Water loss glossary

Every term used in the calculator and the IWA water balance, in plain language.

Apparent losses: Water consumed but not recorded: unauthorised consumption, customer meter under-registration and data-handling errors. Also called commercial losses. Valued at the tariff.
Authorised consumption: Water used legitimately by registered customers and the utility, billed or unbilled, metered or unmetered.
AWWA M36: The American Water Works Association manual that codifies the IWA water balance as the US 'water audit'.
Background leakage: Small, undetectable leaks at joints and fittings that run continuously; part of the unavoidable annual real losses.
Billed authorised consumption: Consumption that generates revenue — billed metered plus billed unmetered. Equivalent to revenue water.
CARL: Current annual real losses: the real losses a system actually has, usually expressed in m³/day or litres per connection per day.
Commercial losses: Another term for apparent losses, used by the World Bank.
DMA: District metered area: a hydraulically discrete part of a network, typically 500–3,000 connections, with all inflows metered so that leakage can be measured from night flow.
Economic level of leakage (ELL): The level of leakage at which the cost of reducing it further equals the value of the water saved.
ILI: Infrastructure Leakage Index: CARL ÷ UARL. A ratio of current to unavoidable real losses; 1.0 is the technical floor. Banded A1–D by the World Bank Institute.
Intermittent supply: Supply that is not pressurised 24 hours a day. Indicators are corrected to 'when the system is pressurised' for comparison.
Marginal cost of water: The variable cost of producing and distributing one more cubic metre — energy, chemicals, bulk purchase. Used to value real losses.
Minimum night flow (MNF): The lowest flow into a DMA, usually between 2 and 4 a.m., when legitimate use is smallest and leakage dominates.
N1 exponent: The exponent relating leakage to pressure (L ∝ P^N1); typically 1.0–1.5, higher for flexible pipes and background leakage.
Non-revenue water (NRW): System input volume minus billed authorised consumption: unbilled authorised consumption plus apparent losses plus real losses.
Pressure management: Controlling network pressure with valves and zoning to reduce leakage and burst frequency.
Real losses: Physical leakage from mains, service connections and storage up to the customer meter. Calculated as the residual of the top-down balance.
Revenue water: Water that is billed — the same as billed authorised consumption.
Service connection: The pipe connecting a main to a customer property; the unit for per-connection indicators. One block of flats on one pipe is one connection.
Service pipe length (Lp): Total length of service pipe between property boundaries and customer meters; zero when meters are at the boundary.
System input volume (SIV): The volume of treated water entering the distribution system in the balance period, from all sources, corrected for bulk-meter error.
Top-down / bottom-up: Top-down: the water balance, real losses as a residual. Bottom-up: night-flow measurement in DMAs. A sound programme reconciles both.
UARL: Unavoidable annual real losses: (18 × km mains + 0.8 × connections + 25 × km service pipe) × pressure (m) litres/day. The technically achievable leakage floor.
Unauthorised consumption: Illegal connections, meter bypasses and tampering, hydrant theft — water taken without authorisation.
Unbilled authorised consumption: Legitimate use that is not charged: firefighting, mains flushing, exempt premises, utility works.
Under-registration: The share of consumption a customer meter fails to record, mostly at low flows and on aged meters.
Water losses: System input volume minus authorised consumption; the sum of apparent and real losses.
w.s.p.: 'When the system is pressurised' — an indicator corrected for hours of supply per day so intermittent and continuous systems compare fairly.

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