Off Allocation SRG

Document History

Overview

Description and rationale

Off-allocation flows are typically those flows that are in excess of regulated requirements. A number of different types of rules are used to share off-allocation flows in practice. The off-allocation node (OAN) in Source enables these rules to be replicated.

Scale

The off-allocation node manages access to flows in a specified reach of the river and can also share flows with downstream off allocation reaches.

Principal developer

Several jurisdictions in Australia have developed model representations of off-allocation, 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 off-allocation have been implemented by various Australian states over many years. Within a given jurisdiction it is not uncommon for there to be differences in the way off-allocation is implemented between river valleys. The representation of off-allocation in Source is designed to replicate these practical implementations and also provide sufficient flexibility to accommodate new changes if required.

Version

Vx.x.x has complete functionality

Dependencies

• Requires a Resource Assessment System (RAS).

• Requires that the account distribution method is used at the Water User Node and other relevant node types.

• If a splitter is used, an off-allocation node cannot manage off-allocation orders down both arms of the splitter. This means that an off allocation node has to be placed separately downstream of each arm of the splitter.

Assumptions and Limitations

Table 1. Assumptions and constraints applying to off-allocation

Definitions

Theory

Overview

An off-allocation flow sharing system is described in Source by:

• An accounting and sharing system

• Surplus flow allocation

• Off allocation requests and withdrawals
  
  
  

Off allocation accounting and sharing is specified as an account type within a resource assessment system. On this basis, where applicable, it inherits the ownership of the resource assessment system, which is taken into account when water is allocated. The off allocation system (OAS) properties that can be configured include:

• The start of the off allocation water year,

• Whether the sharing arrangement is on the basis of trying to equalise access opportunity to account holders, however, the Equalise Shares function is no longer accessed by the user since version V4.10.2 of Source.

• Optional annual system cap on the amount of off allocation water that can be allocated in a water year,

• Optional rules for resetting account balances when specified storages spill,

• Configurable list of storages that are considered for off allocation access, and

• Off allocation accounts.
  
  
  

The off allocation accounts are related to off allocation reaches which are defined by off allocation nodes (OANs). An OAS may be associated with multiple OANs. The off allocation properties for an associated OAN that can be configured include:

• Optional annual usage limit,

• Table of off allocation volume thresholds and associated priorities for downstream water users, and

• Table of downstream water users and associated shares, initial usage, optional use limit and optional delivery efficiency override.
  
  
  

The OAN may only be associated with water users that are downstream of the node. The type of water users that place off allocation requests include:

• Water User supplied via Supply Point Nodes,

• Bulk Licensing Nodes,

• Environmental Node,

• Storage Nodes, and

• OANs. 
  
  
  

The OAN is used to determine off allocation volumes and to share this volume according to the rules specified in the OAS. The OAN properties that can be configured include:

• The off allocation trigger method,

• Start flow threshold for off allocation to be declared,

• Optional end flow threshold for off allocation to cease,

• Optional maximum flow threshold to cease off allocation,

• Optional proportion of off allocation to reserve i.e. not allocate,

• Optional start and end dates for the off allocation season, and

• Optional limit on off allocation requests to be passed to upstream OANs.
  
  
  

Modelling of off-allocation flow sharing in Source comprises three major components: firstly, the water user off allocation request to the system (source requests) which are adjusted for anticipated losses and gains in the ordering phase and reach the OAN as a target request. The ratio between the source and target request is considered to be the delivery efficiency, which may be optionally overridden. Secondly at the OAN volumes of water available to share are considered, and finally, the accounting and sharing of that water to meet water user requests. Off-allocation flow sharing is triggered when the flow volume at an OAN exceeds the defined criteria for an off-allocation flow sharing event. Noting that this may be further constrained by exceeding an optional maximum flow limit. A proportion of the volume of water above either orders or the flow threshold is shared between water users according to the rules specified in the OAS.

As a downstream OAN may have an account at an upstream OAN. In the case of a confluence it is possible for a downstream OAN to have accounts in both upstream branches. In such cases the OAN requests are sent to both upstream OANs. As the requests are met at one upstream branch the subsequent request in the other branch is correspondingly reduced ensuring that the request is not exceeded i.e. not double accounted. Note this will give priority of allocation to the upstream branch which is processed first.

The priority of access of different water users is associated with off allocation volume thresholds. There may be multiple increasing thresholds with different priorities at each threshold. This functionality has been included to represent the extent that events may reach. The first threshold represents a volume that is sufficient to meet the first extent of sharing e.g. the current off allocation reach. Consequently if the volume to be allocated is less than this amount then no sharing occurs. Subsequent thresholds recognise that the event may be able to reach further down in the system and also that the additional users may have a different priority e.g. filling of a storage. The thresholds are treated as a step function i.e. once equalled or exceeded then the current threshold priority applies.

Allocations to accounts depend on the available water, allocation priorities, ownership in the system, usage caps and water user requests.  Conditions for off-allocation flow sharing such as water quality or environmental health factors can be configured by the modeller entering a function to define a flow threshold based on reference water quality or flow at a point or points in the river.  Environmental requirements can be treated as water users who have an off-allocation requirement that can then be prioritised by the modeller or, alternatively, the modeller can define a function to restrict the triggering of an off-allocation flow event based on water quality, account allocation percentages or water sharing rules.

