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Overview

Scale

The allocation to each account type, and to individual accounts within these, are updated each time there is a reassessment.  On a spatial scale, an Annual Accounting system can apply to selected water users spread through all or part of the length of a river system.

Principal developer

Several jurisdictions in Australia have developed model representations of Annual Accounting systems, starting from the 1980's with refinements continuing since then in response to evolving water management needs. The representation in Source has been developed by eWater, based on these earlier representations.

Scientific Provenance

A number of permutations of Annual Accounting have been implemented by various Australian states and territories over many years. Within a given jurisdiction it is not uncommon for there to be differences in the way Annual Accounting is implemented between river valleys. The representation of Annual Accounting in Source is designed to replicate these practical implementations and also provide sufficient flexibility to accommodate new changes if required.

Version

Source version 3.8.12

Dependencies

 Annual Accounting can be used by any of the following node types which are referred to as "Account Hosts":

1. Water User Nodes (with attached Supply Point Node)
2. Environmental Demand Node
3. Bulk Licensing Node

Exact configuration within each of these Accounts Hosts varying but the underlying Resource Assessment system is consistent.

Assumptions

The assumptions and limitations are summarised in Table 1.

Annual Accounting

The Annual Accounting implementation in Source follows the same basic concepts as other types of resource assessment systems, namely:

• An available resource;
• A defined water year;
• A set of account types;
• Accounts to which water is allocated, and for which usage is tracked; and
• Possibly an owner (for example, the state "NSW"), when ownership is enabled.

The modeller is given the flexibility to determine rules such as:

• Dates of the water year.
• The frequency of the resource assessment and allocation process. By default this occurs annually at the start of the water year.
• How the available resource is determined. The main feature of this type of system is that the flexibility of expressions is provided within a structure that is common to Annual Accounting methodologies in different jurisdictions.
• Rules of allocation.
• Usage limits (in addition to the account balance)
• Carryover type (spillable or non spillable)

Event Triggers are provided to allow modelling of carryover and other less commonly used methods in Annual Accounting.

There are some features that differentiate Annual Accounting from other types of resource assessment system. In Annual Accounting, a forward projection of usable future inflow to the end of the water year may be included in the available resource and subsequently influence the allocation announcement. Allocations may be reviewed and adjusted throughout the water year, typically as events occur. This type of review looks at the ability of the system to supply the demand for the rest of the water year. There may be some point within the water year when no further allocation announcements are made, typically at the end of the growing season.

To model Annual Accounting, Source performs resource assessment at a user-specified frequency (via the Reassessment Trigger), adjusting the available resource for demands supplied to date. It provides the modeller with a simple mechanism to define the relationship between the resource and allocation - this is Available Resource versus Allocation (ARA) table. At each reassessment this relationship is used to determine the volume of water allocated to each account type represented, based on that time-step’s available resource estimate (See the section on Account Types and the ARA Table). Functions can also be used instead of the ARA table to determine an Account Type’s allocation. In cases where the Account Type has an increase in allocation from the previous resource assessment, the change in allocation (increment) is assigned to its accounts.

Available Resource

The available resource for an Annual Accounting system is calculated as shown in Equation 1.

Active Storage

Tributary Inflow

Tributary Inflows are summed together and added to the Available Resource during reassessment.

Commitments are specified via functions and the modeller may specify an arbitrary number of them. They can be used to represent (or reserve water for):

• Carryover - An example is the environment’s carry-over volume from the previous water year in the Wimmera / Glenelg system in Victoria. This is the remaining volume after allowing for the environment’s share of storage losses and spills in current allocation year.
• Forecast Losses for Remainder of Water Year - There may be several different losses that could all be added into the resource assessment calculation separately. These could include river losses, channel losses and evaporation losses.
• Fixed Adjustments - Examples include environmental flows, compensation flows and water quality dilution flows. In the Wimmera / Glenelg system, there is an adjustment for amount of borrowed water returned to the Minister of Environment under sub-clause 6.1(c) of the Flora and Fauna Bulk Entitlement Order.
• Reserve - An example in the Wimmera / Glenelg system is the amount of water to be retained in head-works reservoirs this water year to provide for a basic supply next water year.

