Quantities of the various water loss volumes occurring in a water utility can be approximated by employing a mathematical representation, or model, of the loss values. Depending upon the type and nature of the apparent or real losses being modeled, a model can be a simple spreadsheet of estimates of loss volumes attributed to a specific type of loss occurrence, or it can be a complex set of calculations that rely upon a number of data inputs to calculate a reliable quantity of loss. Models are an excellent tool to assist the operator with the preparation of a water audit and water loss management planning; however they should be used with care and due diligence. Models are not magic nor do they give us hind sight or act as a crystal ball; they are only as good as the concepts they employ, the data that is put into them, and the skill and experience of the user; training in their use is essential. So care should be taken to ensure that field data captured and coefficients and variables used represent real conditions as closely as may be necessary for a result of required accuracy. If accountable data is not available estimated data may be used, however, the model should be notated with comments reflecting the estimated inaccuracy for each component and calculating the final weighted potential inaccuracy. Many industry standard water loss control models now incorporate the use of 95% confidence limits, which are applied to each component of data input and calculated for each component of data output. Further information on the use of 95% confidence limits is covered in Chap. 7. This chapter presents examples of some basic water loss models.

Modeling flows in pipe networks and components of consumption has been an integral part of hydraulic network analysis modeling (hydraulic models) for over 30 years, but in

5% confidence limits is used in order to assign confidence to each input component and to calculate aggregated confidence in the final result.

these models nonrevenue water has generally been treated very simplistically as a fixed residual. Accordingly, a separate series of concepts for modeling water loss has been developed since the early 1990s for the following components of nonrevenue water:

• Apparent loss (customer meter inaccuracy, systematic data-handling error in billing systems, and unauthorized consumption)

• Real loss (leakage and overflows)

• Pressure/leakage, pressure/consumption, and pressure/break (frequency relationships)

The reliability and effectiveness of water loss modeling makes it a standard part of the loss management practitioner's tool kit.

It is important to emphasize that water loss management models are not the same tools as hydraulic models. Many water utility personnel, consultants, and contractors have used or seen a hydraulic model, which mathematically calculates values of water flow and pressure in a distribution network, subject to specific inputs and consumption patterns. Hydraulic models are an extremely powerful tool for distribution system analysis, allowing the operator to simulate varying operating scenarios within the system. However, the concepts used for simulating water loss management in most hydraulic models are often oversimplified, to the point where the estimated current leakage is nominally distributed globally around the nodes of the model; and assumed to be fixed over time and pressure-invariant. While such simplified assumptions may be valid for modeling flows and pressures in water distribution piping systems, they are not valid for models which seek to quantify key water loss components.

The water loss modeling approaches discussed include

• Top-down water audit spreadsheet models

• Component analysis of apparent (nonphysical) losses

• Component analysis of real (physical) losses, such as the breaks and background estimates (BABE) model

• The fixed and variable area discharge (FAVAD) concept for modeling pressure/leakage rate relationships and pressure/consumption relationships and making predictions

• Pressure/break frequency analysis concepts for making predictions of the reduction in break frequency on mains and services with reduction in operating pressure

• Application of component analysis and FAVAD concepts for night-flow analysis in discrete zones or district metered areas

• Consumption analysis models

• Short run economic leakage levels

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