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Overview

Description and rationale

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Borrow and Payback has been modelled in predecessors to Source, such as IQQM and MSM, for many years. The concepts in these models have been updated and enhanced to suit the needs of Source.

Version

Source version number 2number 4.191.1.

Dependencies

The model configuration in Source must include at least one Ownership System with at least two Owners for borrow and payback to be modelled.

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Automatically included with the full version of Source.

Structure & processes

Theory

Introduction

The Murray Darling Basin Agreementenables the operators in the River Murray to exchange the ownership of volumes of water between the states of New South Wales (NSW) and Victoria when the lending state has water surplus to its requirements. An example of this is when state owned tributary inflows are greater than expected. These excess tributary flows result in a state’s share of water in the river being surplus to that required to meet to meet its ordered requirements. Where possible, this excess water is utilised by the other state (owner) and the amount of water that needs to be released from storage is reduced. The water used by the other state (owner) is termed a borrow and is reconciled against the state’s water share. In the case of the River Murray, payback and account balances are reconciled at Lake Victoria.

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• Inflow and confluence nodes
• Wetland hydraulic connectors
• Loss nodes
• Supply point nodes
• Controlled splitter nodes
• Storage nodes (Weir upstream reach not included)
• Links

Borrow and Payback accounting occurs mainly during the flow distribution phase, although some aspects of Borrow and Payback are also considered in the order phase. For a weir node, Borrow and Payback only occurs in the weir storage component and not the upstream reach.

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Assumptions and constraints Assumptions and constraints

Assumptions and constraints

Table 1. Assumptions and constraints

Definitions

Other definitions can be found in the eWater glossary.

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The structure of borrow and payback systems is illustrated in Figures 1 and 2, below.

Figure 1. Entity relationship diagram: Borrow and payback

Image Modified

Figure 2: Example of storage allocation to a scenario's borrow and payback systems

Figure 2 illustrates how each storage that falls within a scenario’s ownership system can only be a member of one borrow and payback system, except that where there is reconciliation/payback at a storage for the global system, this storage must also be in a local system.

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Table 2. Variables and entities: general

The following variables relate to a Borrow and Payback system:

Table 3. Borrow and Payback System Variables

*Recorded values

The following variables are used by the Borrow method in the order and/or flow phases:

Table 4. Variables & entities: Borrow Method

The following variables are used by the Payback method at a Storage Node in the flow phase:

Table 5. Variables & entities: Payback Method

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:

1. Details of the global borrow and payback system.
2. 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:

1. Distribution hierarchy.
2. Payback details
3. 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):

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.:

Equation 1	Image Added

Equation 2	Image Added

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:

Equation 3	Image Added

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	Image Added

Equation 5	Image Added

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.

1. 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.
2. 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.

Input parameters required

 BPSystem - The borrow and payback system to use in determining distribution rules and, where relevant, updating accounts. Borrow and payback system(s) are associated with each model node and link during model initialisation.

component - Model component using this borrow method, needed for updating accounts.

Surplus(owner), Deficit(owner) - The current ‘surplus’ and ‘deficit’ volume/capacity for each owner. At a storage node, surplus water is any water that is not required to meet downstream requirements in the current time step. This definition may vary between model components, and between the model phases (order or flow). Refer to the SRG entry for the appropriate model component for more information.

Airspace - Each owner’s share of airspace in the borrow and payback system’s payback storage (when there is one). This is needed to limit lending.

UpdateAccounts - Borrow accounting only needs to occur in some circumstances, as described above. This parameter is set or reset according to whether or not borrow accounts are to be updated. 

Output parameters

The borrow method returns the volume each owner borrowed and lent for the time step (OwnerBorrowed(t, owner), OwnerLent(t, owner), OwnerBorrowed(t, owner, other_owner), OwnerLent(t, owner, other_owner)) to the relevant component so that the payback requirement can be adjusted (see examples at the end of this SRG entry). Borrowed volumes are subtracted from, and loaned volumes added to, the requirement.

Steps

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.

1. Start with zero borrow between all owners:

   Equation 6	Image Added

   
   

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). 

   Equation 7	Image Added

   (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):

      Equation 8	Image Added

      
      

      Equation 9	Image Added

      
      

   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:

      Equation 10	Image Added

      
      

   4. Update the surpluses & deficits left over for sharing at the next priority level:

      Equation 11	Image Added

      
      

      Equation 12	Image Added

      
      

4. Find the owner’s borrow and lending totals to return to the relevant model component:

   Equation 13	Image Added

   
   

   Equation 14	Image Added

   
   

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:

   Equation 15	Image Added

   
   

   Equation 16	Image Added

   
   

   Equation 17	Image Added

   
   

   Equation 18	Image Added

   
   

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 Owner Capacity Share Proportion Method.

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:

Equation 19	Image Added

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

Table 8. For Storages: Additional Parameter for Borrow and Payback Systems

Output data

Table 9. Recorded parameters (Borrow & Payback)

Model Element	Parameter	Units	Methodology variable	Frequency	Display format
Borrow & Payback System – model component	Time step Borrow	volume	ComponentNetBorrow(t, component, owner, other_owner)	Time-step	Table of owner-other owner values for a selected time step.
		volume	ComponentBorrowBalance(t, component, owner, other_owner)	Time-step
Borrow & Payback System	Borrow	volume	NetBorrow(t, owner, other_owner)	Time-step	Time Series table and Graph/chart – see examples below.
		volume	BorrowBalance(t, owner, other_owner)	Time-step
Ownership System	Borrow	volume	Image Added	Time-step
		volume	Image Added	Time-step

Example Model Component Borrow & Payback Report Table

Hume Dam Storage Node: Murray Global Borrow & Payback System

Time Step: 10    (30/1/2010)

Example Borrow & Payback System Report Views Table

Murray Global Borrow & Payback System

When a ‘View Time Series’ function/link is selected from an ‘Other Owners’ column, a window allows borrow and payback data for the relevant owner-other owner combination to be displayed as follows:

Murray Global Borrow & Payback: NSW-Vic Net Borrow

Image Added

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:

Murray Global Borrow & Payback: NSW Balance

Image Added

When the ‘View Time Series’ function/link is selected in the ‘All Owners’ row, a window allows borrow and payback totals for all owners to be displayed as follows:

Murray Global Borrow & Payback: All Owner  Balances

Image Added