Constituents

Introduction

Constituents refer to materials that are generated, transported and transformed within a catchment and affect water quality. Common examples include sediments, nutrients and contaminants, such as salt or dissolved solids. 

Processes that act on these constituents to generate, transport and transform them can be modelled in Source. These models are categorised into:

• Constituent generation models (only for catchment scenarios) - describe how constituents are generated in the functional unit and the resulting concentrations or loads delivered to the sub-catchment node;
• Constituent filtering models (only for catchment scenarios) - represent any transformation of constituents between generation within the FU and arrival at the link upstream of the sub-catchment node; and
• Constituent routing (conservative constituents) models - describes the movement of constituents along a river channel network, including exchange of constituent fluxes between floodplains, wetlands, irrigation areas and groundwater.

Overview of configuring constituents

Constituents are created in 

Configuring constituents

The Constituents Configuration dialog (Figure 1: accessible via Edit » Constituents...) is used to configure 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 below for more information on these. For the latter, 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.
• Name the constituents and constituent sources.

You can 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.

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

In Source, routing of conservative constituents can be undertaken using a marker tracking method (Close, 1996). Markers, representing constituents, are created at defined locations in the schematic and their downstream movement is modelled to determine the concentration at model components. These concentrations are adjusted for rainfall, evaporation and inflows from tributaries and groundwater systems. Refer to the Source Scientific Reference Guide for more information.

There are two types of constituent routing available as shown in Figure 1:

• Lumped routing is the simplest approach, where conservative 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 conservative 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 time step, 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).

Configuring constituents at nodes and links

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 in-stream processing model, click on the cell once. Then, click on the drop-down arrow that appears and choose the required model from the menu.