4.2 Units
The WQ Module allows for simulation construction in one of two unit systems (see Section 4.5.1):
- A milligrams per litre system (which includes micrograms per litre for phytoplankton) that aligns with typical units of laboratory reporting, and
- A millimoles per cubic metre system
A single units system must be adopted for a WQ Module simulation, and unit systems cannot be mixed within a simulation. This specification has implications for at least the setting of numerical values for:
- Initial conditions
- Boundary conditions
- Computed variable parameters
When the milligrams per litre system is used, the WQ Module executes internal conversions between milligrams and millimoles, because millimoles are the units expected by the core water quality calculation routines. This conversion follows the form:
\[\begin{equation} n = \frac{m}{M} \tag{4.1} \end{equation}\] where \(n\) is millimoles (mmol), \(m\) is mass (mg) and \(M\) is molar weight (mg/mmol). A similar concept applies to concentration and flux quantities. As such, the WQ Module assigns values to \(M\) in converting from milligrams to millimoles, i.e. from user specified milligram initial conditions, boundary conditions and computed variable parameters, to the equivalent millimole quantities. The complete list of values of \(M\) for all simulated constituents is provided in Table P.1. The two possibilities for units for each parameter are provided in Appendix R, where relevant.
As an example, if a user specifies the milligrams per litre units system (or allows the same default to be set) and sets
Given the above, users opting to construct WQ Module simulations in the milligrams units system need to clearly understand what is being reported in:
- Field and/or laboratory measurements, and
- Other numerical model predictions that may be used as WQ Module boundary conditions (such as catchment pollutant export models)
In the case of laboratory nutrient measurements, it is conventional to report concentrations of the typical quantities used by the WQ Module as silicate-Si, ammonium-N, nitrate-N, FRP-P, organic-C, organic-N, and organic-P. Whilst these quantities might be referred to informally as (for example) “nitrate concentrations”, the reported number is most commonly the concentration of elemental nitrogen contained within nitrate. To be clear, if a nitrate-N concentration is reported by a laboratory to be 3.5 mg/L, this means that in every litre of water, there are 3.5 milligrams of elemental nitrogen contained within nitrate molecules. Given the molar masses of elemental nitrogen and nitrate, this 3.5 milligrams of elemental N means that there are 15.5 milligrams of nitrate molecules (15.5 = 3.5 x 62/14) in the same litre of water. In this example, the concentration with which the WQ Module expects to be provided is 3.5 mg/L, not 15.5 mg/L.
The use of other model data to force WQ Module boundary conditions is also interpreted in this way. For example, a catchment model might report that an inflow to the WQ Module at an upstream riverine boundary condition has a “total nitrogen” concentration of 10.0 mg/L, and a user may have then speciated this to nitrate by applying a multiplicative factor of 0.15. If this resulting nitrate concentration of 1.5 mg/L is applied to the WQ Module as a boundary condition, it must be a concentration of elemental nitrogen within nitrate, which is consistent with what would be reported by a laboratory. This means that the user must be sure that the “total nitrogen” reported by the catchment model is in fact an elemental nitrogen concentration.
If users elect to construct WQ Module simulations in units of millimoles, then no internal WQ Module conversions take place, and the nature of the quantity (molecular or elemental) being specified is not relevant. For example, the user could specify millimoles units and