The Minimum Flow Requirement node is used to ensure that there is a minimum flow rate at a point in a river system to meet various demands (stock, domestic or environmental needs). It is also used to generate transfers between an upstream and downstream storage. This node imposes a constraint which must be met at that point in the river model, even if additional water must be released from a storage. The requirement can be specified as:
- A single value;
- A time series using Data Sources;
- Via an expression or function using the Function Editor;
- As monthly values in megalitres under Monthly Pattern; or
- Setting up a rule using Rule Curves (this is described in further detail next).
Figure 1. Minimum flow requirement node
You can also enter minimum daily flow requirements manually or load the data from a file. Table 1 shows the format of a comma-separated (.CSV) file for importing minimum flow requirement data.
Table 1. Minimum Flow Requirement node (data file format)
Row | Column (comma-separated) | |
|---|---|---|
1 | 2 | |
1 | Month | Daily Minimum Requirement (ML) |
2 | month | value |
Rule curves
You can set up a rule by choosing Rule Curves as the flow method. Rule Curves allow you to manage releases using a lookup table. Figure 2 shows an example of a Trigger Function configured as a function.
They are set up as follows:
- The node is set up where the ‘sensor’ is located;
- You set the node where the release is made from (a reservoir);
- The first column in the lookup table is the volume or water level in the sensor node. This lookup table can be imported and exported using the corresponding buttons (as shown in Figure 2), with the format of the comma-separated file shown in Table 2; and
- Depending on the month and the level/volume in the sensor, the flow rate or volume is released according to the table. It is a step function in time.
| Info | ||
|---|---|---|
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| Note: Units for the Trigger Function are not specified. These can be changed using the units buttons next to each month. |
The Interpolate checkbox calculates the releases based on an interpolation method. For instance, assume that the output of the trigger function is 34000 and the Interpolate checkbox has been enabled. Then, flow will be calculated based on two trigger values in the table (258695 and 347195) for a particular month.
Figure 2. Minimum Flow requirement node, Rule curves
Table 2. Minimum Flow Requirement node, Rule Curves table (data file format)
Row | Column (comma-separated) | |
|---|---|---|
1 | 2..n | |
1 | Trigger | Month |
2..n | value | month |
Ordering and ownership
In the ordering phase, when the amount of ordered water expected (ie. forecast) to flow down the river past a minimum flow requirement is less than the requirement, then an order is placed such that the forecasted flow meets the requirement. If orders are more than the value specified by the requirement, no adjustment is made. In the flow phased, orders cannot be adjusted, so where the actual amount of water flowing down the river past a minimum flow requirement is more than forecast, there may be more water than is required to meet orders.
When ownership is enabled, you can define a sharing hierarchy to control how the constraint is apportioned between downstream owners. Refer to Minimum Flow Requirement Ownership for more information.
Checking constraints
When a minimum flow requirement is enforced, there may be a difference between the amount of water ordered and water released. The differences are recorded by the Recording Manager. For more information, see Identifying Constraints.