Term Definition

Term Definition
% of Volume Input Percentage of system input volume is an appropriate measure to define the financial and ecological views of water losses, as defined in the IWA Manual of Best Practice: Performance Indicators in Water Supply Systems which supports the use of % for the Financial Non-Revenue Water by Volume, and Water Resources Indicators.

Apparent Losses consist of unauthorised consumption (theft or illegal use), and all types of inaccuracies associated with production metering and customer metering. Under-registration of production meters, and over-registration of customer meters, leads to under-estimation of real losses. Over-registration of production meters, and under-registration of customer meters, leads to over-estimation of Real Losses.

Authorised Consumption is the volume of metered and/or unmetered water taken by registered customers, the water supplier and others who are implicitly or explicitly authorised to do so by the water supplier, for domestic, commercial and industrial purposes. It includes water exported.

Note that authorised consumption includes items such as fire fighting and training, flushing of mains and sewers, street cleaning, watering of municipal gardens, public fountains, frost protection, building water. These may be billed or unbilled, metered or unmetered according to local practice.

CARL Current Annual Real Losses - total losses from the system as calculated using an IWA water balance.

Infrastructure Leakage Index (ILI) CARL/UARL

Non-Revenue Water is the difference between the System Input Volume and Billed Authorised Consumption.
Real Losses are physical water losses from the pressurised system, up to the point of customer metering. The volume lost through all types of leaks, bursts and overflows depends on frequencies, flow rates, and average durations of individual leaks.

System Input Volume is the volume of water input to a transmission system or a distribution system

TIRL Technical Indicator of Real Losses - this term is no longer used and has been replaced by CARL.
Traditional PIs the simple traditional Technical Performance Indicators for Real Losses which are most widely used in different parts of the world to make comparisons of the Annual Volume of Real Losses are:
As a % of Input Volume
As a figure per length of mains per day or hour
As a figure per service connection per day or hour
As a figure per property per day or hour
As a figure per length of system per day or hour (where length of system = length of mains + length of service connections up to point of
customer metering).

Unavoidable Average Real Losses (UARL) It is recommended that the calculation of the UARL in litres/service connection/day is based on the following form of equation. This recognises separate influences of Real Losses from length of mains (Lm in km), number of service connections (Nc), total length of service connections from the edge of the street to customer meters (Lp in km), and average pressure (P in metres) when the system is pressurised. UARL = (A x Lm/Nc + B + C x Lp/Nc) x P (litres/service connection/day when the system is pressurised)

Water Abstracted is the volume of water obtained for input to raw water mains leading to water treatment plants
Water Losses of a system are calculated as: Water Losses = System Input Volume - Authorised Consumption

Water Losses can be considered as a total volume for the whole system, or for partial systems such as raw water mains, transmission or distribution. In each case the components of the calculation would be adjusted accordingly. Water Losses consist of Real and Apparent losses, and are effectively identical to the previous IWSA definition of Unaccounted-for Water.

Water Produced is the volume of water treated for input to water transmission mains or directly to the distribution system

Active Leakage Control (ALC): The process by which unreported leaks are detected and repaired. This contrasts to Passive Leakage Control. Non-visible leaks account for more than 90% of the total volume of real losses. Small hidden leaks, which often run for years, lose more water than even the most dramatic surface bursts. Active Leakage Control is comprised of well-targeted and properly-managed leak detection activities to locate hidden leaks, communication strategies to encourage the utility's customers to immediately report visible leaks, and effective work management to ensure speedy and good quality repairs.


Automated Reading (AMR): AMR enables frequent and accurate data transfer of metered information. It enables improved analysis of the status of individual meters, customer leaks and frauds. At the network level, AMR provides tools for enhanced Water Loss analysis, ongoing water balance and identification and localization of bursts. Consequently, AMR improves NRW management at all levels.


Apparent Losses/ Commercial Losses: Includes all types of inaccuracies associated with customer metering as well as data handling errors (meter reading and billing), plus unauthorized consumption (theft or illegal use). Apparent Losses are called "Commercial Losses" by the World Bank and in some countries the misleading term "Non- Technical Losses" is used.


Authorized Consumption: The volume of metered and/or unmetered water taken by registered customers, the water supplier and others who are implicitly or explicitly authorized to do so by the water supplier, for residential, commercial and industrial purposes. It also includes water exported across operational boundaries. Authorized consumption may include items such as fire fighting and training, flushing of mains and sewers, street cleaning, watering of municipal gardens, public fountains, frost protection, building water, etc. These may be billed or unbilled, metered or unmetered.


