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Chartered Institution of Water and
Environmental Management (CIWEM)

106-109 Saffron Hill, London, EC1N 8QS  
Tel: 020 7831 3110   Fax: 020 7405 4967

Water Distribution Network Leakage in the UK


This policy position statement outlines the key issues associated with loss of water from the distribution network between the point of water treatment and delivery of water (potable and non potable) to the customer. This loss of water is known colloquially as "leakage".

CIWEM's Position on Leakage:

1. All water supply distribution networks suffer some leakage. Zero leakage is the ultimate goal. However it is generally accepted that it is not, currently, technically possible, or indeed economically viable, to achieve zero leakage. CIWEM recognises and applauds the considerable reductions in leakage made by water companies in recent years.

2. CIWEM recommends that the targets set for leakage reduction by water utilities should take greater account of environmental and social factors as well as economics.

3. CIWEM recognises that although some of the water lost from buried water pipes does return to the environment, this may not be to the same aquifer from which it was originally abstracted and the treatment process may mean that the water chemistry of potable water can be substantially different from that in the aquifer to which it returns.     

4. CIWEM recognises that a considerable amount of energy is used to abstract, treat and pump potable water and as a result leakage contributes to the carbon footprint of water supply.

5. CIWEM recognises that not all the leakage currently reported is lost from pipelines owned by water utilities; around a third to a quarter of total leakage is from customer owned supply pipes. There are a number of options available to reduce the losses from these privately owned pipes and CIWEM advocates further studies to identify the best course of action.

6. CIWEM recommends that in the long term, all water use should be metered for the purposes of water conservation and more accurate leakage measurement.

7. CIWEM recommends that leakage should not be quoted in terms of percentages as this is misleading. The most appropriate units for quoting leakage are in l/property day or l/km pipe/hour.

8. CIWEM supports an holistic approach to leakage control, by considering the components of leakage and selection of appropriate policies.


The Chartered Institution of Water and Environmental Management (CIWEM) is the leading professional body for the people who plan, protect and care for the environment and its resources, providing educational opportunities, independent information to the public and advice to government. Members in 98 countries include scientists, engineers, ecologists and students.



Leakage is an important element in the supply-demand balance for most water supply companies. In the recent past droughts have exposed the vulnerability of some UK companies in maintaining supplies such that the question of what levels of leakage should be accepted has been raised by the media, the public and specialist groups such as the All Party Parliamentary group for Water.

Currently water utilities set their own annual targets which are agreed with the economic regulator OFWAT, based on achieving or maintaining the sustainable economic level of leakage (SELL) . Most companies have now reached SELL. OFWAT now assesses whether targets have been passed or failed against a 36 month rolling average.

The high profile of leakage in the UK media and the visibility of some leaks has led to a high level of public and customer awareness.  The profile of water utilities is, to a significant degree, judged by their behaviour on leakage.  Generally, the UK water utilities are  significantly better than in many other countries, and the lowest leakage companies in the UK compare well internationally.

Leakage levels are generally quoted in Ml/d which is the leakage lost per day averaged over a year. Two additional leakage figures are also quoted in OFWAT's annual report, litres lost per property connected per day (l/prop/d) and litres lost per km of distribution pipe owned by the company per hour (l/km/hr). These last two allow some comparison between companies and it is generally recognised that the l/prop/d is more appropriate for urban and semi-urban contexts. In some countries leakage is still expressed as a percentage. This is very misleading as leakage can vary with consumption without any change in the actual leakage. A unit proposed by the IWA Water Loss Task Force is the infrastructure leakage index (ILI) which is a unitless figure based on actual leakage compared with a theoretical minimum. Whilst this has some use in international benchmarking the ILI does not take pressure management in to account, does not consider the overall condition of the pipes and due to the use of theoretical values, is not suitable for regulatory targets.

There have been significant reductions of leakage achieved by UK water companies in recent years. However what is not widely recognised is that, if left unattended, leakage rises steadily. Research demonstrates that there is a need for an increasing level of resourcing to attain progressively lower leakage levels.

