IWA Water Balance

IWA  Best Practice Standard Water Balance

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Drawing on the best practice from many countries, the first IWA Water Loss Task Force, and the Performance Indicators Task Force, produced a standard approach for Water Balance calculations with definitions of all terms involved. First published in 2000 as shown below, this was a major step forward, and it is now used and recommended (with minor modifications) by technical organisations, regulators, and international funding agencies in an ever-increasing number of countries.

IWA Standard Water Balance as originally published in 2000

IWA Standard Water Balance as originally published in 2000* Parts of ASEAN region use ‘Commercial’ instead of ‘Apparent’, and ‘Physical’ instead of ‘Real’

Due to widely varying interpretations of the term Unaccounted-for Water (UFW) worldwide, it was recommended in 2000 that use of UFW be discontinued, and is now much reduced internationally. If the term UFW is used at all, it should be defined and calculated in the same way as Non-Revenue Water in the IWA Water Balance.

CheckCalcs, EurWB&PICalcs and other water balance software have been used at National, State and Utility level to introduce the IWA Water Balance to countries in the Americas, ASEAN Region, Europe and Middle East. Some practical additions that can help to avoid errors and misunderstandings in applying and interpreting the IWA Water Balance, as used in the free CheckCalcs and EurWB&PICalcs software, are shown in the enhanced form of the IWA Water Balance below.

Enhanced IWA Water Balance used in the free European CheckCalcs software

The CheckCalcs version of the Water Balance requires System Input Volume components (‘Volume from Own Sources’ and ‘Water Imported’)

These more specific versions of the Water Balance requires System Input Volume components (‘Volume from Own Sources’ and ‘Water Imported’) to be shown, then split into ‘Water Supplied’ and ‘Water Exported’. This assists auditing of water transfers, and highlights the difference between System Input Volume (which includes Water Exported) and Water Supplied (which does not). Known errors in System Input Volume should be corrected at the start of the Water Balance, otherwise they will appear as hidden positive or negative errors when the Real Losses are calculated.

Problems occur when %s of System Input Volume are used as a technical performance indicator for Non-Revenue Water or its components. The IWA 2000 Water Balance did not require the volume of Water Exported to be explicitly shown, so Utilities that export water can appear to have better NRW and Real Loss management performance (as % of System Input Volume) than Utilities that do not export water. Some Utilities which export water calculate %s as %s of Water Supplied, others do not; this is one of many reasons why comparisons using %s are unreliable – see WLRandALtd for references on this topic.

Another problem with the misuse of %s arises because some Utilities start the Water Balance at the source, and some at the output of Water Treatment Works. The IWA Water Balance definitions are unclear on this point, defining System Input Volume as ‘the annual volume relating to that part of the Water Supply System to which the Water Balance relates’. It is recommended to carry out separate water balance calculations for transmission and treatment systems, and distribution systems.

The use of confidence limits in Water Balance calculations provides valuable insights into the reliability of the calculated components of NRW, and the priorities for action for improving the reliability of these calculations. As some users may lack confidence in using this technique, a ‘Concepts’ webpage explaining this approach has been created. Meanwhile, EurWB&PICalcs is designed to permit basic sensitivity testing of the effect of changing infrastructure and volume input parameters on the calculated NRW, Real Losses and KPIs.