In the Method of Characteristics, the pipes in the network are broken into segments so that a sharp pressure-wave front can travel the length of one of the pipe's interior segments in one time step. However in systems with a mix of very long and short pipes, it is not always practical to use very small time steps since this can significantly increase the time it takes to complete a simulation. Therefore, it is possible to adjust either the length or wave speed parameters for each pipe so that a larger time step can be used while still satisfying the requirement that a sharp pressure-wave front can travel the length of one of the pipe's interior segments in one time step.

For example, if a pipe has a length of 10 ft and the wave speed is 1000 ft/s, then the time step required to simulate this pipe without adjustment is 0.01 seconds (= 1 ft / 1000 ft/s). However, if the time step was set to 0.02 seconds, the pipe length would need to be adjusted to 20 ft (= 0.02 s x 1000 ft/s), or the wave speed would need to be reduced to 500 ft/s (= 10 ft / 0.02 s) to satisfy the requirement that a sharp pressure-wave front can travel the length of one of the pipe's interior segments in one time step.

In general, a smaller calculation time step will produce a more accurate solution but will take longer to compute. However, using a larger time step (and adjusting pipe lengths or wave speeds) can produce accurate simulation results with much shorter simulation times, so this is generally recommended.

The calculation time step used in WaterGEMS CONNECT can be defined by the user, or the user can elect to have WaterGEMS CONNECT automatically select a time step for them. If WaterGEMS CONNECT selects the time step, it will attempt ensure the time step provides a good trade off between solution accuracy and the time taken to compute the simulation. The time step selected by WaterGEMS CONNECT generally requires some adjustment to the pipe lengths or wave speeds. The adjustments are done automatically by WaterGEMS CONNECT, but the user is able to select whether they want the length or wave speed adjusted. Similarly, if a user enters their own time step, WaterGEMS CONNECT will adjust the pipe lengths or wave speed accordingly and once again the user can select which of these parameters is adjusted.

In addition, many short pipes in a model will prompt WaterGEMS CONNECT to select a smaller time step - increasing the time taken to compute a simulation. (Note: it may be possible to remove short pipes from the model using the Skelebrator tool.)

Regardless of whether a user-defined, or automatic time step is used, users are advised to conduct a sensitivity analysis using a run with a very small user-defined time step to satisfy themselves that the time step they are using produces satisfactory results. (The appropriate time step to use for this will depend on the model, but a value like 0.01 s is suggested.) If the run using a very small time step produces results that correlate well with results obtained using a larger time step, then it should be valid to adopt the larger time step.

Likewise, there is no hard and fast rule which determines the maximum amount of adjustment that can be applied to pipe lengths of wave speeds without adversely affecting the results, so users should investigate the sensitivity of results to different levels of adjustment. However, users should keep in mind that, if the mean pipe length adjustment is significant, this means that the mass of liquid analyzed in the model is significantly different to the mass of liquid in the real system.