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| Variable or method | Purpose/description | Units |
|---|---|---|
| Airspace(owner) | Owner’s share of the airspace in the current time step (input and output parameter). Airspace is the difference between the volume in storage and the full supply volume – each owner has a configured fixed percentage of this. | volume |
| BPSystem | System to perform paybacks for (input parameter). This must be associated with the current storage. | n/a |
| CanPayback(owner, pl) | Maximum volume of debt owner can pay back to lending owners that share at priority level pl in BPSystem. This is limited by the owner’s storage share. (Calculated) | volume |
| CanReceive(owner, pl) | Maximum volume owner can receive this time step as payback for loans to other owners that share at priority level pl in BPSystem. This is limited by the owner’s airspace. (Calculated) | volume |
| component | Model component accessing the payback method (input parameter). | n/a |
| PaidBack(owner, pl) | Volume of debt owner paid back to all lending owners that share at priority level pl in BPSystem. (Calculated) | volume |
| Received(owner, pl) | Volume owner received as payback for loans to other owners that share at priority level pl in BPSystem. (Calculated) | volume |
| TotalCanPayback(pl) | Total volume all debtor owners that share at priority level pl in BPSystem can pay back lending owners using their storage share. (Calculated) | volume |
| TotalCanReceive(pl) | Total volume all lending owners that share at priority level pl in BPSystem can receive as payback from debtor owners into their airspace. (Calculated) | volume |
| Payback(owner, other_owner, pl) | The volume an owner paid back to the other_owner for sharing at priority level pl (Calculated). | volume |
| Storage(owner) | Owner’s share of the storage volume in the current time step (input and output parameter). | volume |
Model Configuration Phase
When an ownership system is created (e.g. when ownership is turned on), a default global borrow and payback system is also created that contains all storages within the ownership system’s boundaries.
The modeller then configures:
- Details of the global borrow and payback system.
- A local borrow and payback system for each storage for which one is required. Once a local borrow-payback system is configured, the storage is removed from the global system (e.g. see Figure 2).
The following must be configuredfor each borrow and payback system:
- Distribution hierarchy.
- Payback details
- Initial borrow balance between owners
Distribution Hierarchy
The list of owners that can share surpluses is specified for each priority level in each borrow and payback system. Source ensures that for a global system, each owner is represented, and can share with every other owner. In a local system, owners may opt out of sharing, or only share to some owners. In the methodology, this distribution hierarchy is described as BPSystemShare(pl, owner).
Example global system distribution hierarchy (where every owner can share with every other, but not necessarily at all priority levels):
| Owners | |||
| Priority | A | B | C |
| 1 | Yes | Yes | No |
| 2 | No | Yes | Yes |
| 3 | Yes | No | Yes |
In this example, with owners A, B and C, the connections required are AB, AC, BC. AB applies at priority level 1, AC at priority level 3, and BC at priority level 2.
Payback Details
The modeller must specify the global borrow and payback system’s reconciliation type to determine the ‘location’ at which payback occurs and, if required, the storage where this is to occur, as discussed in the section Repayment of Borrow, above.
Initial Balance
An initial net borrow value is configured for each owner / other_owner combination in each borrow and payback system: BPSystemBorrow(0, owner, other_owner). A negative value indicates that an owner owes other_owner, a positive value indicates that the other_owner owes the owner. During the configuration phase, Source calculates each owner’s initial borrow balance, and ensures the total of all these balances adds up to zero, i.e.:
eqn
eqn
Model Initialisation Phase
During model initialisation, Source associates all nodes and links with the global borrow and payback system except storage nodes that have local borrow and payback systems which are not the global system payback location.
Start of Time Step
Borrow and payback accounts are cumulative, so it is necessary to carry over borrow and payback totals from the previous time step to the next one. This is done for each borrow and payback system being modelled:
eqn
The accounts for each individual model component that falls within a given borrow and payback system boundary are NOT cumulative, so these start at zero every time step:
eqn
eqn
Each model component uses the methods described in the following sections to update the relevant borrow and payback system’s accounts in the flow phase for any borrow/lending that occurs at the model component.
During Time Step: Order and Flow Phases
Borrow
Borrow needs to be considered in both the order and flow phases. However, borrow and payback accounting occurs in the flow phase, as it is only at this stage that the flow borrowed/paid back is known.
- Order phase - There is a potential to reduce upstream orders through borrowing of tributary inflow. See Ownership at Inflow and Confluence Nodes - SRG for details.
- Flow phase - An owner can have a delivery shortfall as a result of higher than expected losses, lower than expected tributary inflows, or the use of its water by another owner that was planned in the order phase. If there is surplus water in the river at the point where the shortfall occurs, the owner with the shortfall can borrow from other owners.