Some of the above examples could also be represented by Account Types. The volume of water reserved for purposes such as water quality might be independent of allocation, while that required for reservoir evaporation and transmission losses might vary with allocation. The water uses defined under Commitments should generally be those that are independent of allocations within the Annual Accounting system, and for which no water user accounts are required within the system. An example could be water in the system’s storages that is to be managed by another resource assessment system.

Commitments are summed together and the total subtracted from the Available Resource during reassessment.

Other resources

Other Resources are specified via functions and the modeller may specify an arbitrary number of them.  They can be used to add to the available resource a wide range of water resource types and facilitate representation of the following inflow volumes commonly used in Annual Accounting:

• Minimum historic usable cumulative tributary inflow volume.
• Minimum historic cumulative headwater inflow volume

These volumes are based on historic (rather than modelled) data for each month, and are forecasts of the total minimum inflow volume from the current month to the end of the water year or season.

Other applications include inter-basin transfers and groundwater resources. These resources may be managed by another resource assessment system. An example of an inter-basin transfer is the Wimmera / Glenelg system where water is taken into the Wimmera-Mallee system from the Goulburn-Broken system via the Waranga Western Channel, and water availability is managed via the Goulburn-Broken resource assessment system.  Source allows functions to be entered to allow for this linkage between systems, as well as to provide access to relevant model variables.

Other Resource are summed together and the total added to the Available Resource during reassessment.

Usage to date

This parameter represents how much of the water allocated has been ordered or used by Account Hosts in the water year to date. Usage for individual accounts is tracked by Source and may be accounted when water is ordered, delivered, or diverted. The timing of usage accounting depends on whether water order debiting or water usage debiting applies (associated with the resource assessment system).

When the modeller selects the "Usage from Account Host" option, the total of usage in all accounts that has been tracked by Source is used, as in, it uses the value taken from orders made by Account Hosts. Alternatively, the modeller may input an expression to determine the usage value (the "Usage from expression" option). An example application would be to adjust the tracked value to represent a conveyance loss.

Usage to Date is added to the Available Resource during reassessment.

Spill to date

Spill to date represents water spilled from accounts in the current water year up to the reassessment date.  Spill reduction for each account is tracked in Source, this is the total spill reduction from all accounts within this accounting system from start of water year to date.

Total Spill to Date is added to the Available Resource during reassessment

Adjustment

Adjustments represent changes to the accounts that have occurred via Triggers (not just the "Adjustment" Trigger)  Triggers allow changes to Account to occur based on specified Conditions and may result in adjustments to many account properties.  When these changes to occur they are tracked to ensure that the modifications are factored into allocation.

Adjustments are subtracted from the Available Resource during reassessment.

Account Types

As with other types of resource assessment system in Source, Account Types in an Annual Accounting system represent a type of water to be allocated or assigned. Some examples are:

• Types of entitlement or licence, sometimes referred to as "Products". Examples include High Reliability Water Share (HRWS) and Low Reliability Water Share (LRWS) in Victoria, and High Security and General Security in NSW;
• Basic rights, such as stock and domestic rights, and native title;
• Environmental allocation;
• Reserve. For example, in the Murray, Goulburn, Lachlan and Murrumbidgee systems, water is set aside for following seasons. In the Thomson/Macalister system, allocations are made to a Drought Reserve, which has much the same purpose;
• Transmission and operational losses (TOL) in the Lachlan system;
• Spill entitlement. An example of where this occurs is the Thomson / Macalister system in Victoria where, if Lake Glenmaggie spills, Southern Rural Water can declare a period that spill entitlement is available and the water delivered to irrigators during this period is not counted as part of their seasonal allocation;
• Carryover account types, such as the Spillable Water Accounts (SWAs) in Victoria. Carryover is widely available in annual accounting systems. It allows individuals to carry over part of their unused water allocation (up to a prescribed limit) for use in the following season. It was introduced to annual accounting in northern Victoria in 2006/07 as an emergency drought measure- see DSE (2009), with a limit of 100 per cent of entitlement (ie an individual’s carryover plus allocations cannot exceed 100 per cent). SWAs were introduced on 30 June 2010 to reduce the risk of entitlement-holders losing their carryover in full allocation years. As allocations increase throughout the following season, these are added to users’ accounts until carryover plus allocation equals 100 per cent of the entitlement volume. After this, any additional allocations are credited to the SWA. Throughout the season, the storage manager assesses the risk of the storage spilling during the remainder of the season. Only when the storage manager declares the status of system as "spilling", can the entitlement holders access the water in the SWA;
• Bulk entitlements (BEs) in Victoria, and Conveyance licences used in the Murrumbidgee and the Murray in NSW. In Victoria, BEs are issued to Water Authorities to define their respective rights to water (NRE, 1995). The NSW Conveyance licenses are assigned to irrigation companies, which then allocate water to individual irrigators;
• Nested or linked resource assessment systems (known in Victoria as allocation groups). One example is the Goulburn system link to the Loddon system. Allocation for the Loddon HRWS account type is calculated firstly based on available resource in the Loddon system and then compared with the Goulburn allocation. The lesser of the two is the final Loddon allocation; and
• Off-quota water, or Off-allocation systems (a type of nested resource assessment system).

