The concept of hydraulic equivalence is useful in simplifying network models. It refers to the idea of defining an equivalent pipe to replace two or more pipes in parallel or two or more pipes connected in a series, while preserving hydraulic integrity. The equivalent pipe will produce the same head loss as the pipes it represents and will most likely have a nonstandard diameter. Methods for defining an equivalent pipe for parallel and series pipes follows.

Series Pipes

Pipes connected in series might have different C-values and/or lengths and well as different sizes as shown in the figure below.

It is possible to replace series pipes by an equivalent pipe. Noting that the same flow must pass through each pipe in a series, the equivalent pipe is determined as the pipe which will carry this flow rate and produces the same head loss as the series pipes (sum of all individual head losses).

Parallel Pipes

An equivalent pipe can be used to replace two or more pipes in parallel. Noting that the head loss in each parallel pipe must be equal, the basic principle behind the concept of an equivalent pipe is to replace parallel pipes by a single pipe which will transport the same total flow rate for the same head difference between the connecting nodes.

Equivalent pipes were once commonly used by modelers to minimize the number of pipes in the model. However, the limitations (hardware and software capacities) that made this process necessary have been eliminated and it is better engineering practice to avoid using equivalent pipes, especially for water quality modeling. The reduced network model will contain equivalent pipe diameters that do not represent actual pipes, so there can be confusion regarding what is real and what is equivalent. In addition, the equivalent pipes will be different from those in the CAD and GIS data sets, which will make it difficult to maintain data integrity and significantly limit data updating, transfer and maintenance.