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- Borrow is permitted to occur anywhere in the system. This applies to local systems as well as global systems. Note that where borrow in a storage is concerned, this is required only when one owner has run out of storage share.
- A borrow network/distribution system defines how any owner’s surplus water is shared amongst other owners. See the section on ‘Distribution of Surpluses’ Distribution of Surpluses for more information on how this is done. In a global system, every owner must be able to share to every other owner.
- In scenarios where ownership is configured, all storages must be members of a borrow and payback system. By default they are members of the global system.
- Individual storages can be configured to either:
- Account for borrows at a local level, which means that payback can only occur by ‘recolouring’ storage volumes. For example, any borrow in Hume Reservoir on the River Murray, must be paid back by reassigning water stored in Hume Reservoir.
- Account for borrows as part of a global borrow and payback balance. Note that borrows recorded under local borrow and payback systems are not included in the global set of accounts.
- For local borrow and payback systems and for global borrow and payback systems that are reconciled at a storage, the amount of borrow from other owners is limited by how much surplus the other owners have.
- A record of the net borrow between each of the water owners in an ownership system is maintained for:
- The global borrow and payback system (exclusive of storages configured to maintain local accounts)
- Each storage configured to maintain a local account of borrows. A storage configured to account for borrows locally does not participate in the global borrow account (i.e. borrows are not accounted in two places).
- Global borrow is either:
- Paid back in a storage: A user defined input parameter identifies the storage where balances are reconciled. This payback storage must have a separate, local borrow-payback system configured.
- Not explicitly paid back. Borrow balances adjust, in due course, as a result of the action of resource assessment systems (RAS). See Interaction with Resource Assessment Systems for a description of how this occurs.
- For local systems, or for global systems where a payback storage is nominated: Payback is attempted each time step at the storage. See Repayment of Borrow for more information on how this is done.
Distribution of SurplusesAnchor Distribution of surpluses Distribution of surpluses
The Murray Darling Basin Agreement only requires that borrow and payback occur between two state owners, so no specific rules have been required to determine how an owner shares out their surplus water when there is more than one other owner which could require it – i.e. those with a deficit. To allow the flexibility to meet future requirements, Source allows the modeller to configure for each borrow and payback system a ‘distribution system’ (otherwise referred to as a borrow network, see Assumptions and Constraints 9 and 10).
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Owner A allocates 800 GL to its users and B 1200 GL. Later in the season after the users of owner B have used 1000 GL, B runs out of water and by borrowing starts to take back water from A (which has 200 GL in storage it can't allocate).
Methodology
Variables and Entities Used
Variables and entities used are listed in Tables 2-5, that follow. Rows highlighted in grey refer to entities. Each entity may have its own set of variables and/or methods, which are described in a separate table.
Table 2. Variables and entities: general
| Symbol | Purpose/Description | Units |
|---|---|---|
| Allocation(RAS) | Allocation for resource assessment system RAS | volume |
BPSystem BPSystem(RAS) BPSystem(ownersys) | Borrow and payback system. See Table 3 for variables and methods specific to this entity. BPSystem(RAS) is used to manage the same water resources as resource assessment system RAS BPSystem(ownersys) belongs to ownership system ownersys. | n/a |
| nBPSystem(RAS) | Number of borrow and payback systems that are used to manage the same water as resource assessment system RAS | n/a |
| nBPSystem(ownersys) | Number of borrow and payback systems that belong to ownership system ownersys. | n/a |
| component | Model component index. | n/a |
| nowner | Number of owners in the ownership system the borrow-payback system belongs to. | n/a |
| nowner(pl) | Number of owners that have a borrow & payback sharing relationship at priority level pl. | n/a |
| n_other_owner | Number of owners, other than the ‘current’ one, a total is being calculated for. | n/a |
| n_other_owner(pl) | Number of owners, other than the ‘current’ one, a total is being calculated for that have a borrow & payback sharing relationship at priority level pl. | n/a |
| ownersys | An ownership system | n/a |
| owner | Current owner in system | n/a |
| owner(pl) | Owner that has a borrow & payback sharing relationship at priority level pl. | n/a |
| other_owner | Owner that is not owner | n/a |
| other_owner(pl) | Owner that is not but has a borrow and payback sharing relationship with at priority level pl. | n/a |
| RAS | Resource assessment system | n/a |
| pl | Priority level for sharing | n/a |
| t | Time-step index | n/a |
The following variables relate to a Borrow and Payback system:
Table 3. Borrow and Payback System Variables
| Variable or method | Purpose/description | Units |
|---|---|---|
| *BorrowBalance(t, owner) | Owner’s cumulative borrow balance at time step – equals the sum for all other owners: NetBorrow(t, owner, other_owner) | volume |
| *ComponentBorrowBalance(t, component, owner) | Owner’s time step t borrow balance for borrowing done at component. | volume |
| *ComponentNetBorrow(t, component, owner, other_owner) | Net borrow between owner and other_owner for time step t, performed at model component - +ve value indicates owner does the borrowing, -ve value indicates owner does the lending | volume |
| *NetBorrow(t, owner, other_owner) | Cumulative net borrow between owner and other_owner at time step | volume |
| Share(pl, owner) | Flag (Yes or No) specified by modeller indicating whether an owner, owner, shares with other owners at priority level pl. For any two owners, owner and other_owner, they can share at priority level pl if Share(pl, owner) = Yes and Share(pl, other_owner) = Yes. | n/a |
| Connected(owner, other_owner) | Flag (Yes or No) indicating whether two owners, owner and other_owner, are able to share using the configured distribution hierarchy, and is derived from Share(pl, owner). These flags are set up and used for validating the distribution hierarchy used in a global borrow and payback system. | n/a |
*Recorded values
The following variables are used by the Borrow method in the order and/or flow phases:
Table 4. Variables & entities: Borrow Method
| Variable or method | Purpose/description | Units |
|---|---|---|
| Airspace(owner) | Owner’s share of the airspace in the payback storage for BPSystem in the current time step (input 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 |
| Borrow(owner, other_owner) | Amount owner borrows from other_owner in this instance of sharing (calculated). This may be limited by airspace if it is within a storage. | n/a |
| component | Model component accessing the borrow method (input parameter). | n/a |
| BPSystem | Borrow system to use for the current round of sharing (input parameter). | n/a |
| Deficit(owner) | Deficit volume/capacity/flux per owner in the current time step (input parameter). | n/a |
| OwnerBorrowed(owner) | Total each owner borrowed from all other owners in this instance of sharing (output parameter). | n/a |
| OwnerLent(owner) | Total each owner lent to all other owners in this instance of sharing (output parameter). | n/a |
| Surplus(owner) | Surplus order/flux/capacity for owner in the current time step (input parameter). | n/a |
| TotalDeficit(pl) | Total of owner deficits at priority level pl (calculated). | n/a |
| TotalSurplus(pl) | Total of all owner surpluses remaining to be shared to other owners with a deficit at priority level pl (calculated). | n/a |
| UpdateAccounts | Input parameter that indicates whether borrow and payback system accounts should be updated. | n/a |
The following variables are used by the Payback method at a Storage Node in the flow phase:
Table 5. Variables & entities: Payback Method
| Variable or method | Purpose/description | Units |
|---|---|---|
| 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 | |