The AWWA M36 water loss audit: a guide for utilities and cities.
An AWWA M36 water audit is a standard balance that accounts for all water a utility puts into its system: authorized consumption on one side, apparent and real losses on the other. It needs measured volumes for system input and for each kind of consumption, and a live water balance keeps those volumes measured all year instead of rebuilt at audit time.
Keep every AWWA M36 audit volume measured all year, not rebuilt at deadline.
The top of the balance, measured instead of assumed
- Source meters live
Production and import meters are read continuously, so system input volume is measured every day.
- Meter health shown
Connection rate and data gaps on each meter are visible, so a missing day is caught early.
- Data quality noted
Periods with weak coverage are flagged, so audit validators see where the data is strong.
Billed and unbilled consumption, reconciled against input
- Billed metered use
AMI customer reads reconcile against district input, so the authorized side of the balance stays current.
- Unbilled use counted
Flushing, firefighting and municipal use are recorded as measured volumes where meters exist.
- Bills checked
Billed volumes are compared with metered volumes, so apparent losses from billing gaps surface.
Losses localized by district, then verified after repair
- District balances
Real losses are tracked per district metered area, so the audit total has addresses behind it.
- Night flow baselines
Minimum night flow per district shows whether losses are rising between audits.
- Repairs verified
After each repair, the district balance confirms the loss fell, with records kept for the next audit.
Preparing a water audit, before and after Aqueduct
- Audit volumes are gathered from many systems at deadline.
- System input rests on meters nobody checked all year.
- Real losses are a single total with no location.
- Apparent losses are estimated with default values.
- Repairs between audits leave no measured trace.
The Aqueduct way- Audit volumes are measured all year from a live balance.
- Source meter health and data gaps are visible every day.
- Real losses are tracked per district metered area.
- Billed and metered volumes are compared continuously.
- Every repair is verified in the district balance.
What the M36 audit is.
The American Water Works Association publishes Manual M36, Water Audits and Loss Control Programs. It sets out a top-down water audit: a standard balance that starts with all the water a utility puts into its distribution system and accounts for where it went. AWWA also publishes free water audit software that follows the same structure.
The balance has two sides. On one side is system input volume: water the utility produces or imports, less water it exports. On the other side is everything that happens to that water: authorized consumption, billed and unbilled, and water losses, split into apparent losses and real losses.
Non-revenue water is the part of system input that earns no revenue: unbilled authorized consumption plus apparent losses plus real losses. Several states require utilities to complete and submit an M36 audit, in some cases with validation. The rules differ by state, so check the current rule for yours.
The lines of the balance.
A simplified view of the M36 structure. Your audit software and your state's rule set the exact inputs.
| Line | What it covers | How it is usually measured | What a live balance supplies |
|---|---|---|---|
| System input volume | Water produced from own sources plus water imported, less water exported | Production and bulk meters, often read through SCADA | Continuous measured volumes per source and per district |
| Billed authorized consumption | Metered and unmetered use that is billed to customers | Customer meters and billing records | AMI reads summed per district for comparison with supply |
| Unbilled authorized consumption | Authorized use that is not billed, such as firefighting, flushing and some municipal use | Often estimated | Metered use where municipal buildings and facilities are metered |
| Apparent losses | Customer meter inaccuracy, unauthorized consumption and data handling errors | Meter testing, billing analysis and estimates | Meters reading outside their usual range, flagged for testing |
| Real losses | Physical leakage from mains, service connections and storage | Calculated as what remains after the other lines | Night flow per district metered area, showing where real losses sit |
What the audit needs from you.
The audit is only as good as its inputs. The largest single input is system input volume, so production and import meters need to be accurate and their records complete. The audit also asks how confident you are in each number, through data validity grading.
Real losses are usually derived, not measured directly: they are what is left once system input, authorized consumption and apparent losses are accounted for. That makes them sensitive to errors on every other line. The better the measured inputs, the more the real loss figure can be trusted.
Performance indicators follow from the balance. The Infrastructure Leakage Index compares current annual real losses with an estimate of unavoidable losses for a system of that size and pressure. It is a useful benchmark once the inputs behind it are sound.
Why audits get hard.
Most of the effort goes into rebuilding a year of data at the deadline.
Volumes gathered once a year
Production, import and billing data are pulled together at audit time, gaps and all.
A total, not a location
The audit gives a system-wide real loss figure. It does not say which district is leaking.
Validation turns into reconstruction
When inputs were not kept measured, validating them means rebuilding them.
Repairs that are not checked
Leak repairs close as work orders. Whether district loss actually fell is rarely confirmed.
A water audit should describe the system as it is, not as it was guessed.
Talk to our teamHow a live water balance feeds the audit.
Aqueduct keeps the audit inputs measured all year. Your team completes, validates and files the audit.
Monitor
Read production, import and export meters through SCADA, and customer reads through AMI.
Measure
Keep a water balance and a night flow baseline for each district metered area, every day.
Identify
Flag the district where loss rises above its model, and meters reading outside their usual range.
Act
Route the work order into your CMMS or EAM with the likely location.
Verify
Confirm the repair when night flow returns to baseline. The measured volumes carry into the annual audit.
What Aqueduct does and does not do.
Aqueduct supplies continuously measured volumes per district for each line of the balance it can measure, and shows where real losses sit. It works alongside your SCADA, AMI and historians and reads their data. Control of the network stays with SCADA.
Aqueduct does not complete, validate or file the audit, and it does not replace the AWWA software or a validator. Your team, or the consultant you use, remains responsible for the audit and for meeting your state's rule.
Related glossary entries.
Each links to its glossary entry.
Where teams put it to work
Audit inputs ready any day
Measured system input, authorized consumption and loss volumes kept current, so the audit is a review rather than a rebuild.
Learn more
From audit total to district
Real losses split by district metered area, so the audit number becomes a list of districts crews can act on.
Learn more
Terms behind the balance
Plain definitions of system input, real losses, apparent losses and non-revenue water, for staff new to the audit.
Learn moreRelated
Common questions
A standard top-down water balance from the American Water Works Association. It accounts for system input volume against authorized consumption, apparent losses and real losses.
Real losses are physical: water that leaks from mains, service connections and storage. Apparent losses are water that is used but not correctly measured or billed, such as meter inaccuracy, data errors and unauthorized use.
Several states require utilities to submit an M36 audit, some with validation. The rules differ by state, so check the current rule for yours.
No. Aqueduct supplies measured volumes per district and shows where real losses sit. Your team completes, validates and files the audit.
District metering lets the balance be kept per district, which is what localizes loss. Aqueduct baselines the districts you have and shows where added metering would help.
See Aqueduct on your own facility.
Bring a site, a utility bill or a question. We will show you where the loss is.