Overview of configuring constituents
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- 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)Groundwater - see XXX.
- Configure how constituents are processed by the model, options are:
- Constituent filters , choose which model is used by each functional unit/sub-catchment combination (for catchment models only);
Constituent routing, choose between lumped and marker routing for the scenario;
- Storage processing models, choose which model is used by each storage node; andInstream processing models, choose which model is used by each storage routing link; and
- Instream processing models.
After you have defined constituents, the Constituent Model Configuration dialog is useful for viewing, selecting and editing:
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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.
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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
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If using gauge flow to override modelled flow, then leave as modelled, and constituent loads will be calculated using the gauged data concentration. Enabling 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 compare any differences between the observed and modelled concentrations of constituents during a model run by recording Constituents » Constituent Name » Downstream Flow Concentration and Constituents » Constituent Name » Gauged 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 noderesults in the modelled constituent concentration at the gauge being overridden by the observed concentration (Figure 3). When enabled, the constituent mass (ie. load) will be calculated based on the observed concentration. In a gauge node, you can also override modelled flow with the observed flow by enabling Set Flow on the Observed Flow menu. If you do this, but leave Set to Gauged disabled, the modelled constituent concentration will remain the same as when using modelled flow, but constituent mass will be calculated using the observed flow volume. If you override either the modelled constituent concentration and/or the modelled flow with observed data, there may be a change in constituent mass, concentration or mass balance. These may be recorded by navigating to Constituents » Constituent Name in the Parameters Pane and selecting the Unaccounted Mass, Unaccounted Concentration and Mass Balance parameters.
Figure 3. Gauge node (Constituents)
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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. Whenexchange 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
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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.
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| 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 a 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 a 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
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 a 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
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 a 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.
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