Unavoidable Annual Real Losses

& Infrastructure Leakage Index

How Low Could You Go?

The first IWA Water Loss Task Force developed a system-specific equation for the lowest technically achievable Annual Real Losses, for well managed infrastructure in good condition. The UARL concept is similar to the calculation of ‘par’ score for a golf course, based on the length of fairways (around 1 shot per 180 metres) and the number of greens (2 shots per green).

Substituting appropriate parameter values in a simplified Component Analysis of annual Real Losses, for well managed infrastructure in good condition, an equation to calculate Unavoidable Annual Real Losses (UARL) was derived (1999M):

Meter location on service connections in water Leakage and Pressure Management

UARL (litres/day) = (18 x Lm + Ns x (0.8 + 25 x Lp/1000)) x P

Lm = mains length (km),

Ns = number of service connections (main to property line)

Lp = average length, property line to meter (metres),

P = average pressure (metres)

UARL equations can be shown in a variety of units and formats. In  Europe, many Utilities know the length of their service connections (main to first meter) and calculate their annual real losses as a volume per year, so the equation for UARL used in the EurWB&PICalcs free software is :

UARL (m3/year) = (6,57 x Lm + 0.256 x Nc + 9.13 x Lt) x P

Lm = mains length (km), Nc = number of service connections (main to first meter)

Lt = total length of service connections, main to first meter (km)

The UARL equations can be used to predict, with reasonable reliability, the lowest technically achievable annual real losses for any combination of mains length, number of connections, customer meter location and average operating pressure – assuming the distribution system infrastructure is in good condition with high standards for management of Real Losses.

The graphs below show UARL in litres/service connection/day, and m3/km mains/day, for systems with customer meters located at the property line, for a typical range of connection densities and average system pressure.

   UARL for distribution losses in litres/service connection/day                        UARL for distribution losses in m3/km of mains/day

These graphs show UARL in litres/service connection/day

Since 1999, UARL has been calculated for hundreds of international Utilities; some have been able to achieve it, but only a very few (with smaller systems, or fewer unreported leaks than assumed in the UARL formula) can consistently beat it.

Lower limits on system size and average pressure for use of the UARL formula have gradually been reduced since 1999, as shown in the Table below, from Ref. 2009J.

Lower limits on system size and average pressure for use of the UARL formula have gradually been reduced since 1999

However, in 2014 research into good quality data small systems in Austria (2014M, 2014N) explained why small stand alone systems with fewer than 3000 service connections could achieve leakage less than the UARL (ILIs less than 1.0) particularly at pressures below 40 metres with flexible pipes having high FAVAD N1 values. This paper now represents ‘State of the Art’ for lower limits for system size and pressure when calculating UARL, and explanations of how ILIs less than 1.0 can occur.

UARL is also used in the calculation of the Infrastructure Leakage Index (ILI):

ILI = Current Annual Real Losses (CARL)/UARL

In the ASEAN Region, the CARL is also known as the CAPL (Current Annual Physical Losses) and UARL is also known as the MAAPL (Minimum Achievable Annual Physical Losses); so ILI = Current Annual Physical Losses (CAPL)/MAAPL

The Global ILIs webpages show ILIs from Australia, Europe, North America and individual countries.

The ILI is used in an International Leakage Performance Classification system for Real Losses management performance, and identifying recommended strategies for improvement. The following Tables are from the free CheckCalcs software.

uarl 1

The International Leakage Performance Category (LPC)  classification system is used to identify general recommended strategies for improvement. The LPC classification has 8 categories (A1 to D2);  LPC boundary values of ILI for Low and Middle Income Countries (LAMICS) are twice as large as boundary values for High Income Countries (HICs) , so it is necessary to identify if a particular country is an HIC or a LAMIC.A list of High, and Low/Middle income countries (with High/Middle break point around $US 12 to $US13k median per capita income) can be found at

UBL: Unavoidable Background Leakage: an equation for calculating Unavoidable Background Leakage, for component analysis of night flows, is also shown in the ten-year review of the international application of UARL and ILI (2009J).