The largest component of non-revenue water in utilities around the world is leakage. Leakage increases due to asset aging, high operating pressures, surge events, corrosion, external pressures, etc. It can account for more than half of the produced water in the worst cases. Leaks are classified based on their size, either as detectable or as background (undetectable through acoustic means).
As opposed to regular demands, leakage flow has a much closer relationship with pressure. During periods of peak consumption with lower pressures, leaks are at their minimum. During the early morning, when the consumption is minimal and pressures are highest, leakage is at its maximum. For this reason, models should incorporate the pressure dependence of leakage to produce accurate estimates. When modeling leakage as standard demands, even if the associated patterns follow the typical daily pressure dynamics, the model will fail to accurately mimic the real behavior of leaks—especially as a response to anomalous pressure conditions.
Enabling pressure-dependent leakage in water distribution modeling contributes towards important goals, such as:
- Quantifying more accurately the amount of leakage throughout the network and throughout time
- Assessing the impact on leakage reduction when implementing pressure control programs (like lowering valve settings at night), or other interventions
- Narrowing down the estimated location of potential detectable leaks to be addressed by surveying and maintenance crews
- Narrowing down the list of potential assets to be replaced due to high background leakage
Assigning emitters to junctions is one way in which pressure-dependent leakage can be modeled. However, introduced in version 2025.4, the Leakage module further facilitates pressure-dependent leakage modeling, with several advantages over emitters:
- Can be assigned to pipes directly and not only to junctions
- Input values are expressed in terms of leakage area (of cracks, fissures, openings, punctures, etc.), which eases comprehension by representing a straightforward physical property
- Input area values at pipes are normalized by length, so that the numbers are more indicative of the relative condition of the assets
- Much higher accuracy when simulating flexible pipes, such as those made of PVC, through the adoption of the fixed and variable area discharge (FAVAD) paradigm
Fixed and variable area discharge (FAVAD) paradigm
The leak flow (Q) using the established emitter paradigm is computed with this equation: Q = ChN1.
C is the emitter coefficient,
N1 is the global emitter exponent (defined in Simulation Options Demand and usually set to 0.5), and
h is the pressure. However, it has been shown that the exponent is larger for flexible materials, and also that it changes with pressure. This is because leaks are usually flexible and do not behave like an orifice does in that their area also increases with pressure. Totally rigid leaks have an
N1 value of 0.5, while totally flexible ones with an initial area of zero have a value of 1.5.
To overcome this limitation, the FAVAD paradigm utilizes this extended equation for Q:
![](https://help.autodesk.com/cloudhelp/ENU/INFWP-UserGuide/images/GUID-0EDD7F5D-8A0B-4F94-880F-32660A4E4B4F.png)
Reference paper: van Zyl, J. E., Asce, M., & Cassa, A. M. (2014). Modeling Elastically Deforming Leaks in Water Distribution Pipes. Journal of Hydraulic Engineering, 140, 182-189.
Cd is the known discharge coefficient and g is the acceleration due to gravity. A0 is the area of the leak when the pressure is zero. M is the expansion rate of the leak area as pressure increases. Under this paradigm, and following EPANET's 2.3 current unreleased implementation, the values for A0 and M need to be assigned to each pipe. More specifically, the user needs to input unit values a0 and m that are normalized by pipe length (l): A0 = a0l; M = ml.
Background leakage parameters for rigid pipes
Rigid pipes, such as those made of cast iron or steel, have a very small unit expansion rate m, which can be assumed to be negligible (m = 0.0) for practical purposes. That is, it is assumed that the leakage area does not increase in any meaningful way when pressure increases. The initial leakage area a0, on the other hand, is always positive because, in practice, there is always an unavoidable level of background leakage. a0 ranges between 0.1 mm2/1000ft (ideal conditions) to values of around 5.0 mm2/1000ft. Values can be much larger if there are leaks that become detectable through acoustic or other means (and thus cannot be said to be background leaks anymore).
