Constituents

For an introduction to Water Quality in Source, please see Water Quality under Fundamental Concepts in the Getting Started chapter of the Source User Guide.

In Source, the term constituents refers to materials that are generated and transported within a catchment or river system and affect water quality. Common examples include sediments, nutrients, salts and other dissolved solids. 

Processes that act on these constituents to generate and transport them can be modelled in Source and are broadly categorised as Catchment Water Quality models and Storage and Link Water Quality models.

Catchment water quality models include:

• Constituent generation models - describe how constituents are generated within a functional unit (and any associated constituent sources) and the resulting concentrations or loads are delivered to the sub-catchment link

• Constituent filtering models - represent any reduction in constituents between generation within the FU and arrival at the link upstream of the sub-catchment link.

Storage and link water quality models include:

• Constituent routing models - describe the movement of constituents along a river channel network, including exchange of constituent fluxes between floodplains, wetlands, irrigation areas and groundwater. Constituent routing models are conservative, meaning that they do not alter the total mass of constituent stored in the system

• Constituent processing models - describe processes that can alter the mass of a constituent in a storage or river reach (link), such as via a decay process.

Open

To configure constituents:

• Enable and define constituents and constituent sources using the Constituent Configuration dialog at Edit » Constituents.... See Defining constituents for details on how to do this.

• Configure how constituents enter the model. Options are:
  
  

  • As a point source - see Configuring constituents at nodes.

  • Laterally through a link  - see Configuring constituents at links.

  • from the surface as a diffuse source - see Constituent generation models (for catchment models only)

• Configure how constituents are processed by the model, options are:

  • Constituent filters - see the Constituent Filters - SRG for more details on the filter models available and their configuration (for catchment models only);

  • Constituent routing;

  • Storage processing models; and

  • Instream processing models.

After you have defined constituents, the Constituent Model Configuration dialog is useful for viewing, selecting and editing:

• the filter and generation models and constituent source(s) for each sub-catchment/functional unit combination;

• the instream processing model for each storage routing link; and 

• the storage processing model for each storage node.

See Constituent Model Configuration for more details.

The Constituents Configuration dialog (Figure 2; accessible via Edit » Constituents...) is used to enable constituent modelling, and define both constituents and constituent sources:

• You can choose to enable constituents in a scenario using the Constituents Enabled checkbox. Note that disabling constituents results in constituent recorders being disabled as well;

• Specify the type of routing - there are two constituent routing options in Source - Lumped and Marker. These are relevant at Link level. Lumped constituent routing is the default. 

• Define and manage constituent type as follows:

  • To add a constituent type, enter its name in the Name field and click the Add button

  • To change its name, choose it from the list, then enter the new name in the Name field and click the Edit button, or

  • To remove a constituent type, choose it from the list, then click the Remove button.

After you have defined constituents, the Constituent Model Configuration dialog is used for viewing, selecting and editing:

• the filter and generation models and constituent source(s) for each sub-catchment/functional unit combination;

• the instream processing model for each storage routing link; and 

• the storage processing model for each storage node.

Open

There are two types of constituent routing available, Lumped and Marker routing. (Figure 2) and the choice made here will affect how constituents are routed at a link. Both of these are conservative routing models, which means that they do not change the total mass of constituent in the system. 

• Lumped routing (default) is the simplest and most common approach applied in Source. Constituents are routed within a link based on kinematic wave theory. Assuming fully-mixed conditions within a link, the constituent flux and concentration simply move from the top of a link to the downstream end of a link within a time step, preserving the mass balance. Constituent concentrations in a link can be altered by the addition of constituents generated from sub-catchments, external inflows, and losses defined within a reach; and

• Marker routing considers constituents as particles and tracks their movement within a link, which can be divided into divisions for hydrologic routing purposes. While available to all users this method is less commonly used. Initially, the model will start with a marker at the end of each division in every link. At every time step, a new marker for each constituent will be created for each division, and the distance a marker moves is driven by the velocity in the division over the current time step. While the flow rate is assumed constant over the timestep, the velocity within the division will change as a result of a change in reach storage. Markers will travel through the river network until they are either merged with adjoining markers or leave the river network (i.e. via extractions, decay within the reach, evaporation, groundwater inflows/losses and rainfall). Although available to all users, this method is less commonly used. Refer to Marker routing (Particle tracking) - SRG for more information about Marker routing.
  For marker routing, you must specify two additional parameters:

  • Minimum Marker Gap – defines the spacing between markers as either a fraction of the model time-step or fraction of the reach division. This parameter can improve model efficiency by reducing the number of markers that require processing at each model time step. The allowable range is from 0 to 1, with 0 not deleting any markers, while a value of 1 will ensure that at the end of each time-step, there is only one marker defined for each reach division; and

  • Minimum volume – minimum volume required to maintain constituent mass balance within the links.