, confirming the results of previous top-down assessments, and
3. Leakage from service connections in the North are much worse, 2-3 times, than in the South.
Service connections are the major contributor to leakage within the network with 56% of the total losses occurring through these

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