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• irrigating crops;,
• urban, industrial and rural stock and domestic use; , and
• managing environmental water entitlements, which may or may not result in an extraction from the river.

The Water User node provides functionality needed in Source to be able to model water use. The functions performed by the Water User node include managing orders and extractions (either with or without an accounting system), setting priorities to determine the sources used for extraction and directing the return of surplus water to rivers, groundwater (by infiltration) and water user storages.

A water user is modelled as comprising three components: a water distributor, a demand model and (in some cases) one or more water user storage units. Of these, the water distributor and the water user storage components are integral parts of the Water User node but the demand model is separate. These three components exchange information as illustrated in Figure 1.

The water distributor component manages orders, extractions and returns. It manages the distribution and carries out its function through an exchange of information with the other two components;

The demand model can be chosen by the modeller from one of the options available in Source. Alternatively, new demand models can be developed and linked to the water distributor using the ‘plug-in’ concept; and

The water user storage component is used to simulate an off-river storage associated with a given water user. It can represent

;

• an on-farm storage (OFS) used

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  used for irrigation;
• an off-river storage used for urban demands; or
• an off-river storage used to meet environmental demands.

At least one Supply Point node is also required to determine where water is to be extracted. Extractions may be from a regulated river, from an unregulated river or from groundwater. The Supply Point node is also used to specify a number of other factors, including delivery efficiency, travel time, pumping capacity and the flow rate above which overbank flows occur. More details are available in the Supply point node - SRG entry.

The Water User node provides the same input to both the rules-based and optimisation-based ordering schemes available in Source. Consequently, in the model run sequence, the processing associated with the water user occurs before the ordering phase starts.

Return Flows

The Return Flow module enables the user to specify:

• Return flow properties, such as constituent concentrations, at the point where the flow leaves the water user
• Return flow processes, such as Linear Storage Routing, that occur after the flow leaves the water user

Linear Storage Routing

Conceptually, Linear Storage Routing describes overland flow processes; it is intended to model return flow from the point where it leaves the water user to the point where it re-joins the river system. 

The Linear Storage Routing model routes return flow through two parallel linear-stores:

• The first store represents quickflow, the proportion of return flow that becomes surface runoff, also known as direct runoff
• The second store represents slowflow, the proportion of return flow that becomes shallow-subsurface flow

The total return flow that reaches the river system is the sum of quickflow and slowflow.

The Linear Storage Routing equations are the same as for the Linear Routing Module described in the IHACRES-CMD - SRG, except that effective rainfall U is replaced with return flow at the point it leaves the water user.

Demand Constraints

Demand constraints allow the user to restrict demand at a water user node based on either:

• Limit curve – prevents a demand node from using all of its available allocation too early in the season; 
• Usage limit – limits account usage for a specified time period (eg. water year).

Limit Curves

Limit curves are designed for annual accounting resource assessment systems. Limit curves are based on the following user-defined parameters:

• Allocation percentage –  the available allocation in relation to the total entitlement for the accounts associated with this water user. If there are n account types, then each account receiving 100% of its entitlement will be expressed as an allocation percentage of  n × 100%. An allocation percentage of 150% indicates that the highest reliability/security account has been allocated 100% of its entiItlement, and the next highest reliability has been allocated 50% of its entitlement. It can be defined as a constant, a function, or based on each account associated with this water user.
• Limit  – the maximum total water delivery for the irrigation season. How Limit varies with available Allocation percentage is specified by the user as a piecewise linear relationship.
• Fraction – represents a  proportion (0 – 1) that determines the coefficients a, b and c of the limit curve equation (Equation 1); 
• Irrigation End – the date corresponding to the end of the irrigation season.
• Water Year Start – the date corresponding to the start of the water year.
• Desired Carryover – expressed as a percentage of the account host's total allocation for the water year, this is the volume of unused water to be carried over at the end of the irrigation season to the next season. It is defined as piecewise linear relationship with allocation percentage. Configuring this parameter is optional.

If Function is enabled:

• Allocation Percentage is either the user-defined constant or value calculated from the user-defined function. 
• Limit (ML) for the current time step's Allocation Percentage is found using the piecewise linear relationship between allocation percentage and limit specified by the user.
• Limit is then used to calculate cumulative demand (Equation 1).

If From Accounts, is enabled:

• Allocation Percentage is the sum of current allocations (%) for each account associated with the water user.
• Unadjusted limit (ML) for the current Alllocation Percentage value is calculated from the piecewise linear relationship between allocation percentage and limit.
• Carryover and Spillable water is accounted for as follows:
  • If carryover is non-spillable, by adding Carryover Start of Water Year for each associated account to Unadjusted Limit.
  • If carryover is spillable, and spillable water is available, by adding Carryover Start of Water Year and deducting Spill Reduction Water Year for each associated account to Unadjusted Limit; or
  • If carryover is spillable, and spillable water is not available, by adding Carryover Start of Water Year and deducting both Spill Reduction Water Year and Spillable Water for each associated account to Unadjusted Limit.
• If a piecewise linear relationship between Allocation and Desired Carryover has been defined, then:
  • Effective Allocation Percentage for the current Adjusted Limit value is calculated from the piecewise linear relationship between allocation percentage and limit.
  • Desired Carryover (%) for the current Effective Allocation Percentage value is calculated from the piecewise linear relationship between allocation and desired carryover.
  • Desired Carryover volume (ML) = Desired carryover (%) × total account entitlement for the account host (ML).
  • Finally, Limit (ML) = Max(0, Adjusted Limit − Desired Carryover volume).
• If Desired Carryover has not been defined, then Limit = Max (0, Adjusted limit).
• Limit is then used to calculate cumulative demand (Equation 1).

The cumulative demand (Y) is calculated using the limit curve, Equation 1.

Where:

X is the number of timesteps between the current time step and the irrigation end date and a, b and c are defined in Equations 2, 3 and 4. 

You can record results for:

• Limit, the limit calculated using the Allocation percentage to Limit relationship.
• Cumulative Demand, result from the limit curve equation (Equation 1).
• Accumulated Supply,  the total volume of water supplied to the water user node since the start of water year. It is reset to 0 at the start of each water year. 
• Leftover Cumulative Demand, Accumulated Supply − Cumulative Demand. 

Scale

Point scale, any time-step except that the choice of time-step may be constrained by the demand model time-step, such as when used in conjunction with the IQQM Crop Model (see IQQM Crop Model SRG) which is daily time-step only.

Principal developer

eWater CRC.

Scientific provenance

The Water User node is purpose-built for Source. It is based not so much on scientific principles as on distribution rules that can be independently verified.

Version

Source version 2.17

Dependencies

The Water User node must always be connected to at least one Supply Point node (it may be connected to many). It also needs to be connected to a demand model or data.

Data

Details on data are provided in the Obsolete - Source User Guide.