Off allocation requests

Off-allocation requests are generated by a range of previously mentioned downstream water users. The purpose of the request is largely dependent on the type of water user.

Off allocation requests from water users connected via a supply point are constrained to on farm storages, irrigation demands and maximum extraction rate at supply point. Note other demand types do not have functionality to make requests for off allocation water. Where specified, the on farm storage has an option to fill the storage to maximum storage volume with off allocation water. Each time step the on farm storage will place an additional request to bring the storage to maximum capacity. Depending on configuration settings, each time step the irrigation demand model may request off allocation water to fill rice ponds to maximum pond levels or other crops to opportunistic soil moisture targets (specified as depletions).

The bulk licensing node represents a point in the system where a portfolio of licences are aggregated together and are accounted together. In terms of off allocation water the off allocation requests of users associated with this node are combined together and sent to the associated OAN as a bulk request. The amount of the request that is allocated and delivered is then shared to associated licences according to specified sharing rules. Noting that delivery efficiency is taken into consideration from the bulk licensing node and not the portfolio of licences. The bulk licensing node also provides and optional off allocation use limit.

Storage nodes may optionally specify an unregulated operating target. Each day an off allocation request will be made to bring the storage up to the unregulated operating target. Note this target may be a function.

OANs group together requests from downstream water users and other OANs and pass these requests on to upstream OANs. These requests may optionally be capped to an upper limit. This functionality is provided to share different sized events amongst multiple downstream reaches. For example small events may only be shared to the most upstream reaches and very large events might be shared with all downstream reaches.

When off allocation reaches a water user the water may be used for three purposes, in the following priority order:

1. avoiding shortfalls in meeting the current regulated requirement

2. offsetting use debit orders

3. supplying water user node opportunistic requirements of crops and filling on farm storages

Off-allocation flow event triggers and volume estimation

In general, off-allocation events are triggered when flows at a location exceed a set of conditions, but these may also be subject to constraints when extraction of flows will result in some other conditions not being met. For example, the following types of off-allocation flow access conditions exist across New South Wales valleys:

• river operators judging when flows exceed regulated and essential requirements (operator triggers)

• exceedance thresholds

• water account triggers

• storage volume triggers

• water quality constraints

• environmental constraints

• duration and time of year triggers
  
  
  

Flow event triggers are based on flows exceeding flow thresholds at an OAN. These thresholds may be fixed numbers or a function and different thresholds can be specified for commencing and ending the off allocation event. The event triggers may be optionally constrained by a seasonal period and a maximum flow threshold, which may be a fixed number or function. The threshold can be configured to be based on total flow (Figure 1) or flow above orders (Figure 2). In the case where triggers are based on flow, the larger of orders and event trigger is adopted as the threshold. The off allocation volume may be determined as flow above threshold (Option 2 and 3 in respective Figures 1 and 2) or flow above orders (Option 1 and 4 in respective Figures 1 and 2).

Note the thresholds are based on flow and consequently do not take ownership into consideration. This is dealt with when water is shared to downstream users. The off allocation volume is split according to the share of each owner above regulated requirements. Then shared to downstream water users according to sharing rules for the OAN in the OAS associated with the owned share of the off allocation volume.

Off-allocation system components

As stated in Overview, an off-allocation flow sharing system is described by:

1. Resource assessment system (RAS), which it may inherit ownership from

2. An off-allocation system, which is an accounting type within the RAS

3. Off-allocation node (OAN), which contains the water users that send requests for off allocation water

4. Accounts, which are the resource shares and associated usage of account holders/water users associated with the OAN

Figure 3 shows the relationships between the major components of an off-allocation system in a Source model.

For each RAS there can only be one OAS, each OAS can have one to many account types (there will be one account type for each OAN created in the scenario). An OAN can only have one account type per RAS but may have other account types as part of other RAS / OAS's (this allows for a single OAN to be used to model off-allocation flows for multiple owners at a single point in the river). For each account type there can be zero to many accounts, but each account has one account holder.

An account holder can be any of the following types of nodes downstream of the OAN which have a demand for off-allocation water:

• Water User Node,

• Bulk Licensing Node,

• Environmental Demand Node,

• Storage Node or

• Off-Allocation Node

The OAS is defined at three different levels:

1. system wide properties are defined at the off-allocation system level,

2. properties of each OAN are defined at the account type level, and

3. individual node/user settings are defined at the account level.

Note that although Ownership is not configured directly for an off-allocation system, off-allocation flow sharing is always used in conjunction with a resource assessment system. When off-allocation sharing is configured for a scenario, it must be linked to an associated resource assessment system by the modeller. If ownership has been configured for the linked resource assessment system then that owner’s share of flow at the OAN is subject to owner shares of the off allocation volume.

Methodology

Variables Used

Variables used are listed in Table2, excluding some temporary variables used locally in some equations; these are defined where they are used.

Table 2 — List of Variables Used