Links

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.

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; these are summarised in Table 1.

Table 1. Link types in Source

Using links in Source

Once links have been added a model, several attributes can be edited in a similar way to 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.

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:

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. For more detail on the wetland link, refer to Wetland 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 Types of link routing for more detail.

Link elevation

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

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.

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 4 shows that both lagged flow and storage routing are in use.

Figure 4. Project Hierarchy (link models)

Straight through routing

All links are assigned straight through routing by default. This link has 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.

Straight through routing links are represented in the Schematic Editor using black, dashed lines.

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 that link at some whole number of time-steps in the future. This type of link is represented in the Schematic Editor as a black line, with alternating dots and dashes. Once you have enabled lagged flow routing, double click the link to configure the settings.

Figure 1 shows the feature editor for a lagged flow routing link and Table 1 describes the parameters required to configure this link.

Figure 1. Link (Lagged flow routing)

Table 1. Parameters for lagged flow routing

Storage flow routing

This type of link is represented in the Schematic Editor as a solid black line. Storage routing is based on mass conservation and the assumption of monotonic relationships between storage and discharge in a link. 

This is a simplification of the full momentum equation and assumes that diffusion and dynamic effects are negligible. The method uses index flow in flux, storage and mass balance equations. A weighting factor is used to adjust the bias between inflow and outflow rate, hence allowing for attenuation of flow. The storage routing equation is shown below and some of its terms are represented diagrammatically in Figure 2:

Equation 2

where:

S is the storage in the reach,

K is the storage constant,

m is the storage exponent, and

q‾ is the index flow, which is given by 

Equation 3

where:

I is inflow to the reach during the time-step,

O is outflow from the reach during the time-step, and

x is the inflow bias or attenuation factor.

Figure 2. Prism and wedge storage

Refer to the Source Scientific Reference Guide for more details.

Travel time in the reach is computed as follows:

Equation 1

A link configured for lagged flow routing is treated as a series of sub-reaches of equal length, with the travel time in each sub-division equal to one time-step. Water moves through the link progressively, without attenuation. You cannot configure fluxes, constituents or ownership on a lagged flow routing link. If lateral flows are significant and/or there is dead storage in the reach, you can approximate lagged flow routing using generalised non-linear storage flow routing, as follows:

• Compute the number of divisions, n, by dividing the average wave passage time by the model time-step and round the result to a whole number. The result must be at least one (ie n ≥ 1).
• Configure a storage flow routing reach where:
  • n = number of divisions;
  • x = 1;
  • m = 1; and
  • K = model time-step.
• If you need to account for lateral flows where n=1 and the average travel time is a fraction of the model time-step (eg. a reach with a one day lag in a model with a monthly time-step), you can adjust K to a smaller value without affecting the shape of the hydrograph.

Figure 3 shows the feature editor configured for storage link routing. Table 1 outlines the parameters required.

About dead storage

Dead storage refers to the capacity of a storage that is below the minimum operating level. At this water level, there is no outflow. The level of the reach with respect to dead storage at the beginning of the time-step affects its level in subsequent time-steps as follows:

• The reach is at or below dead storage and the fluxes during the time-step are insufficient to raise the level above dead storage; or
• The reach is above dead storage but fluxes during the time-step would lower the level in the reach to below dead storage; or
• The reach is above dead storage and remains above dead storage during the time-step.

To determine if the reach is at or below the dead storage level, Source:

• Computes an initial storage estimate by using inflows to fill the reach up to but not exceeding the dead storage level;
• Computes a revised storage estimate based on any remaining inflows and fluxes but ignoring outflows; or

If the revised storage estimate is above dead storage then outflows are computed, otherwise the initial storage estimate is used and outflows are set to zero.

The next section provides an overview of the parameters shown in Figure 5.

Equation 1

Storage constant

When using linear routing (m=1), the units of the storage constant K are in seconds. For models using daily time-steps, the recommended starting value is 86400 (the number of seconds in one day). When using non-linear routing (m≠1), the recommended starting value should be calculated as follows:

Equation 2

For example, if the reach length is 1000 metres, the time-step is one day, and m=0.8:

Equation 3

The parameters for storage flow routing are summarised in Table 1.