Average Zone Night Pressure (AZNP): The property-weighted average pressure in a zone during the minimum night flow period.


Awareness Time: The time between the occurrence of an unreported leak and the Water Undertaking becoming aware of its existence.


Background Leakage: The component of leakage that is not affected by ALC. This usually consists of very small leaks.


Billed Authorized Consumption: Those components of Authorized Consumption which are billed and produce revenue (also known as Revenue Water). Equal to Billed Metered Consumption plus Billed Unmetered Consumption.


Billed Metered Consumption: All metered consumption which is also billed. This includes all groups of customers such as domestic, commercial, industrial or institutional and also includes water transferred across operational boundaries (water exported) which is metered and billed.


Billed Unmetered Consumption: All billed consumption which is calculated based on estimates or norms but is not metered. This might be a very small component in fully metered systems (for example billing based on estimates for the period a customer meter is out of order) but can be the key consumption component in systems without universal metering. This component might also include water transferred across operational boundaries (water exported) which is unmetered but billed.


Bottom-up: This term refers to assessments of leakage made from night flows measured in DMAs and added together to produce an area leakage level.


Burst: A failure of a pipe or service leading to leakage. This term is interchangeable with leak, although it typically refers to large / high pressure leaks.


Bursts And Background Estimates (BABE): Component-based estimate of annual losses in different parts of the distribution system for any particular combination of local circumstances, i.e. pressure, burst frequency, burst flow rate, number of properties, length of mains, method of leakage control, standards of service, and waste notice service/enforcement policy.


CARL: Current Annual Real Loss. The actual level of real losses per annum. CARL always tends to increase as the distribution networks grow older. This increase can be constrained by a successful leakage management policy.


Cascade: A method of supplying DMAs where water flows through one DMA into another one. This necessitates more than one meter on some DMAs; a situation that is best avoided.


Commercial Losses: see "Apparent Losses"


Current Annual Real Losses (CARL): CARL always tends to increase as the distribution networks grow older. This increase can be constrained by a successful leakage management policy.


Customer Metering Inaccuracies and Data Handling Errors: Apparent losses caused by customer meter inaccuracies and data handling errors in the meter reading and billing system.


Customer Night Use: The water used by customers during the minimum night flow period.


DM: District Meter


District Metered Area (DMA): A small metered area within the distribution network.


DMA's Establishment: Large distribution networks, especially those with poor infrastructures, cannot be managed efficiently without breaking them down into network zones and subzones (District Metered Areas - DMAs). This is essential in order to obtain a comprehensive understanding of the water consumption and water loss within the various parts of the network. When this information is available appropriate actions can be taken. DMAs will substantially reduce leakage awareness time and are a prerequisite for effective prioritization of leak detection and repair activities.


Economic Level of Leakage: The level of leakage at which the net present cost of operation of the network is a minimum.


Energy Management: Water is heavy and delivering water from the source to consumers' taps has a large amount of embedded energy. Energy efficiency can be achieved by physical improvement of the network, reducing real and apparent losses in the system and optimizing the operation of the network, thus minimizing the carbon footprint of water supply.


Flushing: The induction of flows in pipes by opening hydrants or washouts.


GIS: Geographic Information System


Hydraulic Balance Point: In a complicated network fed by several trunk mains, there will be points within the distribution mains network where the net flow is close to zero at a given time, as flows from different routes feed the customers on either side. These hydraulic balance points are often suitable for sector or DMA boundaries as the closure of a valve here will cause little disruption.


ILI (Infrastructure Leakage Index): The ILI is a measure of how well a distribution network is managed (maintained, repaired, rehabilitated) for the control of real losses, at the current operating pressure. It is the ratio of current annual volume of real losses (CARL) to unavoidable annual real losses (UARL). This is how it is calculated: ILI = CARL/UARL.


Leak: The escape of water through a hole, crack, fissure, or other aperture. See also Burst.


Leakage and Overflows at Utility's Storage Tanks: Water lost from leaking storage tank structures or overflows of such tanks caused by e.g. operational or technical problems.


Leakage on Service Connections up to point of Customer Metering: Water lost from leaks and breaks of service connections from (and including) the tapping point until the point of customer metering. Leakage on service connections might be reported breaks but will predominately be small leaks which do not surface and which run for long periods (often years).


