Constituents

Overview of configuring constituents

To configure constituents:

Enable and define constituents and constituent sources using the Constituent Configuration dialog. 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)
Configure how constituents are processed by the model, options are:
- Constituent filters (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 for each sub-catchment/FU 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

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.

Note:

Once a constituent is defined, Edit and Remove operate as described. However, once the Constituents Configuration dialog is closed, the existing constituents cannot be edited or removed.
For constituent sources, the default source is indicated by a green tick (Figure 1). It cannot be deleted and is automatically assigned to each functional unit/sub-catchment combination. You can change which source is the default using the Set as Default contextual menu.

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.

Note: When using lumped routing the following applies for storage routing links, storages and weirs that have volumes close to or equal to zero during the run. The working volume is the sum of the initial storage volume and all input flows, minus evaporation. The minimum volume is 0.01 m³, and is not currently user-configurable. When the working volume drops below the minimum volume, constituents are deposited as mass and removed from the system. The deposited mass is recorded in the Deposited Mass parameter (located at Constituents » Constituent Name » Deposited Mass).

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 2), 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 below).

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

Figure 2. Constituent Model Configuration dialog

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 3). This behaviour is similar to flow.

Note: Only constituents with units of concentration (mass/volume) can be added or replaced using the Inflow node.

Figure 3. 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 3). You can choose to override the modelled constituent concentration with the observed concentration by enabling Set to gauged. Refer to Gauge node - Constituents for more information.

Figure 3. Gauge node (Constituents)

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.

Figure 4. Storage node (Constituents)

Inlet channel mixing allows you to introduce mixing of constituents at a conveyance link (Figure 5). You specify a percentage of the wetland/storage volume that is conceptually represented by the conveyance link, and the remaining volume represents the main body of the storage/wetland. When exchange of water occurs between the wetland/river or the wetland/wetland, mixing of constituents is assumed to occur in the inlet channel. If the exchange of water is large enough to flush out the inlet channel, then the constituents will mix with the main body of the wetland, or the river depending on the direction of water exchange.

Figure 5. Storage node (Inlet channel mixing)

Additionally, for each constituent, you can configure various aspects of its concentration (Figure 6):

Additional Inflow Load – specify the amount of constituent to be added to the storage per time-step. It is not specific where this constituent mass comes from;
Groundwater – concentration of constituents entering the node via groundwater flow; and
Gauged Concentration – the recorded concentration at that storage node over time. This can be used to compare against modelled results.

Figure 6. Storage node, Constituent concentration

Configuring constituents along links

Constituents can be configured for storage routing links in the feature editor (Figure 7). In this screen, you can specify the link’s constituent concentration when the simulation begins. This parameter assigns a concentration for each modelled constituent in the scenario for the markers created in that link during the model initialisation. You can also specify the instream processing model, the parameters of which can then be configured by selecting Configure….

For each constituent, you can specify an increase in concentration from different sources, similar to constituents in the storage node (Figure 6). The parameters are:

Additional Inflow Load – specify the amount of constituent to be added to the storage routing link per time-step. It is not specific where this constituent mass comes from;
Groundwater – concentration of the constituent entering the link via groundwater flow; and
Timeseries Flux – concentration of the constituent entering the storage routing link viaTimeseries Flux.

Note: Unless there is either an initial storage or initial flow defined, there will be no constituent mass in the link at the start of the model simulation.

Figure 7. Storage Link Routing (Constituents)

Configuring constituent in catchment models

Configuring constituent sources

The first step when configuring constituent sources is to add sources to the Constituents Configuration dialog (as described in Configuring constituents). By default, a single constituent source is added to the scenario when constituents are enabled. This can be changed by using the Set as Default contextual menu on the desired constituent source. The default source cannot be deleted and is the one that is automatically assigned to an FU/sub-catchment combination in the Constituent Model Configuration dialog (Figure 1).