Some of the above examples could also be represented using the Annual Accounting system’s Commitments or Other Resources functions.  An Account Type should be used if its allocation is dependent on allocations to other Account Types within the Annual Accounting system, or usage of the allocation needs to be tracked.

The sub-sections below describe the use of Account Type parameters or relationships in relation to Annual Accounting systems.

Priority

Unit Shares / Entitlement

Allocation calculated method

This parameter determines the mechanism by which the allocation to an account type will be made.

The default option is the "Available Resource Allocation Table" option. When this is chosen, a piecewise relationship must be specified for the account type that matches the account type’s allocation to a volume of available resource. Example uses of this option are for the Victorian HRWS and LRWS volumetric entitlements.

The other option is "Function", which allows the modeller to specify the expression by which allocation is calculated. This expression may reference a wide range of variables available in the model, including allocations to other account types and resource assessment systems. Example applications of the "Function" option would be for spill entitlement, reserve, carryover, off-quota and nested or linked resource assessment system account types.

An expression could be used to implement an adjustment to allocations to comply with the Murray Darling Basin cap, such as in the Goulburn-Broken system. In this instance, a relationship of the rolling average "cap" diversions minus modelled diversions versus allocation reduction percentage is applied to reduce LRWS allocations. The expression would incorporate:

• A time series of rolling average cap diversions;
• Comparison of modelled Usage to date and the time series, to determine the difference;
• Use of the look-up table determine the reduction amount; and
• Application of the reduction to the percentage allocation.

Available Resource verses Allocation (ARA) Table

Another example is the relationship between HRWS and LRWS account types in Victorian river systems to implement a reserve policy. In this case, the allocation is done according to projections over one year and two year planning periods, as shown in Figure 4.

 It can be seen from Figure 4 that as the resource available increases, allocations increase up to 100% of HRWS entitlements based on the ARA relationship for the one year planning period. There will be no increase in allocation after 100% until enough resource is available to supply 100% of HRWS entitlements in the following season. Then the allocations greater than 100% or LRWS allocations can be announced with further improvements in the available resources, using the ARA relationship for the two year planning period. The ARA tables to implement this type of relationship between account types in two such annual accounting systems (1 year planning, 2 year planning) are shown in Figure 3.

This parameter caters for the different ways allocations are announced over the water year. The options are:

• Percent (with linked volume) - This is designed for cases where allocations are announced as a percentage of the annual volumetric entitlements. This is generally the case in NSW and Victorian systems. The "linked volume" is the total volume of shares for the account type - which represents the volume allocated to the account type at 100% allocation (see the sub-section on the Shares parameter for more detail). In ARA table rows, the modeller specifies a percentage which is multiplied by the share volume and included in the (read only) available resource column total.
• Percent (with Function) - Allocations are also announced as a percentage of annual volumetric entitlement, but modeller has more flexibility to define percentage by function in ARA table rows.
• Volumetric - to be used when allocations are announced as a volume. This is the case in the Wimmera/Glenelg system, and in the Lachlan for basic rights (native title rights, domestic and stock rights).
• Monthly varying - Volumetric allocations to some account types will change over the year. An example is the TOL account type in the Lachlan system. The TOL volume may change each month due to the fact that losses such as evaporation varies according to climatic and other conditions. The modeller can also specify whether the month’s TOL volume should be used for the whole month, or whether it is necessary to interpolate between current and next month values to determine TOL for a particular day. This interpolation is not done for time-steps within the end month of a water year - in this case that month’s value for TOL volume is used throughout the whole month.
• Function - Allocations are announced through a user-defined function.