Ideally, these parameters would be estimated by simultaneously considering the following sources of information:
- Mass balance and derived total leakage (e.g., by contrasting produced measured volumes with totalized measured consumption values)
- Asset information for pipes: age, material, structural vulnerability, chemical vulnerability, maintenance history, etc.
- Hydraulic conditions: historic operating pressure, current average pressure ranges, pressure surge events, etc.
- Additional SCADA measurements (e.g., pressure sensors)
![](https://help.autodesk.com/cloudhelp/ENU/INFWP-UserGuide/images/GUID-972AA0A2-04F4-4ACE-BBB9-BF5B02B29223.png)
Symbols:
- ICF: The Infrastructure Condition Factor. 1.0 under ideal conditions (new pipe, high-quality installation, proper surge protections, intensive active leakage control). Usually, no higher than 3.0. Please refer to the M36 manual for methods on computing the ICF.
- pav : Average pressure at the pipe in psi
- Nc : Number of service connections per 1000 feet
- Lc : Average length of service connections, from the curb stop to the customer meter, in feet
This equation is derived mainly from equation 7-5 for the Target Background Leakage (TBL). Please note that the AWWA's method is intended for background, undetectable leakage only. Detectable leakage is assumed to be considered separately in their methodology. Here are some examples of a0 (in mm2/1000ft):
- 0.14 - A newly installed transmission line with no service connections, ideal operation and maintenance, and an average operating pressure of 60 psi
- 0.30 - A middle-aged transmission line with no service connections, and an average operating pressure of 80 psi
- 0.47 - A newly installed main with 10 service connections every 1000 feet (18 feet each), ideal operation and maintenance, and an average operating pressure of 60 psi
- 0.61 - A sub-urban middle-aged main with 4 service connections every 1000 feet (48 feet each), and an average operating pressure of 95 psi
- 0.84 - An old, low pressure main with 8 service connections every 1000 feet (18 feet each), and an average operating pressure of 50 psi
- 1.12 - A 20-year-old high-pressure main with 10 connections every 1000 feet (18 feet each), and an average operating pressure of 110 psi
- 1.25 - A high-density urban main with 20 service connections every 1000 feet (12 feet each), and an average operating pressure of 70 psi
- 2.11 - An old main in poor condition with 10 connections every 1000 feet (18 feet each), and an average operating pressure of 90 psi
Background leakage parameters for semi-rigid and flexible pipes
For further reference, see: van Zyl, J. E., Asce, M., & Cassa, A. M. (2014). Modeling Elastically Deforming Leaks in Water Distribution Pipes. Journal of Hydraulic Engineering, 140, 182-189.
The rigidity of a pipe depends mostly on two characteristics, the Elasticity modulus (E) of its material, and the thickness of its wall. Cast iron pipes have very large values of E (>1.5x107 psi or 100 GPa) and relatively thick walls. They can thus be considered as fully rigid, with a leakage expansion rate m of 0.0. PVC pipes are usually thinner and have a much smaller value of E (~4.4x105 psi or 3 GPa). They can be considered as fully flexible, with an initial leakage area at atmospheric pressure a0 of 0.0. Pipes of other materials will sit somewhere in the middle. For example, asbestos cement pipes have a thick wall and an intermediate value of E (~1.7x106 psi or 12 GPa). For such semi-rigid pipes, it would be more accurate to assign non-zero values to both a0 and m.
To determine the leakage parameters for flexible or semi-rigid pipes, follow the following steps:
- Compute the initial leakage rigid area a0r with the equation above (derived from the AWWA M36 manual).
- Estimate the elastic modulus E and wall thickness of the pipe.
- Classify the pipe as flexible, semi-rigid, or rigid, based on these attributes:
- If rigid, keep the rigid value of a0 = a0r and assign m = 0.0; finish.
- If flexible, make the variable area leakage contribution v = 100%
- If semi-rigid, assign a variable area leakage contribution 0% < v < 100%
- Compute
a0 and
m by distributing the initial rigid area
a0r at the average pressure
pav according to the ratio
v: