Leakage and non-revenue water
Why leakage figures are so hard to compare, why percentage measures mislead, and why "just fix the leaks" is harder than it sounds.
Leakage is the most cited and least well understood number in the water industry. Two things cause most of the confusion: leakage is not measured directly, and it is routinely conflated with non-revenue water.
The distinction that matters
- Real losses (leakage)
- Water that physically escapes from mains, service connections and storage. This is what people mean by leakage.
- Apparent losses
- Water delivered and consumed but not correctly recorded: meter under-registration, unauthorised use, billing and data errors.
- Unbilled authorised consumption
- Water legitimately used but not charged for: firefighting, mains flushing, street cleaning.
- Non-revenue water
- All three of the above combined. It is a commercial measure, not a physical one.
A utility can cut its non-revenue water substantially by replacing old meters and fixing its billing system, without repairing a single leak. Reporting that improvement as reduced leakage would be wrong, and it happens.
Why leakage is a residual, not a measurement
Nobody meters the water leaving a burst main. Leakage is calculated as what remains after subtracting all measured and estimated consumption from the total put into the network. It therefore inherits the error of every other term in that balance — and the largest of those is usually the estimate of unmetered household consumption.
This is why leakage figures carry genuine uncertainty and why methodology changes can move a company’s reported number without anything physical changing. Hydrionis records these values at low confidence for that reason.
Why percentages mislead
A utility reporting 20% non-revenue water and one reporting 30% may have identical physical losses, if the first serves customers who use more water. The percentage falls as consumption rises, entirely independently of the pipes.
The International Water Association recommends volume-per-connection measures instead — litres per property per day, or per kilometre of main — precisely because they are not distorted by the denominator. Where Hydrionis presents a percentage, it says what the denominator is.
Absolute leakage scales with the size of the company, so it must not be used to rank utilities of different sizes. Use litres per property per day or leakage per kilometre of main for comparison.
View the data behind this chart
| Subject | Value (Ml/d) | Notes |
|---|---|---|
| Thames Water | 570.0 | — |
| Scottish Water | 455.0 | — |
| United Utilities | 415.0 | — |
| Severn Trent Water | 400.0 | — |
| Yorkshire Water | 260.0 | — |
| Anglian Water | 175.0 | — |
| Dŵr Cymru Welsh Water | 165.0 | — |
| Northumbrian Water | 130.0 | — |
Why "just fix the leaks" is hard
- Most leakage is not dramatic bursts but a large number of small, continuous background leaks that never surface
- Finding them requires acoustic surveying, district metering and increasingly satellite and sensor techniques — none of which is cheap per leak found
- Networks are largely buried under roads, so repair cost is dominated by excavation and reinstatement rather than by the pipe
- Higher pressure means more leakage, but lower pressure means poorer service, so pressure management is a genuine trade-off
- There is an economic level of leakage below which reducing further costs more than the water is worth — a legitimate concept that is also, understandably, a difficult one to defend publicly
Sources
- Water Services Regulation Authority — Water company performance and price review data. Open — attribution required · Open Government Licence v3.0
- International Water Association — IWA water loss and performance indicator framework. Public but restricted · Publisher terms — public access, reuse not clearly granted
- American Water Works Association — AWWA standards, water audits and utility benchmarking. Public but restricted · Publisher terms — public access, reuse not clearly granted