Developing hydraulic and water quality equivalent systems

Water distribution system modelers organize system models by balancing model complexity and the effort to set up, solve and calibrate the model. The modeling goal is to represent the true system in the field with the least effort and calibration data. As a model increases in complexity as measured by the number of nodes and pipes, more field data is needed to estimate model parameters and the error associated with that model error increases. On the other hand, a simplified model may not capture the full system under all conditions. In terms of water quality, prevailing thought is that an "all-pipe" model is necessary. Here we examine the use of hydraulic equivalent systems on water quality estimates as represented by water age. A simple system is used to demonstrate the effect of applying existing series and parallel pipe simplification approaches. Although these methods preserve pressure conditions, water age is not and increasing levels of system reduction results in larger errors. We then demonstrate that a two step process can insure hydraulic and water quality equivalence for the full range of flows. This study demonstrates that a model skeletonized based on hydraulics to remove or reduce pipes must be re-calibrated for water quality at each level of model simplification.