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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.:
| Equation 1 |
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| Equation 2 |
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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:
| Equation 3 |
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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:
| Equation 4 |
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| Equation 5 |
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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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The following steps describe how surpluses are distributed within the given borrow and payback system BPSystem. Note that an owner can participate in sharing at a priority level (is in the owner(pl) and other_owner(pl) lists) when BPSystem.Share(pl, owner) = Yes.
Start with zero borrow between all owners:
Equation 6 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).
Equation 7 (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)
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: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):
Equation 8 Equation 9 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.
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:
Equation 10 Update the surpluses & deficits left over for sharing at the next priority level:
Equation 11 Equation 12
Find the owner’s borrow and lending totals to return to the relevant model component:
Equation 13 Equation 14 If the UpdateAccounts flag is set,
Update the reported borrow accounts for the current component and the borrow and payback system as a whole:Equation 15 Equation 16 Equation 17 Equation 18
Payback at a Storage (Flow Phase only)
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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:
| Equation 19 |
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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.
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| Time-step | NSW-Vic Net Borrow |
|---|---|
| 1 | -20 |
| 2 | -15 |
| 3 | 10 |
| 4 | 20 |
| 5 | 5 |
| 6 | -7 |
| 7 | 0 |
| 8 | 30 |
| 9 | 10 |
| Min | -20 |
| Max | 3-30 |
| Average | 3.67 |
When a ‘View Time Series’ function/link is selected in the ‘Owner Summary’ column, a window allows a summary of borrow and payback data for the relevant owner to be displayed as follows:
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