Constituents

Overview of configuring constituents

To configure constituents:

• Enable and define constituents and constituent sources using the Constituent Configuration dialog. This is also where you choose what type of constituent routing your scenario will use. See Defining constituents.
• 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.
  • Through the surface - see constituent generation models (for catchment models only)
  • Groundwater - see XXX.
• Configure how constituents are processed by the model, options are:
  • Constituent filtering (for catchment models only)

  • Constituent routing

  • Storage routing
  • Instream processing

Defining constituents

The Constituents Configuration dialog (Figure 1; 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. Refer to Constituent Routing. 
• Define and manage constituents and constituent sources as follows:
  • To add a constituent or a source, 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, choose it from the list, then click the Remove button.

Figure 1. Configure constituents

Constituent routing

There are two types of constituent routing available, Lumped and Marker routing. (Figure 1). Both assume that the constituents are conservative. 

• Lumped routing is the simplest approach, where 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 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. 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 and cross-sectional area. Markers will travel through the river network until they are either merged with adjoining markers, or leave the river network (ie. via extractions, decay within the reach, evaporation, groundwater inflows/losses and rainfall).  Refer to Marker routing (Particle tracking) - SRG for more information.
  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 – volume to maintain constituent mass balance within the links.

Constituent Model Configurations

You can assign and manage the constituent generation, filter, instream processing and storage processing models for all constituents in the scenario using the Constituent Model Configuration dialog (Figure 1), which is opened by navigating to Edit » Constituent Models.... Before using this dialog, you need to define constituents and constituent sources (as described in Configuring constituents) and also either set up your catchment area using the Geographic Wizard for catchments and assigned FU areas or add constituents to nodes or links. Then, you can use the tree menu on the left to view the filter and generation models for each sub-catchment/FU combination, the instream processing model for each storage routing link, and the storage processing model for each storage node. 

The following operations can be undertaken:

• Change the assigned model,
• Change the parameter values or input data for the assigned model, 
• Filter columns based on their contents
• Sort columns in ascending or descending order; and
• For filter and generation models you can also change, add or remove constituent sources (see below).

Refer to Working with rainfall runoff models for more details on asssigning a model, adding input data and changing parameters. 

Configuring constituents at nodes

In Source, the behaviour of constituents at each node varies. Select Constituents in the node’s feature editor to configure them. Depending on your requirements and the type of node, you can specify either a constituent’s load or concentration at a node. For example, you can only specify a constituent’s concentration on an inflow node.

Inflow node

In the node's feature editor, specify the inflow constituent data (as a concentration) using the Constituents item (as shown in Figure 2). This behaviour is similar to flow.

Figure 2. Inflow node (Constituents)

Gauge node

If using gauge flow to override modelled flow, then leave as modelled, and constituent loads will be calculated using the gauged data concentration. Enabling the Set to Gauged checkbox (shown in Figure 3) allows you to set up the modelling of constituents. Disabling the checkbox will allow the constituents to flow through the node.

Storage node

For the storage node, you must define the initial concentration of each modelled constituent in the feature editor, under Constituents (Figure 4). To change the storage processing model, click on the cell once. Then, click on the drop-down arrow that appears and choose the required model from the menu.