Leakage on Transmission and/or Distribution Mains: Water lost from leaks and breaks on transmission and distribution pipelines. These might either be small leaks which are still unreported (e.g. leaking joints) or large bursts which were reported and repaired but did obviously leak for a certain period before that.


Leakage: The water lost through holes in the pipes and tanks forming the network.


Location Time: The time taken from the point where the Water Undertaking is aware of the existence of a leak to the point when the Water Undertaking is aware of the exact location of the leak.


Losses: Losses can be divided into apparent losses (meter errors and unauthorised consumption) and real losses. Real losses are equivalent to leakage.


Meter Management: Active involvement of utilities in meter testing, maintenance and replacement which ensures that meter underregistration will be kept to a minimum and consequently the utility's revenues will be maximized.


Minimum Achievable Annual Physical Losses (MAAPL): See UARL.


Minimum Night Flow (MNF): The net flow into a metered area during the period of minimum flow: this period is usually one hour.


Night Day Factor (NDF): The factor by which night flow losses, (calculated from the Minimum Night Flow over a one hour period), should be multiplied to obtain the daily leakage. The NDF is usually less than 24, due to lower pressures during the day.


Night Line: See Minimum Night Flow


Non-Revenue Water (NRW): The gap between the amount of water put into the distribution system and the amount of water for which customers are actually billed. The major challenge facing water utilities and municipalities is how to deal with high levels of NRW. High levels of NRW reflect huge volumes of water being lost through leaks (real/physical losses), water not being invoiced or not being accurately measured (apparent/commercial losses) or both.


NRR: Natural Rate of Rise


NRW Management Software: NRW management software provides tools to effectively manage NRW operations. The software tools enable:

Top down and bottom up analysis of DMA data
IWA Water balances at the DMA and higher levels
Prioritizing and managing of leak detection activities
Area performance reporting and thematic mapping tools
An asset performance tool for analyzing network asset performance
A rehabilitation planning tool
Building and maintaining hydraulic models
Recording and viewing the status of events occurring in the network.
These software packages interface with a wide range of existing systems including SCADA, logger databases, GIS, CSS, Work Management and billing systems. Miya's experts have developed leading water audit software tools.


Passive Leakage Control: Leakage control carried out by repairing only those leaks that become and are reported to the Water Undertaking.


PCC: Per Capita Consumption


Physical Losses: See "Real Losses"


Pipe Repair: Non-visible leaks account for more than 90% of the total volume of water losses. Small hidden leaks, which often run for years, lose more water than surface bursts. Good quality repairs, as part of an Active Leakage Control program, are an economic and practical way to contend with leakage.


Pressure Correction Factor (PCF): If leakage (L0) is either measured, or estimated, at one pressure (P0), then in order to estimate the leakage (L1) at another pressure (P1), a relationship of the form: can be used. The PCF is a function of the two pressures P1 and P0. This method is frequently used to translate leakage estimated at 50m head into leakage at the actual pressure experienced in a zone.


Pressure Management: Pressure management involves reducing excess pressures and avoiding pressure fluctuations. It is the most beneficial leakage reduction activity because of the direct pressure/leakage relationship. It helps reduce burst frequency and therefore extends an asset’s lifetime. Miya group companies have developed and implemented the most advanced pressure management systems available today, and Miya regularly invests in ongoing research and development to improve pressure management solutions and technology.


Pressure Reducing Valve (PRV): A control valve within the network which reduces the downstream pressure using various types of control method.


Pressure Zero Test (PZT): See zero pressure test.


Rate of Rise of Leakage: The rate at which leakage increases with time between periods of active leakage control. This can be measured by analysis of long-term flow and repair records. It is usually expressed in litres per connection per day per year.


Real Losses:Real losses involve leakage and overflows at reservoirs, leakage on service connections up to metering point and leakage on transmission and distribution mains, up to the point of customer metering point. Real losses are called "Physical Losses" by the World Bank and in some countries the misleading term "Technical Losses" is used.


Repair Time: The time taken from the point when the water undertaking is aware of the exact location of the leak to the point when the repair is completed.


Reported Burst: A leak that the water undertaking becomes aware of without any detection activity. The reasons for this are typically that the water becomes visible on the surface or the burst leads to loss of supply to customers.


Revenue Water: Those components of Authorized Consumption which are billed and produce revenue (also known as Billed Authorized Consumption). Equal to Billed Metered Consumption plus Billed Unmetered Consumption.


Rota Cuts: Rationing of supply by providing supply to parts of the distribution network for restricted periods, often according to a timed rota.


Run Time of Burst: The total time from the occurrence of a burst to its repair.


