Water Leak Detection with Minimum Night Flow (MNF)

Pipe leaks are rarely visible. A cracked pipe under a road or inside a wall can run for months without a puddle on the surface. All you see is the bulk meter reading climbing while the number of users stays the same. The question is simple: how many litres are lost every night, and in which zone?

Water leak detection with the Minimum Night Flow (MNF) method answers that question without digging. Water network operators in many countries use it because it only needs well-recorded flow data. This article explains how it works, how to calculate it, and what it takes for the result to be trustworthy.

What Minimum Night Flow is

Minimum Night Flow is the lowest flow entering a zone of a water network at night, usually between 2 and 4 a.m. At that hour almost everyone is asleep, factories and buildings have stopped, and water use drops to its lowest point.

If legitimate use is close to zero but the flow into the zone is still high, the remainder is almost certainly leaving through leaks. That is the core of MNF: water that keeps flowing when nobody is using it.

Why measure at night

• Legitimate use is at its lowest. The gap between inflow and consumption is easier to read when consumption is almost nil.
• Leaks keep flowing. A leaking pipe does not sleep. Its flow is fairly steady through the day, and often larger at night because network pressure rises as demand falls.
• A repeating pattern. Because it is measured at the same hour every night, small day-to-day changes show up clearly as a trend.

How to estimate leakage from MNF data

The basic formula:

Night leakage = MNF minus legitimate night use

Legitimate night use covers residents who still use some water (toilets, tanks refilling) and large customers that operate at night, such as three-shift factories or hospitals. Household use is usually estimated per connection, while large customers are best measured directly with their own meters.

Once night leakage is known, daily leakage is not simply night leakage times 24 hours. Daytime pressure is lower, so daytime leakage is smaller too. Network operators use an equivalent-hours factor (often called the night-day factor) that is usually below 24, and its value depends on the pressure pattern in each zone.

Worked example

The figures below are an illustration based on assumptions, not project data. Use your own zone's numbers.

Item	Value
Connections in the zone	400
Assumed legitimate night use per connection	2 litres per hour
Legitimate night use (400 x 2 litres)	0.8 m³ per hour
Recorded MNF (average, 2 to 4 a.m.)	4.4 m³ per hour
Night leakage (4.4 minus 0.8)	3.6 m³ per hour
Assumed equivalent hours factor	20 hours
Estimated daily leakage (3.6 x 20)	72 m³ per day

In this example, about 72 m³ of water is lost every day from a single zone, or more than 2,000 m³ a month. That water has already been treated, pumped, and paid for, but never reaches a user.

What makes MNF reliable

1. An isolated zone. The network is split into closed zones (often called District Metered Areas, or DMAs) with one or two metered inlets. Boundary valves to neighbouring zones must be fully closed.
2. Flow meters that are accurate at low flow. MNF happens exactly when flow is smallest. A meter that is not sensitive at low flow will under-read and hide leakage.
3. Closely spaced data. A reading every 15 minutes or more often is enough to find the lowest point. A single manual reading per day will not do.
4. Pressure recorded alongside. Pressure changes alter leak size. Without pressure data, a rise in MNF can be misread.
5. Known large users. List the customers that use water at night so their consumption is not counted as leakage.

Reading the MNF trend every day

A single night's MNF gives a snapshot, but the trend is what matters most:

• A sudden jump from one night to the next usually means a burst pipe or a failed joint. It needs attention right away.
• A slow rise over weeks points to small leaks that are growing, such as joints starting to seep in many places.
• A drop after a repair proves the repair worked. Without this data, field teams cannot tell whether the leak they fixed was the biggest one.

When a network has many zones, their MNF values can be ranked from highest to lowest. A limited field team can then go straight to the zones with the largest losses.

From numbers to repairs in the field

1. Pick priority zones from the MNF ranking.
2. Narrow down the location by closing valves inside the zone one at a time at night and watching the flow drop (a step test).
3. Pinpoint the leak with acoustic leak detection equipment on the narrowed section.
4. Repair, then verify with the next night's MNF. If it falls as expected, the zone can leave the priority list.

Who needs it most

• Water network operators who want to reduce non-revenue water area by area.
• Industrial estates that supply water to many tenants through their own pipe network.
• Housing estates, apartments, and campuses with their own reservoirs and internal distribution.
• Factories and hospitals with long pipe runs that are mostly buried.

Frequently asked questions

Can MNF be used without splitting the network into zones?

Yes, on a single bulk meter, but it only tells you there is a leak, not where. Zoning is what lets you narrow down the location.

What time should MNF be taken?

Usually between 2 and 4 a.m. Look at a few weeks of data to confirm the lowest hour in your zone, because patterns differ from site to site.

What kind of flow meter is needed?

One that stays accurate at low flow and can send data automatically, such as an electromagnetic or ultrasonic flow meter with Modbus RTU output.

How long until results show?

With data recorded every night, a baseline MNF pattern usually appears within one to two weeks. After that, every change can be compared against that baseline.

Leak detection by zone with InWater

The InWater solution on the INCLUDE platform calculates Minimum Night Flow per zone automatically, ranks zones to prioritise inspections, and raises an alarm when night flow rises beyond its usual range. Reservoir level and water quality are monitored on the same dashboard, and valves can open or close automatically by schedule or threshold.

Flow meters already installed with Modbus RTU output can be read through IncludeGateways and sent over 4G, with no data cable to the control room. For reservoirs and tanks, see the guide to water tank level sensors, and to set alarm limits that do not fire too often, read sensor alarm thresholds. InWater is available from the Starter plan; details are on the pricing page.