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1. 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.
2. A borrow network/distribution system defines how any owner’s surplus water is shared amongst other owners. See 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.
3. 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.
4. Individual storages can be configured to either:
   
   1. 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.
   2. 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.
5. 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.
6. A record of the net borrow between each of the water owners in an ownership system is maintained for:
   
   1. The global borrow and payback system (exclusive of storages configured to maintain local accounts)
   2. 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).
7. Global borrow is either:
   
   1. 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.
   2. 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.
8. 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. 

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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).

The distribution system specifies the list of owners that share at each priority level. Distribution of surpluses is performed in order of priority level, from ‘highest’ to ‘lowest’. Surpluses must fully meet requirements (deficits) at a ‘higher’ priority level before the next priority level is considered. Distribution within a priority level is proportional, i.e.:

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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.:

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