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The functionality provided by the environmental flow node in Source is designed to model existing and proposed water sharing arrangements that include environmental flow rules.

Source models the use of water by a combination of supply point and water user nodes. The environmental flow node (EFN) operates on a time-step basis generating demands for the environment. 

The EFN contains two types of environmental actions and allows modellers to construct an environmental water requirement by using combinations of these, rules can be grouped if required. The two types are:

• Spell based - A spell can be defined as a period of time that the flow has a set of desired characteristics. The environmental flow node can only currently create high flow events i.e. flows over a specified threshold. This type of action can by used to specify a flood freshfloods, freshes (usually associated with a recruitment event such as to trigger fish movement, water floodplain vegetation;), a flow pattern or a minimum flow , (usually applied to maintain minimum habitat requirements); and
• Translucency - specifies the flow requirements in terms of some other time series, usually the release from a dam based on the inflow of the dam. Translucency can also be configured to represent minimum flows

Common concepts to both types of actions are- 

• Season: the period of the year over which the rule should be considered (eg is this a winter flooding rule only);
• Reference Flow: A representation of flow, often higher in the system or an earlier recorded 'natural' flow pattern, that can have various relationships configured in relation to it;
• Start Threshold: Required expected flow for action to be triggered. The expected flow at the environmental flow node is the maximum of the Minimum Constraint at the upstream node and the Order at the downstream node.
• End Threshold: A flow threshold below which the flow should fall to consider the action finished. An action or ordering period will end either when the end threshold is reached or the season has ended.

Scale

The Environmental Flow Node is applied at a point scale and operates on a daily time-step.

Murray Darling Basin Authority and eWater, with assistance from

• Office of Environment and Heritage, NSW,
• Commonwealth Environmental Water Holder,
• Department of Environment, Land, Water and Planning, Victoria, 
• Melbourne Water
• Department of Primary Industry, NSW
• Victorian Environmental Water Holder
• Department of Environment, Water and Natural Resources, SA

Provenance

The Environmental Flow node functionality in Source builds on the eFlow Predictor software tool developed by eWater CRC in 2010. The underlying computational code was written to work within the Source environmentwas conceptualised in consultation with the environmental water management organisations associated with the Murray Darling Basin in 2017.

Version

Source v4.3

Dependencies

The Environmental Flow Node is applied as a stand-alone node, which must be connected to a surface water system. To link environmental orders to accounts, or to prioritise actions across the system, an environmental flow manager and resource assessment system is required. 

Assumptions

• • Water requirements are not additive: Increased river flows and return flows can contribute to other actions (return flows can be re-credited as per normal accounting rules) . 
  •  Actions can be co-dependent: A flow rule can be conditionally contingent on another flow rule also being met. This has to be configured through the importance and cost functions.
  • The highest priority water demand is for flow rules which have commenced but not yet completed. If a flow action has started, then the continuation this flow action has precedence over activation of a new flow rule.

Theory

The Environmental Flow Node is used to generate orders to meet in-stream environmental requirements at an individual asset. It can also be used to monitor defined event patterns. The rest of this document assumes you are intending to place orders based on event outcomes. The desired flow patterns are defined by configuring one or more actions including start criteria, desired flow rate and frequency, as well as criteria for success of the action. 

The Environmental Flow Node (EFN) provides a way of capturing prescriptive descriptions of the patterns of water that the environment requires. These definitions of watering patterns are captured as ‘actions’ within the EFN, many combinations of actions can be prescribed for a single EFN. An action is a combination of a definition of the desired flow, a   The two types of actions presented in the EFN have been designed to capture the most commonly defined environmental flow requirements specified in environmental flow studies and water regulations. These two types of environmental actions allow users to construct a collective environmental water requirement by using combinations of actions. The two action types are:

• Spell-based, which includes:
  • Baseflow: specifies a minimum flow; usually applied to maintain minimum habitat requirements
    
  • High Flow/Flood: specifies a flood fresh; usually associated with a recruitment event such as to trigger fish movement, water floodplain vegetation, moving sediment deposits
  • Pattern: specifies a pattern of flow; used to define multi-peak events.
    
