Comment by lbutler

The GIS system is generally the base for a hydraulic model. You use that data to build a connected graph structure that the hydraulic engine, EPANET, uses to run calculations to figure out the pressure at the nodes and also flow rates in pipes.

There is also a water quality component where you can calculate the age of water in the system or chemical, such as chlorine, or other by-products you may or may not want in the system.

The US EPA site goes into technical details on what the engine can do [0], but the vast majority of modeling is done as part of a water master plan for a water utility.

A water utility will build a hydraulic model of their network and calculate and model the growth of their city over a 30-year period. The model will highlight areas of concern, generally low pressure, and the water utility can propose new infrastructure like larger pipes, tanks, or pumps, and will schedule future capital works to keep service levels acceptable.

They generally repeat this process every 3–5 years, rebuilding the model and rewriting their master plans. Here is an example of a master plan by the City of Kyle [1].

Generally, a water utility is proposing tens of millions of capital works, if not more. So traditionally, the high price tag has just been accepted. But obviously, this doesn't scale down to smaller utilities, and normally consultants will do the work on their behalf, including holding the right software license.

[0] https://www.epa.gov/water-research/epanet

[1] https://www.cityofkyle.com/media/69766