Borrow and Payback Systems, Distribution Systems

To ensure the most efficient use of the available water resource, you must be able to transfer water ownership amongst owners. If one or more owners has surplus capacity or water, they lend this to the owners with a deficit using the Borrow and Payback systems. If multiple owners need to borrow, and there is insufficient water or capacity to meet their combined requirement, a configured Distribution system is used to determine priority of access.

In a distribution system, borrow records the exchange of water from owners with surplus to those with deficit. Borrowing can occur at any node where there is a loss or gain, and results in the ownership of the borrowed water being transferred from the lender to the borrower. An exchange of water indicates that when an owner borrows water, they need to pay it back to the owner they borrowed it from. This is referred to as payback, which occurs when water that was lent to a particular owner is paid back to its original owner. Borrowing owners may payback their debt by transferring their water to the other owner, either in a specified payback storage or indirectly via Resource Assessment Systems.

Both borrowing and payback occur in an order, based on priority levels. An owner’s debts at the highest priority level must be paid back before the next priority level is considered. Payback within a priority level is proportional, ie. owners with a debt will pay back the debt to another owner in proportion to the other owner’s share of the owner’s total debt at the priority level.

There are two types of borrow and payback systems:

• A single global borrow and payback system (Figure 1) where payback can occur in a norminated storage or indirectly through the resource assessment process. By default, all borrow and payback systems created are global systems; and
• Zero or more local borrow and payback systems, each associated with a single storage and both borrow and payback must occur at this storage. The storage cannot be a member of any other borrow and payback system (including global systems).

Each storage that falls within a scenario's ownership system can only be a member of one borrow and payback systems, but where there is reconciliation/payback at a storage for the global system, a local system storage must be used.

Borrow and payback with resource assessment

Borrowing owners may payback their debt by transferring their water to the other owner, either in a specified payback storage (Payback storage) or indirectly via Resource Assessment Systems (System). Figure 1 shows where to configure this. The former indicates the storage where debts in the borrow and payback system are paid back or reconciled, whereas the latter refers to the resource assessment system that is linked to the ownership system. Use the drop down menu to choose the appropriate storage node. A similar window is available for a local borrow and payback system (Figure 2).

Click Add Initial borrow to specify any borrows that occur at the start of the scenario run.

Figure 2. Local borrow and payback

Distribution systems

A distribution system is one where owners with surplus water share their allocated percentage with owners that cannot meet their requirements. Distribution systems define how this resource can be shared amongst owners using priority levels. The proportion that is distributed is determined as follows:

• Owners with a surplus will share their surplus in proportion to their share of the total surplus; and
• Owners with a deficit will receive surpluses in proportion to their share of the total deficit.

Characteristics

The following apply to distribution systems in Source:

• Ownership must be enabled prior to adding distribution systems. This creates a Default Distribution System (shown in Figure 3) with a priority level of 1;
• They have global scope. In other words, they are applicable throughout Source;
• They specify the water recipient (priority levels); and
• They do not define when (time) or how fast (flow rate) the recipient will receive water.

Figure 3. Distribution systems

Priority Levels

Priority levels determine the order of distribution amongst owners that require water, which occurs from highest to lowest. Surpluses must fully meet requirements (deficits) at a higher priority level before the next lower priority level is considered. In other words, they will always be executed for each owner at priority level 1, then priority level 2, and so on. Also note the following:

• There can be any number of priority levels. Priority levels with only one owner will have no effect. Priority levels that are identical to higher levels will also have no effect. If there are 6 owners, there can only be 57 permissible priority levels, 15 containing two owners, 20 containing 3 owners, 15 containing 4 owners, 6 containing 5 owners, and 1 containing all 6 owners;
• Each pair of owners must appear in at least one priority level. That is, it is not possible to have owners that are unable to borrow or lend to or from another owner; and
• If a distribution rule is operating in priority 1 and an owner cannot receive the entire distribution allocated, the rule will move onto priority 2, and so on for the lower priority levels. Consider owners A, B, C, and D, each operating at priority levels 1, 2, 3 and 4 respectively. Now, assume that A gives its spare capacity to B. However, B does not require the entire allocation. Then, A can choose to give the remainder to C, before D.

An example of a priority level description is shown in Figure 4.

Configuration in Source

Choose Edit » Ownership... to open the Ownership setup dialog. Ensure ownership has been enabled (refer Ownership Systems).To add a new distribution system, right click Distribution Systems and choose Add Distribution System (here). You can also rename or delete a distribution system using the contextual menu, as shown in Figure 3.

The table specifies the list of owners that share water at each priority level. Click Add Priority Level to add additional priority levels and tick or untick the relevant checkbox to add or remove an owner from a priority level. One owner can be part of several priority levels.

Results

To view the results of distributed water, as well as that borrowed and paid back, enable recording of the Ownership and Borrow and Payback attributes under Parameters in the Project Explorer. After configuring and running the scenario, click View Results in the Recording Manager to open the Ownership Results dialog (Figure 5).

This window shows all the ownership systems present in the scenario, along with the borrow-payback systems that are part of the ownership system. It shows the borrow and payback balances amongst owners at a particular time-step (as a table in the Current Balances tab or a time series in the Timeseries Data tab). You can also use the date-picker above the time scroll to change the date. The Total column shows the overall balances for each owner at that time-step. Payback occurs from an owner that has a negative balance to another owner with a positive balance.

In the example shown in Figure 5 , Owners 2 and 3 have borrowed 30ML and 50ML from Owner 1 respectively. The Total column indicates that Owner 1 must receive a total of 80ML, whereas Owners 2 and 3 must pay back a total of 30ML and 50ML respectively. These numbers represent the balances for the first time-step in the model.

Figure 6 shows the borrow-payback balances for all three owners as a time series. The graph can be separated into three distinct segments:

• Upto 11/10/2009: Owners 2 and 3 borrow from Owner 1. The graph shows the balance of Owner 1 increasing in this period (he is owed water from the other owners), whereas the other two owners show a fall;
• 12/10/2009: Owner 3 pays back their share of the outstanding balance to Owner 1. This is represented by the increase in balance for Owner 3 and the corresponding decrease for Owner 1; and
• 13/10/2009 onwards: Owners 2 and 3 continue to borrow from Owner 1 in proportion to their storage volume. However, Owner 3 also pays back water to Owner 1, hence the increase in balance for Owner 3. Owner 2 may not have any storage capacity allocated to it, hence is unable to payback.

Figure 6. Borrow-payback balance (as a time series)