The primary components of leakage management are:

• water may be lost from all parts of the network;
• raw water mains to treatment works
• trunk mains to service reservoirs
• water utilities distribution pipes (including communication pipes)
• customer supply pipes
• in addition water may be lost from customer premises through e.g. leaking supply pipes, dripping taps, faulty WC cisterns

The primary benefits of leakage reduction are seen as:

• reduced abstraction and therefore environmental improvement
• increased reliability of water supplies
• deferment of capital expenditure on water resources and supply schemes
• reduced operating costs (if above the ELL)
• improved public perception of water companies and encouragement to conserve water themselves.
• reduced energy and chemicals for treatment and pumping
• reduced infrastructure damage (e.g. weakened road foundations).

Zero leakage is the ultimate goal. However it is not, currently, technically viable, or indeed economically sound, to achieve zero leakage. Although water lost from the distribution network will return to the environment, it may not be available for abstraction, it may be of a different water chemistry and as mentioned above, it will have already required a substantial energy input which is then lost.


Key Issues

Accurate estimation of leakage requires an estimate of the volume of water delivered to customers in their properties. However, with only some 15% of household supplies metered, direct measurement has not yet been achieved. Leakage estimates are therefore built upon components and statistical methods which themselves are subject to potentially large errors in estimation.

Around a third to a quarter of total leakage is from customer owned supply pipes which convey water onwards from the curtilage of the property. Although water companies offer subsidised repair and/or replacement schemes, this approach has clouded the matter of responsibility and brought into the debate future ownership of customer supply pipes.   

Although there has been a substantial degree of research into methods of determining current and future SELL, there is still insufficient clarity as to the methodologies of deriving SELL, for the short and longer term, for them to be agreed in the UK.  The evaluation of the long term SELL is dependent on the cost and timing of other means of maintaining an adequate surplus of water supply over demand, such as demand management and water resource and supply enhancement. In undertaking a holistic approach to the costs and benefits there is a requirement to include social as well as environmental costs.  There remains uncertainty as to the factors to include and how to cost them.

To determine the level of leakage it is necessary to determine, and then deduct, water which is being legitimately consumed. However the majority of consumption in most cases remains unmeasured and has to be estimated thus giving rise to uncertainty in leakage estimates. The main source of error is seen, by the regulators in particular, as the estimate of unmeasured per capita consumption (PCC). There is a suspicion that companies reporting high PCC may in fact have higher than reported leakage. At present the most common way for water companies to estimate unmeasured consumption is by measuring the consumption in detail for a small sample of customers then use statistical methods to assess consumption on a company wide basis. 

In addition the lack of metering means that the level of leakage on the privately owned 'supply pipe'  is also difficult to identify and assess. If there is no apparent impact on the property or the supply of water it can be difficult to persuade the property owner to repair the pipe, although many utilties now offer a subsidized repair service.

Pipe pressures have now been shown to have a clear relation to leakage, although there is still some disagreement about the degree of reduction that a pressure management project will bring about, and whether this reduction will gradually be eroded as the water pipe deteriorates. Pressure control is now widespread in most water companies. However, pressure can raise concerns about availabilities of fire fighting water.

As pipes deteriorate there is a theoretical point at which the cost of replacing the pipe is less than the ongoing cost of repairimng leaks. However, this point may lead to an unacceptably high level of interruptions in supply to certain customers. Therefore any replacement programme must consider capital investment costs, operational costs, environmental and social costs for both repairs and placement as well as customer service.



CIWEM recognises the importance of leakage reduction in the management of water supplies and its contribution to the sustainable management of water resources. In this context it supports the efforts of all stakeholders in the water industry to manage leakage effectively and economically.

CIWEM is concerned, however, that because of the high political and media interest in leakage, the role that future leakage reduction can play in securing reliable water supplies may be over-played in some parts of the UK. In particular it has little impact on the ability to meet peak demand. In this respect there is a requirement to communicate effectively the measurement of leakage e.g. Ml/d etc, loss per km of pipe, or per head served, the economics of leakage and how leakage targets are set and expressed. 

CIWEM recognises the difficulties of quantifying the components of SELL, especially the environment and social externalities. CIWEM supports the recent initiatives to clarify how leakage targets should be set and realises that all the interests need to be balanced. The Institution does however believe that a long term view needs to be taken which has due regard to the long term protection of the water environment and the need to conserve and make best use of water. These have to be addressed in the face of the uncertainties of the impact of global warming, including its effects on the availability of water resources and demand, and the need to minimise waste from the use of chemicals and fossil fuels to treat and distribute water.