Table 1. Link (Storage routing parameters)

Piecewise storage function

Link travel time can also be set using a piecewise linear editor. This describes a series of relationships between reach index flow rate q¯ versus travel time. The slope of the curve is the same as that for index flow rate versus storage so the rating curve can be derived using dead storage (zero flow) as the starting point. The data points can be entered manually or imported from a .CSV file, the format of which is shown in Table 2. Quadratic interpolation is used to find points in each defined segment on the curve (as in BigMod where x=1).

Table 2. Link (Storage routing travel time, data file format)

Piecewise routing allows you to specify how K varies with flow. If x=1, then K must always be less than or equal to the time-step. In BigMod routing, the highest value of K is found in the travel time relationship, and the reach should be sub-divided into sufficient divisions such that the highest value of K for each division is less than the half the time-step.

Link rating curve

Rating curves are used to describe the physical characteristics of the reach and convert a flow into a level, ie. they produce an output of level. They can be specified in one of two ways:

• Using the Cross Section Editor to specify a physical cross section, and then generating the rating curve; or
• Directly entering the rating curve - the piecewise linear editor allows you to define relationships with respect to water level, discharge rate, reach width and dead storage. You can define multiple rating curves for a reach, each scheduled to commence on a particular date.

To define a new rating curve directly:

• Right click Rating Curve and choose Add Rating Curve;
• Today’s date will automatically be entered for Start Date. To change this, click the calendar on the right side (see Working with date-pickers);
• Enter the water level, discharge rate, reach width and dead storage; and
• Enter an appropriate value for Overbank Flow Level.

You can also use the Import button to import a rating curve from a .CSV file the format of which is shown in Table 3.

Table 3. Link (Storage routing, Rating curve, data file format)

Where:  

level is the storage height in the reach in metres above dead storage

rate is the outflow from the reach in megalitres per day in the corresponding level

width is the surface width of the reach in metres at the corresponding level

storage is the dead storage in the reach in megalitres at the corresponding level.

There should be at least one row describing the maximum depth at which there is zero flow, and which quantifies the maximum amount of dead storage in the reach. Thereafter, the dead storage volume should remain constant. Table 4 shows an example of this. A depth of 0.5 metres defines the maximum amount of dead storage (100 megalitres), after which the dead storage remains constant. Note that if discharge is 0, then dead storage must be increasing, or it must be equal to the previous value of dead storage.

Table 4. Link (Storage routing, Rating curve, example)

To edit an exising rating curve, select the curve from the list of available curves under Rating Curve. Edit the data and click OK to close the editor. To delete a rating curve, right click the curve from the list and choose Delete.

You can also export rating curves to .CSV files by clicking the Export button. Table 3 shows the file format.

Link losses and gains

Choose Loss/Gain to specify flux as a function of flow using a piecewise linear editor.

By convention, losses are described using positive numbers whereas gains are specified using negative numbers. In other words, a gain is a negative loss.

Figure 5. Link (Storage routing, Loss/Gain)

You can enter the relationship manually, or import the data from a .CSV file, the format of which is shown in Table 5. This table shows the data file format for both evaporation and rainfall.

Table 5. Link (Evaporation.Rainfall, data file format)

Where:

time is the time of observation in "dd/mm/yyyy hh:mm:ss" format

value is the evaporation rate.rainfall in millimetres per time-step

Link evaporation

Choose Evaporation to specify the rate of evaporation per unit of surface area. Typically, this is done using a time series (loaded using Data Sources), the format of which is shown in Table 5. You can also specify the rate of evaporation as a single value, or as an expression using the Function Editor.

Figure 6. Link (Storage routing, Evaporation)

Rainfall on link surfaces

To specify the rate of precipitation per unit of surface area, choose Rainfall. Just like evaporation, this can be specified as a single value, as a time series (format shown in Table 5) or an expression. A time series can have multiple columns containing rainfall data.

Figure 7. Link (Storage routing, Rainfall)

Timeseries Flux

This allows the input of a time series of total water lost or gained on a link. Values can be positive or negative. A negative value denotes water returned to the link (a gain). See also Link losses and gains.

Figure 8. Link (Storage routing, Timeseries flux)

Constituents

Before you can configure constituents for a link, you must define them first for the scenario using Edit » Constituents. Refer to Links.

Ordering at links

Figure 9. Link (Ordering)

Ownership at links

Ownership must be enabled at the scenario-level (using Edit » Ownership) prior to configuring ownership at storage routing links. Refer to Ownership for details.