Units¶
All data in WNTR is stored in SI (International System) units:
 Length = \(m\)
 Diameter = \(m\)
 Water pressure = \(m\) (this assumes a fluid density of 1000 \(kg/m^3\))
 Elevation = \(m\)
 Mass = \(kg\)
 Time = \(s\)
 Concentration = \(kg/m^3\)
 Demand = \(m^3/s\)
 Velocity = \(m/s\)
 Acceleration = \(g\) (1 \(g\) = 9.81 \(m/s^2\))
 Energy = \(J\)
 Power = \(W\)
 Pressure = \(Pa\)
 Mass injection = \(kg/s\)
 Volume = \(m^3\)
WNTR is compatible with all EPANET unit conventions. When using an EPANET INP file to generate a water network model, WNTR converts model parameters using the units defined in the Units and Quality options of the EPANET INP file. These options define the mass and flow units for the model. Some units also depend on the equation used for pipe roughness headloss and on the reaction order specified. Table 4, Table 5, and Table 6 provide information on EPANET unit conventions (modified from [Ross00]).
Hydraulic parameter  US customary units  SIbased units 

Flow  flow can be defined as:

flow can be defined as:

Demand  flow  flow 
Diameter: pipes  in  mm 
Diameter: tanks  ft  m 
Elevation  ft  m 
Hydraulic head  ft  m 
Length  ft  m 
Emitter coefficient  flow / sqrt(psi)  flow / sqrt(m) 
Friction factor  unitless  unitless 
Minor loss coeff.  unitless  unitless 
Pressure  psi  m or kPa 
Roughness coeff: DW  10 ^{3} ft  mm 
Roughness coeff: HW, CM  unitless  unitless 
Velocity  ft/s  m/s 
Volume  ft ^{3}  m ^{3} 
Water quality parameter  US customary units  SIbased units 

Concentration  mass /L where mass can be defined as mg or ug  mass /L where mass can be defined as mg or ug 
Bulk reaction coefficient: order1  1/day  1/day 
Wall reaction coefficient: order0  mass /ft ^{2} /day  mass /m ^{2} /day 
Wall reaction coefficient: order1  ft/day  m/day 
Reaction rate  mass /L/day  mass /L/day 
Source mass injection rate  mass /min  mass /min 
Water age  hours  hours 
Energy parameter  US customary units  SIbased units 

Energy  kWhours  kWhours 
Efficiency (pumps)  percent  percent 
Power  hp (horsepower)  kW 
When running analysis in WNTR, all input values (i.e., time, pressure threshold, node demand) should be specified in SI units. All simulation results are also stored in SI units and can be converted to other units if desired. The SymPy package can be used to convert between units. The example converting_units.py demonstrates its use:
from sympy.physics import units
# Convert 12 inches to meters
D = 12*float(units.inch/units.meter)
# Covert 0.2 mg/L to kg/m3
C = 0.2*float((units.mg/units.l)/(units.kg/units.m**3))
# Convert 30 psi to m (assuming density = 1000 kg/m3 and gravity = 9.81 m/s2)
if not units.find_unit('waterpressure'):
units.waterpressure = 9810*units.Pa
P = 30*float(units.psi/units.waterpressure)
# Convert 200 gallons/day to m3/day
if not units.find_unit('gallon'):
units.gallon = 4*units.quart
R = 200*float(units.gallon/units.m**3)