SCADA (Supervisory Control and Data Acquisition). A computer system for gathering and analyzing real time data.


Sector: A section of the distribution network, usually much larger than a DMA and often defined by clear natural of manmade boundaries, such as rivers or railways.


Step Test: A test to find the location of a leak. Parts of an area fed through a meter are progressively isolated while the flow is monitored. The drop in flow after each isolation is used to identify the amount of leakage in that isolated section.


System Input Volume: The volume of treated water input to that part of the water supply system to which the water balance calculation relates ( after the corrections of bulk meter inaccuracy).


Top-down: Refers to assessment of leakage levels through a water balance.


Training: It is Miya's mission to propagate knowledge to water utility practitioners, in order to ensure the continuation of best practice Water Loss Management during the project period and even after Miya's service term comes to an end. The sharing and transfer of knowledge is carried out by training the water utility management and staff, maintaining high involvement throughout the process and even integrating with the work teams to ensure that the know-how is used optimally by the utility.


TWL: Top Water Level


UARL: Unavoidable Annual Real Losses. Physical Losses cannot be totally eliminated. The volume of Minimum Achievable Annual Physical Losses represent the lowest technically achievable annual volume of Physical Losses for a well-maintained and well-managed system. The standard equation for calculating UARL for individual systems was developed and tested by the IWA Water Losses Task Force. It allows for:

Background leakage - small leaks with flow rates too low for sonic detection
Reported leaks and breaks - based on average frequencies, typical flow rates, target average durations.
Unreported leaks and breaks - based on average frequencies, typical flow rates, target average durations
Pressure/leakage rate relationships (a linear relationship being assumed)The UARL equation requires data on four key system-specific factors:
Length of mains (all pipelines except service connections)
Number of service connections
Length of service connection between property boundary and customer meter. (Note: this is not the same as the total length of the service connection. Losses on the service connection between the tapping point at the main pipeline are included in the allowance per service connection. The additional allowance for length of connections on private land was included to take the longer leak run-times in situations where visible leaks would not be seen by public into account. In most urban situations, if the customer meter is inside the building, the length of service connection between property boundary and customer meter is obviously nil)
Average operating pressure




Unaccounted-for Water: Non Revenue Water. Because of the widely varying interpretations and definitions of the term "Unaccounted for Water", it is strongly recommend using the term NRW and that this term be no longer used.


Unauthorized Consumption: Any unauthorized use of water. This may include illegal water withdrawal from hydrants (for example for construction purposes), illegal connections, bypasses to consumption meters or meter tampering.


Unbilled Authorized Consumption: Those components of Authorized Consumption which are legitimate but not billed and therefore do not produce revenue. Equal to Unbilled Metered Consumption plus Unbilled Unmetered Consumption.


Unbilled Metered Consumption: Metered Consumption which is for any reason unbilled. This might for example include metered consumption by the utility itself or water provided to institutions free of charge, including water transferred across operational boundaries (water exported) which is metered but unbilled.


Unbilled Unmetered Consumption: Any kind of Authorized Consumption which 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 which is very often substantially overestimated. Theoretically this might also include water transferred across operational boundaries (water exported) which is unmetered and unbilled - although this is an unlikely case.


Unreported Burst: A burst which can be found by active leakage control but not by passive leakage control.


Water Audit: An accurate and detailed water audit details how much of each type of water loss is occurring and how much it is costing the water utility. The key concept behind this approach is that water should not be “unaccounted-for”. In conducting a water balance audit, a quantity is determined for the major components of water consumption and water loss, and a price is placed on each component in order to assess its financial impact on the water utility. A detailed and accurate water balance forms the basis for an effective NRW management strategy.


Water Conservation: A complimentary activity which is important to decrease water demand:
Water use profiling
ICI water audits
Residential end use analysis
Verifications of pre/post-replacement savings
Toilet replacement programs
Outdoor / peak day assessment
Other appliances - flow control
Grey water recycling


Water Loss: The difference between System Input and Authorized Consumption. Water losses can be considered as a total volume for the whole system, or for partial systems such as transmission or distribution schemes, or individual zones. Water Losses consist of Physical Losses and Commercial Losses.


Water Undertaking: A general term for the organisation responsible for operation of the water supply and distribution system.


Zero Pressure Test: A test to identify whether the boundary to a zone is watertight. An area of the distribution system is isolated by closing boundary valves. The pressure is monitored and if it drops to zero this indicates that the boundary is watertight.