EPANET

EPANET, from the US EPA, simulates extended-period hydraulics and water quality in pressurised networks: pipes, pumps with curves and controls, valves, tanks, demand patterns, plus chlorine decay, water age and source tracing. Its INP file format is the lingua franca of the field — practically every commercial distribution package imports and exports it.

Version 2.2 moved development into the open (with the Open Water Analytics community) and added pressure-dependent demand. As with SWMM, the commercial suites largely sell productivity around this engine, so understanding EPANET itself pays off no matter which GUI you use day to day.