At the IWA Performance Indicators (PI) Conference 2017 in Vienna, Austria, on 15-17 May 2017, the following definitive statement was made: “Everyone knows %s of SIV must not be used for target-setting and/or making technical comparisons”.
Nevertheless % of System Input Volume (SIV) and % of Water Supplied (equal to SIV minus Water Exported) continue to be used for these purposes, predominantly because they are traditional and easy to calculate.
The 3rd Edition (2017) of the IWA Manual of Best Practice Performance Indicators for Water Supply Systems lists three reliable indicators that are available for Real (Physical) Losses:
- Op27: Real Losses per connection (l/conn/day when system is pressurised), for urban distribution systems;
- Op28: Real Losses per mains length (l/km/day when system is pressurised), for bulk supply and low service connection density (rural) distribution systems; and
- Op29: Infrastructure Leakage Index (ILI): the ratio between actual real losses and an estimate of the minimum real losses – calculated using the Unavoidable Annual Real Losses (UARL) formula – that could be technically achieved for the system operating pressure, average service connection length and service connection density.
The first Table below shows that the EU Reference document Good Practices on Leakage Management (© European Union, 2015) conforms with Op27, Op28 and Op29. It also provides additional guidance on ‘Fit for Purpose’ PIs for differing operational objectives, and a footnote assists in choice of Op27 (urban systems) or Op28 (rural systems).
The Fit for Purpose PIs are increasingly accepted within Europe and available to be used internationally as:
- volumetric PIs for target-setting and tracking progress;
- the ILI for technical performance comparisons between systems; and
- the UARL as a measure ‘how low can you go’.
This blog seeks to reinforce earlier international messages (for over 3 decades already) that expressing Real Losses as % of SIV gives misleading perspective of true performance because:
- it is strongly influenced by changes and differences in consumption, which may vary substantially seasonally and from one year to another, and is not under the control of the utility;
- it does not make allowance for any system-specific key factors (see second Table above); and
- gives misleading perspective of true performance.
Professionals in the Water Loss Control field have recently commenced an initiative ‘Professionals Abandon Percentages of SIV’ at https://www.leakssuitelibrary.com/pros-abandon-percents-of-siv/. This contains numerous international examples showing that % of SIV is a misleading PI for leakage. We would welcome anyone wishing to join over 120 practitioners from 22 countries and register their support for this initiative. We also kindly invite IWA to further lead a movement to finally end the use of % of SIV – which isn’t a representative indicator of a utility’s performance.
This Blog was prepared by authors: Cor Merks, Allan Lambert, Marco Fantozzi, Mark Shepherd on 29th July 2017 and published on the IWA Website in August 2017. Please note that the weblink in the IWA published version takes the reader to the LEAKSSuite Library landing page, rather than the intended ‘Professionals Abandon Percents of SIV‘ page
Since then, in July 2019 the weblink to the LEAKSSuite website was modified to a weblink to the LEAKSSuite Library.
This article by Cor Merks, Allan Lambert, Marco Fantozzi and Mark Shepherd was originally published in August 2017 as a blog on the IWA Website