In addition to the repair of leaks as they become apparent, it is essential that a long term deterioration of the mains network is not allowed to develop through inadequate rates of mains relining and renewal. CIWEM notes from information published by OFWAT that the implied average age of water mains before relining and replacement is currently 60 years for the water utilities as a whole, (based on the maintenance activity of companies in the period 1990-91 to 1997-98). The most active company has mains with an average asset life of 29 years and the least active 272 years. CIWEM does not consider this range to be acceptable and considers that best practice is likely to be closer to that achieved by the most active company. CIWEM is aware of the costs to customers of increased activity on mains renewal or relining, but believes that customers should pay the full costs of service provision rather than allow assets to deteriorate. It supports the approach detailed in the UKWIR report 'A Common Framework for Capital Maintenance'.

CIWEM takes the view that future leakage targets may be tighter for some water utilities than those currently set in order to take a long term view of the environmental and social issues, and in recognition of the levels achievable by the best performing companies. However there needs to be recognition of the practicalities of meeting these targets, the uncertainties in the analytical models and the practicalities of leakage measurement. In addition it is important that a rate of mains renewal is funded which ensures that leakage levels in the future can be controlled at economic levels.

Leakage practitioners currently exchange information through the Water UK Leakage Network and through professional activities initiated by professional organisations such as CIWEM. It is essential that the continued co-operation of leakage professionals be promoted and supported and that comparative or actual competition does not obstruct this activity.

CIWEM also endorses the general principle of EC environmental legislation of 'no deterioration' which should be applied to leakage targets and that once leakage has been reduced it should not be allowed to subsequently rise. The approach of monitoring rolling averages is sensible to ensure that any exceptional weather conditions do not lead to inappropriate regulatory action.

CIWEM considers that Government should give the economic regulator (OFWAT) guidance on the targets which should to be set for companies so as to ensure that wider social, environmental and practical issues are taken into account. However Government will need to be advised by experts in the industry in doing so. In addition CIWEM supports the use of targets which are not misleading and can be easily understood, e.g. Ml/d, l/prop/day or l/km/hr rather than percentages.

CIWEM supports the introduction of pressure management systems where these are shown to be cost effective as this ensures a consistent standard of service to customers throughout the day. However, it is recognised that discussion with all customers, including the fire service, is required before the introduction of any new management scheme.

CIWEM recognises that there are several mechanisms by which leakage can be reduced, such as by pressure reduction, district metering and associated programmes of active leakage control, and online monitoring, "find and fix" and leakage helplines. All such measures should be pursued to their economic level and best practice developed and shared amongst water undertakers.

October 2010


Note:- CIWEM Policy Position Statements (PPS) represents the Institution's views on issues at a particular point in time.  It is accepted that situations change as research provides new evidence.  It should be understood, therefore, that CIWEM PPS's are under constant review, that previously held views may alter and lead to revised PPS's


Supply pipe
That part of the service pipe which is not the communication pipe and is owned by the property owner whose premises are supplied by that pipe.

Communication pipe
That part of the service pipe which is owned by the Water Company and laid in the same street as the main to which it is connected.

Service pipe
Pipe connection between a water main and the premises being supplied. The Service pipe includes accessories such as the ferrule, boundary box/stopcock or other apparatus.


WRc. 2009. 'Code of Practice for the Self Laying of Water Mains and Services - England and Wales' WRc Swindon UK

OFWAT. 2002. Tripartite 'Best Practice Principles in the Economic Level of Leakage Calculation

UKWIR. 1999. NERA The environmental and social value of leakage reduction

WRc. 2008. Leakage Target Setting - A Frontier Approach WRc

OFWAT. 2007. RPS Providing Best Practice Guidance on the inclusions of Externalities in the ELL Calculation
(Main Report and Guidance)

UKWIR. 2002. Capital Maintenance Planning: A Common Framework


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Chartered Institution of Water and
Environmental Management (CIWEM)

106-109 Saffron Hill, London, EC1N 8QS  
Tel: 020 7831 3110   Fax: 020 7405 4967