Links

About links

Links connect nodes in Source - they link, store and route water passing between nodes. Nodes are connected using links only, and two links cannot be connected to each other without an intervening node.

A reach refers to a stretch of river, or physical section, between an upstream and downstream location. A link, on the other hand, is a logical connection within a river systems model. Routing describes the change in timing and shape of flow as water moves down a river.

Links (or reaches) can have routing configured on them. For links (or reaches) that do not have routing configured, they are used to define the order of execution in the model.

Note: Throughout this document, unless explicitly stated otherwise, the term routing means hydrologic routing, not hydraulic routing.

Links are categorised as either:

Vertical - used to connect most nodes. The first four types of links in Table 1 are all vertical; or
Horizontal (or wetland links) - used to connect the Wetlands Hydraulic Connector node (source) and the Storage node (target) only. The presence of a horizontal link at a storage node indicates that the storage is behaving as a wetland. Figure 2 shows an example of a horizontal link.

The Node Connection Matrix shows the types of links created when you connect different nodes.

Table 1. Link types in Source

Type of link	Used to connect	Representation in Schematic Editor	Example
Demand link	Supply point (source) and water user (target) nodes only	Red, dashed line
Lagged flow routing	Various nodes	Black line, with alternating dots and dashes
Storage routing	Various nodes	Black, solid line
Straight through routing	Various nodes	Black, dashed line
Wetland link (Horizontal)	Wetlands Hydraulic Connector node (source) and the Storage node (target) only	Green, solid line	Shown in Figure 2

Using links in Source

Once links have been added a model, you can edit several attributes in a similar fashion as for nodes:

Node and link default names;
Renaming nodes and links;
Configuring node and link parameters;
Adding notes to nodes and links (only for lagged flow routing and storage routing links);
Searching for nodes and links;
Deleting nodes and links;
Node and link default names; and
Copying and pasting.

Note: Once added to a scenario, links can be viewed and their parameters edited using The Feature Table.

Adding links to a model

To add a link to a model, first refer to Figure 1 which defines the terminology. To create the link:

Position the mouse cursor over the upstream node;
Click and hold on one of its downstream connectors and start dragging;
When you start dragging the mouse cursor, candidate targets are displayed (as large icons) for the upstream connector of a downstream node; and
Release the mouse and the link will ‘snap’ into place.

Figure 1. Node connection terminology

For horizontal links, the node connectors appear on the left and right side, instead of above and below the nodes. Click and drag these connectors together as described above. You can also drag the link vertically once it has been created by clicking on the red dot. This appears in the centre of the link when you click on the link.

Figure 2. Horizontal link

Dragging links

You can disconnect and reconnect a link between nodes rather than having to delete and re-add it using the Allow Link Dragged button in the Schematic Editor options toolbar. Note that not all links can be connected to all types of nodes, and specific nodes require certain links. Refer to /wiki/spaces/SD516/pages/54987323 for more detail.

Link elevation

You can set the elevation for a link using the Location Control window.

Note: While it is usual to use zero storage as the reference point for the elevation of a link or node, there is no convention for a link as to whether that should be at the start or end of the reach, or some point in between. Source has no mechanism for indicating the fall across a reach.

Types of link routing

Source supports three types of link routing - straight through routing (default), a lagged routing model or a Storage routing model. You are responsible for ensuring that you use the correct model for each link.

Note that you can configure constituents, ownership and ordering for a storage routing link only.

To change the link routing type:

Right click on the link to display the contextual menu;
Choose Routing Type; and
Finally, click on the required link routing.

You can check which routing models are in use in a scenario using the Project Hierarchy. The example in Figure 3 shows that there are two types of links in use - lagged flow and storage routing.

Figure 3. Project Hierarchy (link models)

Straight through routing

All links are assigned straight through routing by default and have the following features:

Water enters and exits such a link in the same time-step;
There are no configuration parameters associated with straight through routing links; and
You cannot configure fluxes, constituents or ownership.

Lagged flow routing

Lagged flow routing only considers the average travel time of water in a river reach. It does not consider flow attenuation. The flow entering a link exits at some whole number of time-steps in the future.

Figure 4 shows the feature editor for a lagged flow routing link and Table 2 lists the associated parameters.

Figure 4. Link (Lagged flow routing)

Table 2. Parameters for lagged flow routing

Parameter	Type	Definition
Lag time	Time	This represents the time it takes for water to travel along the link and is a positive real number.
Initial Storage	Volume	The amount of water deemed to be in the link on the first time-step. For example, if there is a lag of two days, and there is 10ML in the link at the start of the run, then 5ML is deemed to be flowing out each day (total initial storage divided by lag).