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All events occurring during a time-step are independent, such as when a function is evaluated, or when the model variable on which the function depends is updated. You can choose the times in which to carry out certain actions. Tables 11 and 12 define the options which are available by default.

If none of the default options is appropriate, you can define your own date ranges using one of the options from the Date range type popup menu (Figure 15).

To calculate a particular date in the water year using the calendar date functions, you can use $now.dayofyear+184 (to indicate the next calendar year) for example, or $now.dayofyear-181 (represents the current calendar year). 

Custom date ranges fall into three categories:

• Date Range Calendar (Figure 13) allows you to define a precise start and end date during which the model variable is considered to be valid. Enabling the Is Recurring checkbox ignores the years. In other words, the period between [start day/start month] and [end day/end month] is valid for every year during model execution.
• Date Range Period (Figure 14) allows you to specify the date range in terms of a period of time that is relative to the current time-step. A period can range from one hour to one year and is controlled via the Period popup menu. The number of periods included in the range is controlled by the X periods ago(start) and Y periods ago(end) fields where end ≥ start. A value of zero refers to the current period. For example:
  • A Daily period where start=0 and end=0 refers to the current day;
  • A Daily period where start=1 and end=1 refers to the previous day.
• The Date Range Time Step option (Figure 15) allows you to specify the date range in terms of time-steps. The number of time-steps included in the range is controlled by the X time-steps ago(start) and Y time-steps ago(end) fields where where end ≥ start. A value of zero refers to the current time-step. For example:
  • If start = 0 and end = 0, this refers to the current time-step;
  • If start = 1 and end = 1, this refers to the last time-step;
  • If start = 0 and end = 6, this refers to the last seven time-steps including the current time-step.

Figure 14. Function Editor (Date Range, Date range calendar)

Figure 15. Function Editor (Date Range, period)

Figure 16. Function Editor (Date Range, time-step)

For all three custom calendar options:

• The Update Frequency popup menu offers a choice of OncePerTimeStep or OncePerIteration. The latter setting is only applicable to NetLP.
• The Return set popup menu controls the granularity of the returned result:
  • If the range represents a single value, that value is returned regardless of this setting.
  • Table 13 summarises what is returned if the range represents more than one value.

Table 13. Function Editor, Date Ranges (Variables)

Where: input is the series [1,2,3,4,5]

Example

As an example, you can used a function or expression at a Maximum Order Constraint node to limit the maximum orders for each time-step. Assume that a channel constraint of 80 ML/day is required to prevent flows going overbank, except during floods (defined as more than 2,000 ML/day) when up to 3,000 ML/day is permitted as an environmental flow.

Figure 16 shows a fragment of the river network. Downstream demands are represented by a supply point and water user node. In the Function Editor, you specify the internal orders and unregulated river gain of the model using modelled variables. Maximum orders are then defined using an expression.

In this case, the following components will be assigned:

• Storage 2: Requested flow rate - say, $Orders with a data range of Last time-step; and
• Inflow 3: Inflow - $predictedInflow, with the same data range as Storage 2.

Note that both variables will have ML/day as designated units.

Then, the following expression will return a value of 3,000 ML/day if flows are above 2,000 ML/day but 80 ML/day otherwise.

If(($predictedinflow+$orders) > 2000, 3000, 80