Announced Increment and Maximum Announced Percent

Carryover 

Spillable trigger - If carry over is spillable, user can enter an function to determine if water remains spillable in the current timestep. This will generally be a boolean function of time of year or storage(s) levels. The format of boolean function is "IF A>B,1,0", "1" indicates the action will occur and "0" means action will not happen.  

Water Available While Spillable -to decide whether water is available while spillable. For most spillable water account, spillable water is not available to user, however Victoria Inter-Valley Transfer accounts are useable while spillable. 

Basis of Spilling - to calculate spill reduction shared among users, three options are available:

1. Emptied on spill or pre-lease is to totally empty spillable water account if a spill or pre-release occurs.
   
2. Reduce Pro-rata is to spill based on proportion (account spillable water / total spillable water) of spill source volume.  
   
3. Equalise spillable carryover proportions is a rule special for Barmah-Millewa Forest spill. The way this spilling rule works is:
   • Spill first from the account with the greatest carryover as a percent of its entitlement.
   • When the percentage carry over of the first account reaches the percentage of the second account, spill from the first and second pro-rata until the third highest account is reached,
   • Continue on as above until all carryover is spilled or all spill is assigned.

Use Account Type Spill Volume - to specify a separate spill volume source for each account type. If specified, the volume specified for an account type would be used in preference to the allocation system spill source. This is to cope with Barmah-Millewa water which receives allocation from both the Victorian and NSW allocation systems.

A flow chart is listed below to show how spillable water and spill reduction are calculated in resource assessment pre and  post phases.

Center
Table 3 - Total Water - 100%

Accounts represent an Account Host's entitlement to a type of water. Hence, they are linked to an Account Type, and have a number of shares in that Account Type’s allocation. A water user may have more than one account in an Annual Accounting system. For example, Victorian water users may hold both a HRWS and LWRS account. In the Thomson / Macalister system, water users may also have a spill entitlement and off-quota account.

Previous usage, usage tracking and usage limits

The modeller can configure the account’s Previous Usage for the water year. Source keeps track of usage over the water year so that it can be taken into account when calculating the available resource (see the Usage to Date sub-section) and allocation (see the Allocation Calculation Method sub-section under Account Type). Account level usage caps may also apply in annual accounting systems - this can be implemented by adding a usage limit to the account.

Unit shares/Entitlement

Although this parameter is expressed as a volume, it usually represents a number of unit shares assigned the account. Parameters for the associated account type determine when Number of Shares represents a volume at 100% allocation:

• The account type is not in the ARA table (Allocation Calculation Method is "Function"); or
• The account type is in the ARA table, and:
  • Method to Apply for this account type in the ARA table is "Percent (with linked volume)"
  • Use Unit Shares/Entitlement From Accounts? is "Yes"

Most general security access licences are expressed as a relative unit share of the available water rather than as an annual volume (CSIRO, 2008a).

For calculations relevant to Number of shares / Entitlement, see the sub-section on Allocate to Account Types and Accounts within each Account Type in Model Phase: Resource Assessment and Water Accounting.

Initial account balance and Account balance

This parameter is the starting account balance. Account balance is used in Source to limit to the volume of water a water user may order and/or access using the account. During resource assessment, the account balance is credited with the account’s share of the allocation to the account type.

Initial Allocation

 Allows the user to set what proportion of the Entitlement that is initially allocated.

initial Carryover Scalar

Carryover Scalar initial settings.  This controls how much of the Carryover Start of Water Year is assigned to the Carryover Balance so far this Water Year,

Carryover Start of Water Year

This represents the total amount of potential Carryover that can be assigned to the Account based on the Carryover Triggers from the previous Water Year.  This is scaled by Carryover Scalar which controls how much of this total Carryover volume that has been assigned to the Carryover Balance.

