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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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Model Initialisation Phase

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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:

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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:

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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.

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1. Start with zero borrow between all owners:
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2. If the borrow and payback system BPSystem is a local system and this method is used in the flow phase to lend water (UpdateAccounts = Yes), limit the surplus amount each owner can lend to their airspace (to ensure payback is possible). 
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   (This should not be done for global systems, as it is possible for ownership to get out of synchronisation at different points in the system, so temporary incompatibility between airspace and balances must be allowed to occur)

3. Share surpluses to owner’s with a deficit. Surpluses are shared to owners that participate in sharing at the same priority level and have a deficit in proportion to their deficit. Where surpluses exceed deficits, the volume of the owner’s surplus distributed to other owners is in proportion to their share of the total surplus:
   For each priority level pl represented in the configured distribution table – PSystem.Share(pl, owner), do the following:

   1. Calculate the total surplus and deficit remaining to be shared at the priority level (this is the sum for all owners sharing at the priority level):
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   2. If there is no surplus and/or deficit (TotalSurplus(pl) = 0 or TotalDeficit(pl)=0), sharing at the priority level is finished, so skip to the next priority level.

   3. For each owner and other owner that share at this priority level pl (Share(pl, owner) = yes and Share(pl, other owner) = yes), calculate the volume owner can borrow from the other owner at the priority level:
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   4. Update the surpluses & deficits left over for sharing at the next priority level:
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4. Find the owner’s borrow and lending totals to return to the relevant model component:
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5. If the UpdateAccounts flag is set, 
   Update the reported borrow accounts for the current component and the borrow and payback system as a whole:
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Payback at a Storage (Flow Phase only)

The payback process at a ‘payback’ storage node is performed at the end of the flow phase, when storage releases have been made (so requirements have been met as far as possible).

A payback storage either has a local borrow and payback system, or is the specified payback storage for a global borrow and payback system. When a storage is the payback storage for a global borrow and payback system, Source runs the payback process twice, first to reconcile its global accounts, then to reconcile its local accounts.

A running account of net borrow between all owners is maintained for each borrow and payback system – BPSystem.NetBorrow(t, owner, other owner). This amount is positive when the owner has a debt to the other owner, and negative when the owner has lent to the other owner. Debt is paid back by reassigning ownership of water in the storage.

Debtors pay back creditors with their share of storage, until no more is owed or the creditor runs out of airspace. Debts between owners that share at higher priority levels are fully paid before any debts at lower priority levels are processed. At the same priority level, where owners have insufficient storage to repay debts, they pay creditors pack in proportion to how much is owed.

For more details on payback at a storage see Ownership in Storages - SRG.

Time step: Resource Assessment – Water Accounting Phase

In this phase, allocation of each owner’s resource assessment systems (RAS) must be adjusted for the net borrow between owners recorded in borrow and payback system accounts. Each RAS will belong to an ownership system, that has a global borrow and payback system. A RAS may also be associated with one or more local borrow and payback systems (if it is used to manage storages). The allocation of each RAS should be adjusted as follows:

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More details regarding the way resource assessment systems are configured and allocate water are given in Resource Assessment - SRG and its sub-pages. 

Data

Input data

Details on data are provided in the Source User Guide.

Parameters or settings

Parameters are listed in Tables 6-8, which follow.

Table 6. Borrow & Payback System Parameters

Table 7. Distribution System Parameters