The Maximum Order Constraint node constrains releases from upstream storages to the defined maximum order at a point in the river network. The process of limiting releases considers the contribution from tributary inflows between the upstream storage(s) and the location of the maximum order constraint.
This functionality is required in eWater Source to enable it to model situations where it is necessary to limit the flow at a location, for example, to prevent it from going overbank.
Scale
This node is implemented at point scale and is used at every model time-step.
Principal developer
eWater CRC.
Scientific provenance
Maximum flow constraints have been implemented in water sharing arrangements in practice in a number of regulated river systems in Australia. The implementation in eWater Source is designed to be able to replicate these practices and be flexible enough to accommodate new variations if required.
Version
eWater Source version 2.10
Dependencies
This node is only relevant to models of regulated systems at locations where there are upstream storage node(s) present.
Structure and processes
When rules based ordering is used, a constraint phase precedes the ordering phase of every model time-step. In this phase, the modelled river network is processed from upstream to downstream to determine at each node the flow range that is possible when upstream management rules are met. This flow range is defined by a minimum and maximum constraint flow volume. The constrained flow range for each node is forecast so that releases can be made at the appropriate time by upstream storages. The forecast time period covers the shortest to the longest estimated time taken for water to travel from upstream storages to the location.
At the head of the river, the constrained flow range is from zero to infinity. At each node, the upstream constrained flow range is known. This flow range is updated for any additional constraints (physical or management) defined at the node. The updated flow range is then passed down to the next node downstream. In this way the constrained flow range at any node downstream of the Maximum Order Constraint will incorporate restrictions imposed by this node.
In the ordering phase, orders are adjusted as required, as described in the SRG entry on Rules Based Ordering, to ensure that total flow at the node will fall within the constrained flow range for the future time-step in which the order is to be delivered. As water is released from upstream storages to meet the ordered volume, the flow volume at a Maximum Order Constraint node will usually not exceed its maximum order constraint for the time-step,
 Note that it is possible to configure the model in a way such that flow exceeds the maximum order constraint for a time-step - for example by configuring storage nodes in a way such that they spill more than the maximum order. |
These concepts are described in more detail in the Scientific Reference Guide entry on the Rules-Based Ordering (SRG).
The way ordering is handled in NetLP, and the application of a maximum order constraint in this context, is described in the Scientific Reference Guide entry on NetLP - SRG.
Time-step: Constraint phase
At a Maximum Order Constraint node, the constrained flow range passed down from upstream is reduced where necessary to the node’s defined maximum order. It is not, however, reduced below the minimum constraint passed from the upstream node, ie:
| Equation 1 |  |
A more detailed description of the calculations performed, particularly where ownership is enabled, is given in the Scientific Reference Guide entries on Rules-Based Ordering (SRG) and Ownership at nodes and links - SRG.
Input data
When ownership is not enabled, the maximum order constraint is specified by the modeller as a value or expression.
Constraint levels are used when ownership is enabled. One or more of these levels are specified, with each level corresponding to a constraint volume (defined via expression), and owner proportions. The highest constraint level represents the total maximum order constraint volume. Owner proportions apply to the volume between the previous level’s volume, and the volume for the level at which they are defined.
| Equation 2 |  |
Output data
The recorded output variables relevant to a maximum order constraint node are described in Table 1.
Table 1. Recorded output variables
Parameter | Description | Units |
|---|---|---|
Constituent Output | If constituent processing is enabled, shows the concentration of constituents passing through the node. | |
Downstream Flow | Rate of flow immediately downstream of the location of the maximum order constraint’s location. | Volume/ time |
Downstream Flow Volume | Volume of flow over a time-step immediately downstream of the location of the maximum order constraint’s location. | volume |
Mass Balance | Difference between inflow to and outflow from the maximum order node | volume |
Maximum River Constraint | Value of the parameter used to constrain orders for future time-steps; between MinTravelTime and MaxTravelTime time-steps into the future. | volume |
Offset Maximum River Constraint | Value of the parameter used to constrain orders for the current time-step (this is the value of the term Maximum River Constraint (above) determined MinTravelTime time-steps prior to the current time-step). | volume |
Rules Based Orders - Full Order Data | View by date and owner of order and constraint volumes. | volume |
Rules Based Orders - Orders | Order volume time series (graph, table, statistics) | volume |
Rules Based Orders - Constraints | Constraint volume time series (graph, table, statistics) | volume |
Total Inflow Volume | Cumulative volume of upstream inflow over the scenario run. | volume |
Total Outflow Volume | Cumulative volume of downstream outflow over the scenario run. | volume |
Upstream Flow | Rate of flow into the location of the maximum order constraint’s location from upstream. | Volume/ time |
Upstream Flow Volume | Volume of flow over a time-step into the location of the maximum order constraint’s location from upstream. | volume |