By default, for a specified constituent, every sub-catchment/FU combination is assigned the default constituent source. To change the constituent source:

Select the constituent you wish to change the source for (the tree view on the left) and the corresponding row in the table. Note that you can choose either the filter OR generation model, as any changes are automatically applied to both; and
Right-click and choose Add Constituent Source » Current Constituent » <source name>. This will add a new row to the table, allowing you to assign and parameterise a new constituent generation or filter model for the selected constituent/FU.

You can also undertake the following actions using the same contextual menu:

The process to remove a constituent source from an FU is identical to adding one. Choose Remove Constituent Source » Current Constituent » <source name>. Note that at least one constituent source must exist at all times. If all sources have been removed, a new row with the default source will be automatically added;
Bulk assignments can be made to the table in one of two ways:
- Using the All Constituents item; or
- Selecting multiple rows, then right clicking and choosing the desired menu item. This becomes increasingly difficult as the scenario grows in size.

Figure 1. Constituent Model Configuration dialog

Note: Using filters in the Constituent Model Configuration dialog is similar to using them in the Feature Table (see Working with filters in the Feature Table). However, 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.

Constituent filter models

Filter models represent any transformation of constituents between generation within the FU and arrival at the link upstream of the sub-catchment node. Filter models process constituents within the FU and as with constituent generation models, are applied to FUs. In the Constituent Model Configuration dialog, click on any of the defined constituents under Filter Models (in the tree menu). Note that in Source, only one filter model can be applied to a sub-catchment/FU combination.

Assign and parameterise filtering models as follows:

First, assign a model to the sub-catchment/FU combination. To do this, change the model from the default (Pass through) to your required model:
- Click on the cell in the Model column that you want to change; and
- Click on the drop-down arrow that appears and choose the required model from the menu;
Then, assign input data (if relevant) to the model in the PET and Rainfall columns (if relevant); and
Finally, parameterise the filter model - depending on the chosen model, the right-side of the table will populate with the associated default parameters. Click on the cell and edit these values.

Constituent generation models

These describe how constituents (eg. sediments or nutrients) are generated within a functional unit and the resulting concentrations or loads delivered to the sub-catchment node. Click on any constituent to view the associated FU and generation model for each sub-catchment. Follow the same steps outlined for filter models to assign, add input data and parameterise constituent generation models. The available constituent generation models are:

EMC/DWC - the Event Mean Concentration (EMC) Dry Weather Concentration (DWC) model applies two fixed constituent concentrations (EMC & DWC) to an FU to calculate the total constituent load.
Export rate - this model applies a fixed constituent generation rate to a functional unit (FU) to calculate total constituent load. It requires only a single parameter so is quick to use and therefore useful for exploring sensitivity.
Nil Constituent - this model is used as a substitute constituent generation model where no constituent load needs to be modelled for a given constituent from a given FU
Observed concentration - this option allows you to assign observed quick flow and slow flow concentrations.
Power Function - this model fits a rating curve describing the relationship between constituent concentration or load and discharges. It is a straight power curve, where flow in ML/d has been used to generate a relationship with the solute concentration (mg/L),
Power Function (flow in mm) - this model is the same as the Power Function model, but uses a normalised power curve, where flow in mm/d has been used to generate a relationship with the solute concentration (mg/L).

In this case, the default constituent generation model is Nil Constituent.

Linking constituent generation and filter models

Note: This functionality is currently under development and not all models can be linked. The description that follows is an illustration of what van be undertaken in Source.

Constituent generation and filter models may require one or more of their parameters to originate from another one, for each time-step. That is, a model's input may depend on the output of another model.

Consider the Nutrient Delivery Ratio (NDR) and Sediment Delivery Ratio (SDR) models. The output of an NDR model depends on the input of the SDR filter model. To configure constituent model linking between these two models for a given FU and constituent source, assign one SDR to the sediment constituent, and one NDR to each other appropriate constituent. A link is then created between the quick flow in parameter on the SDR and the quick flow sediment in parameter on the NDRs. The link system would ensure that the SDR has run before the NDRs, ensuring the correct flow of data at the right time.

Note: Source detects circular dependencies, and will notify you if a defined link needs to be corrected prior to proceeding.