Units

All data in WNTR is stored in the following SI (International System) units:

• Acceleration = \(\rm g\) (\(\rm g \equiv 9.80665 m/s^2\))

• Concentration = \(\rm kg/m^3\)

• Demand = \(\rm m^3/s\)

• Diameter = \(\rm m\)

• Elevation = \(\rm m\)

• Energy = \(\rm J\)

• Flow rate = \(\rm m^3/s\)

• Head = \(\rm m\)

• Headloss = \(\rm m\)

• Length = \(\rm m\)

• Mass = \(\rm kg\)

• Mass injection = \(\rm kg/s\)

• Power = \(\rm W\)

• Pressure head = \(\rm m\) (assumes a fluid density of 1000 \(\rm kg/m^3\))

• Time = \(\rm s\)

• Velocity = \(\rm m/s\)

• Volume = \(\rm m^3\)

When setting up analysis in WNTR, all input values should be specified in SI units. All simulation results are also stored in SI units and can be converted to other units if desired, for instance by using the SymPy Python package [13].

EPANET unit conventions

WNTR can generate water network models from EPANET INP files using all EPANET unit conventions. When using an EPANET INP file to generate a water network model, WNTR converts model parameters to SI units using the Units and Quality options of the EPANET INP file. These options define the mass and flow units used in the file. Some units also depend on the equation used for pipe roughness headloss and on the reaction order specified.

For reference, Table 4 includes EPANET unit conventions [21].

Table 4 EPANET INP File Unit Conventions

Parameter	US customary units	SI-based units
Concentration	mass /L where mass can be defined as mg or ug	mass /L where mass can be defined as mg or ug
Demand	Same as flow	Same as flow
Diameter (Pipes)	in	mm
Diameter (Tanks)	ft	m
Efficiency (Pumps)	percent	percent
Elevation	ft	m
Emitter coefficient	flow / sqrt(psi)	flow / sqrt(m)
Energy	kW-hours	kW-hours
Flow	CFS: ft 3 /s GPM: gal/min MGD: million gal/day IMGD: million imperial gal/day AFD: acre-feet/day	LPS: L/s LPM: L/min MLD: million L/day CMH: m 3 /hr CMD: m 3 /day
Friction factor	unitless	unitless
Hydraulic head	ft	m
Length	ft	m
Minor loss coefficient	unitless	unitless
Power	horsepower	kW
Pressure	psi	m
Reaction coefficient (Bulk)	1/day (1st-order)	1/day (1st-order)
Reaction coefficient (Wall)	mass /ft/day (0-order) ft/day (1st-order)	mass /m/day (0-order) m/day (1st-order)
Roughness coefficient	0.001 ft (Darcy-Weisbach) unitless (otherwise)	mm (Darcy-Weisbach) unitless (otherwise)
Source mass injection rate	mass /min	mass /min
Velocity	ft/s	m/s
Volume	ft 3	m 3
Water age	hours	hours