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Water Balance Calculations

The water balance and all its components as developed by the International Water Association (IWA) Water Loss Specialist Group (WLSG) and are explained in more detail below.

Understanding your water system

The first step in non-revenue water reduction is to understand what happens to the water once it enters the network, using the correct Performance Indicators. 

Drawing on best practice from many countries, the IWA WLSG produced an international ‘best practice’ standard approach for water balance calculations with definitions of all terms involved. Some of these definitions are outlined below:

  • System Input Volume (SIV) is the annual volume input to that part of the water supply system

  • Authorized Consumption is the annual volume of metered and non-metered water taken by registered customers, the water supplier, and others who are implicitly or explicitly authorized to do so (e.g. water used in government offices or fire hydrants). It includes exported water and the leaks and overflows after the point of customer metering.

  • Non-Revenue Water (NRW) is the difference between system input volume and billed authorized consumption. NRW

  • consists of unbilled authorized consumption (usually a minor component of the water balance) and water losses.

  • Water Losses is the difference between System Input Volume and Authorised Consumption, and consists of apparent (commercial) losses and real (physical) losses.

  • Apparent Losses sometimes referred to as ‘commercial losses’, consist of unauthorized consumption and all types of metering inaccuracies.

  • Real Losses sometimes referred to as ‘physical losses’, are the annual volumes lost through all types of leaks, bursts and overflows on mains, service reservoirs and service connections, up to the point of customer metering.

The components of the water balance should always be calculated as volumes before any attempt is made to calculate performance indicators. The separation of non-revenue water into components:

  •     Unbilled authorized consumption

  •     Commercial (apparent) losses and

  •     Physical (real) losses should always be attempted

Basic information is important

Sometimes even the most basic information, such as system input volume, average pressure, supply time, length of mains, and the number of service connections, is not initially available. The process of calculating each of the water balance components and performance indicators will reveal such deficiencies. The utility management should then take corrective action to close these data gaps and improve data quality. Using incomplete or inaccurate data for the water balance calculation will not produce a useful result.

When the entire system input volume is metered, determining the annual system input volume is a relatively straightforward task. Utility managers must collect meter records regularly and calculate the annual quantities of the individual system inputs. This includes a utility’s own sources as well as imported water from bulk suppliers. Ideally, the accuracy of the input meters is verified using portable flow measuring devices. Water utility managers need to accurately measure water produced by the treatment facility. Total water produced is a key input for the water balance.

The different components of the water balance

Billed metered consumption includes all of the water consumption that is measured and charged to domestic, commercial, industrial or institutional customers. It also includes exported water that is measured and charged. The billed metered consumption period used in the calculation should be consistent with the audit period by processing it for time lags. In addition, NRW managers should determine the general accuracy of various domestic and non-domestic consumption meters for a possible 95% confidence limit by taking a sample of existing-working meters from various locations and testing them on a standard recognized and calibrated meter test rig. Independent companies provide testing services if the water utility does not own a meter test rig. If several different customer meter brands are in operation, then the sample selection should include meters from each brand.

Determining the annual billed metered consumption goes hand in hand with detecting billing and data handling errors, information that utilities also require for estimating commercial losses. The volume of unbilled metered consumption should be established using a similar approach to that for billed metered consumption.

Unbilled unmetered consumption is any kind of authorized consumption that is neither billed nor metered. This component typically includes items such as fire fighting, flushing of mains and sewers, street cleaning, frost protection, etc. In a well-run utility, it is a small component that is very often substantially overestimated. Unbilled unmetered consumption, traditionally including water the utility uses for operational purposes, is often seriously overestimated. This is sometimes caused by simplification (e.g. using a percentage of total system input), or by deliberate overestimates to ‘reduce’ the amount of NRW.