Previous Spill from Carryover

Setup the previous spill that has been already accounted for in the current Water Year,.

Model phase: Configuration

Model Phase: Initialisation

Variables are initialised as follows:

• The previous allocation to each account type (AllocPrevious%(type), AllocationPrevious(type)) is zero for the first time-step;
• If the modeller has specified account limitations based on a unit share then the total volumetric limitations for each account are determined; and
• Account usage is initialised by evaluating the previous usage expression specified by the modeller and the total accumulated for the RAS.

Model Phase: Time-step Beginning, Time-step End

At the start or end of a time-step, annual accounting triggers may be executed, depending on configuration. If the trigger condition returns a true value, the trigger action is executed. By default annual accounting systems have a start of water year trigger, that is executed at the start of the first time-step at the beginning of a new water year.

Actions at the start of a water year

At the start of each water year after the first one in a scenario run, the running usage total for every account in an annual accounting RAS and the running usage total for the RAS as a whole are reset to zero.

Model Phase: Resource Assessment and Water Accounting

Calculate Allocation for each Account Type

Model Phases: Time-step - Order and Flow Distribution

The working of these phases is described in detail in the SRG sections on Continuous Sharing - SRG, Continuous Accounting - SRG and the Water user node SRG.

In summary, at water users with the "Account Sharing" option selected, every order placed uses an account. The volume ordered using the account is limited to the account balance. Accounts are debited either for orders placed (if the account is of an order-debit account type), or usage (where the account is of a use-debit account type). Water-order accounts are debited in the order phase and the total is re-credited for shortfalls in the flow distribution phase. Water-use accounts are debited for "usage" in the flow distribution phase.

The water user node and supply point node interact to perform some complex processing to determine the "usage" to assign to accounts. A running total of usage for the water year for each account is tracked by Source. This total may be referenced by annual accounting triggers that cap usage.

The variables used by the model are summarised in Table 2.

Input data

Input data is summarised in the following tables:

• Configuration parameters - Table 3;
• Allocation parameters - Table 4;
• Trigger parameters - Table 5. The parameters listed in this table are a sub-set of those described in Table 8;
• Account Type parameters - Table 6;
• Account parameters - Table 7;
• Per-trigger parameters - Table 8. There is one set of the parameters in this table for each trigger in the Annual Accounting System.

Table 3. Configuration parameters

A default "Start of Water Year" trigger is created with each new annual accounting system:

• Parent: The annual accounting system’s name
• Type: "Start of Water Year"
• Action type: "Re-assessment"
• Execution: "Timestep beginning"
• Time-steps from last assessment: "1"

Table 6. Account type parameters

Table 7. Account parameters

Table 8. Pre-trigger parameters

When modelling an existing continuous accounting system, the values of parameters adopted will be dictated by rules governing the operation of the system in place. When modelling new systems or modifications to existing systems, it will be a matter of professional judgement and consultation with stakeholders as to reasonable values to explore, combined with "trial and error" solutions, to arrive at acceptable parameter values.

Output data

Output data is summarised in the following tables:

• Account Type recorded output variables - Table 10;
• Account recorded output variables - Table 11; and
• Recorded output variables - Table 12

Results are provided for each time-step, although they are only calculated in resource assessment time-steps.

Table 10. Account type recorded output variables

Table 12. Recorded output variables

Reference list

CSIRO (2008a) Water Availability in the Lachlan. A report to the Australian Government from the CSIRO Murray-Darling Basin Sustainable Yields Project.

CSIRO (2008b) Water Availability in the Murrumbidgee. A report to the Australian Government from the CSIRO Murray-Darling Basin Sustainable Yields Project.

DSE (2009) A Proposal for Future Carryover Arrangements. Northern Region Sustainable Water Strategy

James, B., Erlanger, P. D. and Hansen, W. R. (1993) Modelling a Complex Irrigation Restriction Policy Using a General Simulation Package. Hydrology and Water Resources Symposium, Newcastle, June 30 - July 2.

NRE (1995) The Bulk Entitlement Conversion Process. Bulk Entitlements Report Series No. 2, Department of Natural Resources and Environment, Victoria.