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.

Overview of configuring constituents

Figure 1 Where to find constituents

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.

Defining constituents

Constituent configuration

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.

Figure 2. Configure constituents

Constituent routing

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.

Constituent Model Configuration

Constituent generation models, filter models and instream and storage processing models define where constituents originate and how they move through the modelled system. You assign and manage these models in a scenario using the Constituent Model Configuration dialog (Figure 3), which is opened by navigating to Edit » Constituent Models.... 

Before using this dialog, you need to define constituents and constituent sources (as described in Defining constituents) and also either set up your catchment area using the Geographic Wizard for catchments and assigned FU areas and/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 Configuring constituent sources.

Refer to Working with rainfall-runoff models for more details on assigning a constituent model, adding input data and changing parameters. For more information on using filters see Working with filters in the Feature Table. There is also a sub-catchment filter to help you find sub-catchments either by name or by using the sub-catchment map, see Sub-catchment filter.

Figure 3. Constituent Model Configuration

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 at an inflow or a gauge node.

Inflow node

Only constituents with units of concentration (mass/volume e.g. mg/L) can be added or replaced using the Inflow node (Figure 4). Since constituent mass is a function of flow volume, the constituent mass will then be internally calculated by Source, depending on the volume of inflow at that point.

Figure 4. Inflow node (Constituents)

Gauge node

For each constituent, you can specify its observed concentration by entering a value, supplying a time series or defining a function (Figure 5). If required you can choose to override the modelled constituent concentration with the observed concentration by enabling Set to gauged. Applying an observed value at a Gauge node is useful for comparing modelled and observed values. Refer to Gauge node - Constituents for more information. 

Storage node

Constituents are assumed to enter the Storage node at the same concentration as the inflow to the node. If no Processing Model is chosen the stored mass in the storage will be dependent on the initial concentration of the stored water volume after any inflows (gains)/outflows (losses) to the storage.

By default the initial concentration in the storage is assumed to be 0. It is recommended that the user must define an initial concentration of each modelled constituent in the feature editor, under Constituents (Figure 6). By clicking on the ellipsis button, parameters within the Storage Decay model can be assigned as a single value, from a Data Source or based on a Function (Figure 6). 

Figure 6. Storage node, Constituents

Water User node

Constituent contributions can be added as a concentration at a Water User node and can be associated with all the available demand models. Figure 9 illustrates assigning constituent concentrations for an Irrigator demand model and a time series demand. These contributions represent e.g., constituents associated with irrigation return flows or urban treatment plant outflows and require that return flows are configured for the Water User. Concentrations can be assigned as a single value, from a Data Source or based on a Function.

Figure 9. Water User node, constituent concentration

Weir node

Constituent modelling at the Weir node has the same options as the Storage node (see Figure 6 and Figure 8 above). The Weir node has a configurable storage routing upstream link and a constituent decay model on this link is configurable through the Constituent Models table.

Scenario Transfer node

Along with flows, constituents are also transferred when using a Scenario Transfer node.

Constituents and Ownership

Constituent concentrations are assumed consistent across all owners, according to the concentration of the associated water body.

Figure 10. Storage routing link, constituents

Figure 11. Storage routing link - Decay model configuration

Figure 12. Accessing the Constituent Models table

Figure 13. Accessing the Constituent Generation models

Figure 14. Accessing the Constituent Filter models

More about the Constituent Models table

The Constituent Models table provides a central location from which all constituent models which have been assigned to catchments, nodes or links in a model can be accessed (Figure 15). This is a quick way to assign or modify constituent models. The Import/Export buttons allow the user to Export the current configuration of constituent models, apply bulk modifications, and then Import the table, provided column headings and the number of rows does not change. The Export/Import file format is .csv.

The Constituent Models table provides a central location from which all constituent models which have been assigned to catchments, nodes or links in a model can be accessed (Figure 15). This is a quick way to assign or modify constituent models. The Import/Export buttons allow the user to Export the current configuration of constituent models, apply bulk modifications, and then Import the table, provided column headings and the number of rows does not change. The Export/Import file format is .csv.

Figure 15. Constituent Models table

Figure 16. Constituent Models table

Figure 17. Constituent Models table

It is possible to Export/Import a Constituent Table containing linkages. When importing into the data grid, the number of rows in the data grid and in the csv file must match prior to importing the csv. When the value of a linkage cell is imported / pasted in, the linkage may not be valid at the time of importation / pasting. An example of this temporary invalidity would be a case where several data grids worth of values (several csv’s) needs to be imported, one after the other. The first csv of a data grid may contain a linkage to a model which does not exist yet because that model type is yet to be imported via the second csv of another data grid. In this case, the linkage is pending validation. It will remain pending until one of two things happen. The first is that the user imports the second csv, creating the expected model type for the linkage to connect too and the situation is resolved. The second is that the user attempts to edit the linkage by clicking the “…” button before the second csv is imported. If this happens the user is presented with a question bar at the top of the Define Constituent Model Linkage dialog. They can choose to keep the pending linkage and close the dialog, or throw away the linkage and define a new one. All pending linkages will be thrown away and revert to “Not Linked” upon the clicking of the “OK” button on the main Constituent Model Configuration screen.

Figure 18. Define constituent sources

Once a constituent source has been defined, every sub-catchment/FU combination is automatically assigned the default constituent source for both constituent generation and constituent filtering.  You can change the constituent source from the default or add or remove additional constituent sources in the Constituent Models table by right-clicking on the constituent source cell and using the dropdown to select an alternative source (Figure 19).

Figure 19. Change constituent sources

Multiple constituent sources can be assigned to a FU/sub-catchment combination, with a different generation or filter model applied to each (Figure 20).

• In the Constituent Models table, select the constituent to change the source for under either the filter or generation model (any change to the source is automatically applied to both models for that constituent)
• Right-click the row with the appropriate sub-catchment/FU combination
• Choose Add Constituent Source » Current Constituent » <source name>. This will add a new row to the table, allowing the user to assign and parameterise a new constituent generation or filter model for the selected constituent/sub-catchment/FU.

The same contextual menu can also be used to Remove a Constituent Source. At least one constituent source must exist at all times. If all sources have been removed, a new row with the default source is added automatically.

Figure 20. Multiple constituent sources

The bulk assignment of a source to multiple constituents can be made using the Add/Remove Constituent Source » All Constituents. This will add that constituent source to that sub-catchment/FU combination for all constituents.

The user can also assign the same constituent source to several sub-catchment/FU combination by selecting multiple rows, then right-clicking and choosing the desired constituent source menu item. Selecting all the desired rows in one go can become more difficult the larger the scenario. To get around this, the user can use the column filtering functionality of the data grid to trim the number of rows down to a more manageable number or they can break the changes down into several smaller changes.