.. raw:: latex

    \clearpage

Units
======================================

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

* Acceleration = :math:`\rm g` (:math:`\rm g \equiv 9.80665 m/s^2`)
* Concentration = :math:`\rm kg/m^3`
* Demand = :math:`\rm m^3/s`
* Diameter = :math:`\rm m`
* Elevation = :math:`\rm m`
* Energy = :math:`\rm J`
* Flow rate = :math:`\rm m^3/s`
* Head = :math:`\rm m` 
* Headloss = :math:`\rm m` 
* Length = :math:`\rm m`
* Mass = :math:`\rm kg`
* Mass injection = :math:`\rm kg/s`
* Power = :math:`\rm W`
* Pressure head = :math:`\rm m` (assumes a fluid density of 1000 :math:`\rm kg/m^3`)
* Time = :math:`\rm s`
* Velocity = :math:`\rm m/s`
* Volume = :math:`\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 :cite:p:`jcmg11`.  

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, :numref:`table-epanet-units` includes EPANET unit conventions :cite:p:`ross00`.  

.. _table-epanet-units:
.. table:: EPANET INP File Unit Conventions

   +----------------------+-------------------------------------+------------------------------------+
   |   Parameter          |   US customary units                |   SI-based units                   |
   +======================+=====================================+====================================+
   | Concentration        |  *mass* /L where *mass* can be      |  *mass* /L where *mass* can be     |
   |                      |  defined as mg or ug                |  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 :sup:`3` /s               | - LPS: L/s                         |
   |                      | - GPM: gal/min                      | - LPM: L/min                       |
   |                      | - MGD: million gal/day              | - MLD: million L/day               |
   |                      | - IMGD: million imperial gal/day    | - CMH: m :sup:`3` /hr              |
   |                      | - AFD: acre-feet/day                | - CMD: m :sup:`3` /day             |
   +----------------------+-------------------------------------+------------------------------------+
   | Friction factor      |  unitless                           |  unitless                          |
   +----------------------+-------------------------------------+------------------------------------+
   | Hydraulic head       |   ft                                |   m                                |
   +----------------------+-------------------------------------+------------------------------------+
   | Length               |   ft                                |   m                                |
   +----------------------+-------------------------------------+------------------------------------+
   | Minor loss           |  unitless                           |  unitless                          |
   | coefficient          |                                     |                                    |
   +----------------------+-------------------------------------+------------------------------------+
   | Power                |   horsepower                        |   kW                               |
   +----------------------+-------------------------------------+------------------------------------+
   | Pressure             |   psi                               |   m                                |
   +----------------------+-------------------------------------+------------------------------------+
   | Reaction             |   1/day (1st-order)                 |  1/day (1st-order)                 |
   | coefficient (Bulk)   |                                     |                                    |
   +----------------------+-------------------------------------+------------------------------------+
   | Reaction             | - *mass* /ft/day (0-order)          | - *mass* /m/day (0-order)          |
   | coefficient (Wall)   | - ft/day (1st-order)                | - m/day (1st-order)                |
   +----------------------+-------------------------------------+------------------------------------+
   | Roughness            | - 0.001 ft (Darcy-Weisbach)         | - mm (Darcy-Weisbach)              |
   | coefficient          | - unitless (otherwise)              | - unitless (otherwise)             |
   +----------------------+-------------------------------------+------------------------------------+
   | Source mass          |   *mass* /min                       | *mass* /min                        |
   | injection rate       |                                     |                                    |
   +----------------------+-------------------------------------+------------------------------------+
   | Velocity             |   ft/s                              |   m/s                              |
   +----------------------+-------------------------------------+------------------------------------+
   | Volume               |   ft :sup:`3`                       |   m :sup:`3`                       |
   +----------------------+-------------------------------------+------------------------------------+
   | Water age            |   hours                             | hours                              |
   +----------------------+-------------------------------------+------------------------------------+