• Translucency: specifies the flow requirements in terms of some other time series, usually the release from a dam related to the natural inflow to the dam. Translucency is effectively a simplified version of the spell based action. 

Spell Based Action

A High flow/Flood action defines a flow requirement above a set level. The parameters for defining a flood/fresh flow component are illustrated in Figure 4 and described below.

Figure 4. Illustration of parameters used to describe a High Flow / Flood action

A spell can be defined as a period of time that the flow has a set of desired characteristics. The environmental flow node can only currently create high flow events i.e. flows over a specified threshold. 

A high flow spell is defined by the period of time the flow is above the target. In figure 4, (7) represents the target flow rate, and (2) represents the length of a spell.    

A season (1) is the period of time within a water year, that you desire spells to occur. Users can specify characteristics to determine the occurrence of spells desired to complete a successful season within a water year, including the number of spells, minimum independence interval (4) and desired successful season frequency (return interval). 

Once you have specified what characteristic you want, the next step is to specify the intervention you are going to take to achieve the desired outcomes defined above. The start threshold defines the point after which an order may be used to augment the flow. The order is based on the rise (5) and fall (6) characteristics, the target flow (7) and minimum duration required (3). Intervening to create a successful spell also includes the ability to extend beyond the minimum duration, by way of a Boolean function, and criteria to indicate when to stop intervening with spell end criteria based on duration below a flow threshold. 

Generate of demands and orders for actions

The environmental flow node will generate demands and orders for actions. An Environmental Flow Node can be assigned to an Environmental Flow Manager or can operate in isolation. In case the actions are managed by an EFM, the EFN will only order actions that are flagged by the EFM as active. If no EFM is used, the EFN will activate actions depending on their desired return interval. In both cases, the EFN will try to piggy bag on flows based on a user-defined flow threshold. That is, an action will still calculate the demand for an action even if the EFM has disabled the action. This is to allow for natural spells to be validated. A user can also determine if and when an action should be forced. If a flow threshold for piggy bagging is not reached and an action is able to be forced, the EFN will start the action at the last possible day of the season.

Evaluate success and condition of action events

The success of an event is calculated by three user-configurable properties:

1. Proportion of target
2. Proportion of duration
3. Proportion of volume

The user can specify a condition function, but by default, the condition is calculated as

EXP(-time since last successful / average return interval).

Rise and Fall Calculations:

There are 3 methods provided to calculate the rise and fall duration and demand volumes:

• Days
• Rate
• Percentage

Each method simply calculates a duration and a demand based on two flows values. In the case of rise, these flow values are the expected flow and the target volume on the first timestep the spell was in the Rise Phase state. The expected flow will be either minimum constraint or downstream order depending on what triggered the spell (i.e if triggered by an upstream constraint, the expected flow will be the upstream constraint). In the case of fall, the two flow values are the target flow and the Fall Target.

Days

The Days method simply calculates a linear increase (or decrease for fall) between two flows

Rise:

Duration =  day value or zero if the expected flow >= target flow.

let increment = (target flow - expected flow) / days

Demand = Min(target flow, expected flow +  increment * days in rise phase)

Fall :

Duration = day value or zero if the target flow <= Fall Target

let decrement = (target flow -Fall Target) / days

Demand = Min(target flow, expected flow  - decrement * days in fall phase)

Rate

Rate method increments at a constant rate regardless of the initial expected flow.  

Rise:

Duration =  (target flow - expected flow)/ Rise Rate or zero if the expected flow >= target flow.

Demand = Min(target flow, expected flow +  Rise Rate * days in rise phase)

Fall :

Duration =  (target flow - expected flow)/ Fall Rate or zero if the target flow <= Fall Target

Demand = Min(target flow, expected flow -  Fall Rate * days in fall phase)

Percentage

The percentage method calculates increases or decreases the demand by a constant percentage every day.

Rise:

Duration =  day value or zero if the expected flow >= target flow.

Demand = expected flow * (1 + increment) on the first day

                 last timesteps demand * (1 + increment) on the following days.

Fall :

Duration = day value or zero if the target flow <= Fall Target

Demand = expected flow * (1 - decrement) on the first day

                 last timesteps demand * (1 - decrement) on the following days.