Supply point node - SRG

At various locations in a river system, whether regulated or unregulated, there can be water demands which could be either volume based or water level based. These demands can be either extractive or ‘in-stream.' Water taken from the river for irrigation, town water, industrial and other uses is an extractive requirement. ‘In-stream’ demands include environmental, recreational and regulatory requirements, and perhaps hydro power.

In Source a combination of demand models, the water user node and the supply point node are used to model the generation and meeting of these demands. Demands, both extractive and in-stream, are generated by demand models. The water user node is used as an interface for water demand models, and it also has one or more supply point nodes associated with it. The primary function of the supply point node is to identify a location at which water is to be delivered, and model aspects of the delivery of water at that point. The water user node distributes demand between supply point nodes associated with it and, in models of regulated systems, generates orders at each supply point node to meet these demands and uses accounts to track and limit these orders.

If the system is regulated, the supply point node can place water orders on storages and determine location dependent features, such as delivery efficiency and travel time. In unregulated systems, supply points cannot place orders, but licences limit the volume they extract, represented by an account balance. Supply point nodes can be ‘extractive’ or ‘in-stream’.

The supply point may also be used to model supply of water from groundwater. Groundwater supply points do not order (and are therefore always classed as unregulated) but accounts may limit extractions. Groundwater supply points are always extractive.

This section describes how the supply point node is used in modelling the delivery of water to a location to meet demand generated at a water user node, and support water use accounting.

Scale

Point scale; calculations are updated at every model time-step.

Principal developer

eWater

Scientific provenance

The supply point node representation in Source builds on concepts in predecessor models, IQQM, MSM and REALM, and experiences with using these.

Version

Source version 4.5

Dependencies

Regulated or unregulated systems in Source can use the supply point. A supply point needs a water user node associated with it.

Context

The supply point node is the point at which the water management functionality in Source connects with a particular location in the river network. Specifically, it is the point where:

• Location dependent parameters, such as estimated order delivery time and operation efficiency are either calculated or specified;
• Orders created by the connected water user node enter the water ordering system;
• The water ordering system communicates constraints on delivery back to the water user node;
• In-bank and overbank ‘deliveries’ (flow rate/volume) for a time-step are recorded at the supply point. Water users receive information (In-bank ‘Available flow’, ‘Overbank flow’)  from the supply point on the amount that can be extracted. 
• ‘Extractive’ supply points: Extractions to meet current water user demand are calculated (subject to availability and other constraints, as mentioned above);
• ‘In-stream’ supply points: The amount of in-stream ‘usage’ to deduct from accounts is determined and communicated to the water user node;
• Unused (not extracted) ordered water becomes unallocated (i.e., available for other uses). The volume of ‘unallocated water’ is updated.

Assumptions and constraints

Refer to Table 1.

Types of water source

A supply point may source water from a river or groundwater. Supply point options are described in the user guide: Supply point node.

Demand distribution

In any time-step, the water user’s demand model generates a future minimum requirement and a future ‘opportunistic’ requirement. The water user’s demand delivery component makes orders for the minimum requirement and off-allocation requests for the opportunistic requirement. It also generates a minimum and opportunistic requirement for the current time-step, and these amounts may differ from those predicted earlier. The modeller specifies the rules as to how these demands are to be distributed when they are configuring the model.

Resource assessment systems (RAS) may be used to manage sources of water (both from the river and groundwater). Every RAS is associated with a water owner. In a scenario with no ownership specified, all RAS uses the ‘not specified’ owner. RAS can manage both regulated and unregulated water sources. A regulated RAS always manages off-allocation water via off-allocation account types.

Where one or more RASs is configured for the modelling scenario, the water user node can be configured to associate a supply point’s orders and extractions with one or more resource allocation accounts, known as ‘Account Sharing’. When this type of demand distribution is used, the modeller specifies the priority of accounts to be used, via the RAS or directly at the water user node. Accounts (and hence demand) are associated with an owner via the RAS. The volume of orders and off allocation requests that can be made at the supply point in any time-step is limited by the balances of its accounts at that time-step. A supply point may be associated with multiple accounts but at most one off-allocation ‘account’ per owner.

The other method of demand distribution is ‘Non-Account Sharing’. When this is selected, the water user’s demand is distributed to each owner at each connected supply point node either (a) on a proportional basis, or (b) using fixed proportions specified for each owner by the modeller.

Order supply path

The account or owner will determine the water source and delivery path used for the order at the supply point.

• Account sharing water users: Orders can be provided from any storage associated with the ordering account’s RAS. The order system determines where to source water from according to storage levels and other constraints at each time-step during the model run. Off allocation requests are always supplied from the off-allocation sharing (OAS) nodes associated with the supply point’s off-allocation accounts (if it has any).
• Non-account sharing water users: Any storage the ordering owner has a share of water and outlet capacity in can supply orders.

When there are multiple supply path options, the water ordering system determines where to source water from according to storage levels and other constraints at each time-step during the model run. For more information on how the water ordering system directs orders to storages:

• Refer toRules Based Ordering; or
• Refer to NetLP - SRG for this type of ordering.

Updating Account balance for Water 'Use'

When account sharing is used at a water user node, the node has accounts that are debited for water ‘usage’. This usage may be for water extraction or delivery at its supply point(s), depending on whether the use is extractive or not. The timing of debiting an account and the amount debited depends on whether its RAS type is ‘order debit’ or ‘use debit’, and on whether the associated supply point is extractive or not. Note that a groundwater or unregulated river supply point must be extractive and, if it is associated with an account, the account is use-debit.

The method of account balance update is outlined in Table 26 below.

Table 3. Updating account balances at supply points

The rationale for allowing the modeller to define an expression for account debiting at in-stream supply points is that these have differing applications. In-stream supply points can be used to place orders to ensure flow for a downstream location, such as a wetland, so account debiting should not necessarily occur at the time the flow arrives at the supply point but perhaps instead when the water arrives at the downstream location. There may also be variations to how accounts are debited - e.g.,. in the case of bulk water entitlements.

Input data

Details on data are provided in the Source User Guide for the Supply point node and in the ownership section Ownership at nodes and links.

Some output that may be displayed in the form of graphs, tables and statistics for the variables listed in Table 6.