Environmental Flow Node - SRG

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 has been designed to deliver additional (environmental) flow i.e. flows over a specified threshold. Limits to flows may be set up elsewhere in the Source functionality. This type of action can by used to specify floods, 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.

Provenance

The Environmental Flow node functionality was conceptualised in extensive 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 that will supplement existing flows to create specific patterns of flow events. 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 rate, duration, frequency and timing of beneficial flow events. 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.

A high flow spell is defined by the period of time the flow is above the target. In figure 4, (8) 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 (7) 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 (8) and minimum duration required (2,3). It is possible to extend the Minimum Spell Duration by way of a Boolean function. This allows variable spell duration's to be defined to represent various environmental flow requirements throughout the year.

Note:

• Previously, the Fall Phase is within the season. However, Source now allows the Fall phase to extend its boundary beyond the end of the season, and the method is for both Rule Based Order and for NetLP ordering.

• If Spell Phase cannot complete its ordering within the season, Spell is not triggered.

• When leading up to the start of a Season, and travel time exists, ordering may happen before the start of a season so that the water arrives within the Season.

Generate demands and orders for actions

The environmental flow node will generate demands and orders for actions. An Environmental Flow Node (EFN) can be assigned to an Environmental Flow Manager (EFM) 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 back on flows based on a user-defined flow threshold. That is, an action will still calculate the demand for that 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 backing 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).

Antecedent Condition

with Average Recurrence Interval being calculated from the Desired Frequency:

For example, desired frequency is 2 in 3 years, the Average Return Interval is 365 / (2/3) = 547.5 (days).

The Antecedent Condition equation returns a value between 0 and 1 (0 is poor condition and 1 is good condition). The user-defined function is not restricted to return a value between 0 and 1, however, it is strongly recommended to consider the implications before changing the output range.

Antecedent Condition is used by the EFM to determine the priority of an action. 

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 flow 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

Environmental Flow Requirement = 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

Environmental Flow Requirement = 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.

(expressed in  the code or a function: Duration = Math.Max(0, Math.Round(target flow - expected flow)/ Rise Rate)) )

Environmental Flow Requirement = 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

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

Percentage

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

Rise:

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

Environmental Flow Requirement = 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.

Translucency Action

The logic around decisions to create demand associated with a translucency action is as follows

1. Are we in the specified season?
2. Are we in event?

The action is 'in event' if Reference flow > Start threshold 

Any time the reference flow < end threshold, the event is ended. 

Target flow is reference flow * % translucency.

Where:
reference flow = the flow defined by a function (such as reservoir inflow) 
Water demand(t) = min(Target flow - ForecastFlow(t), available water)

Note. Where a Translucency action uses forcing environmental water rather than held environmental water, the action should not be associated with a flow manager.

Translucency Reference Flow 

A translucency reference flow is often used to represent an inflow to a storage. The translucency action is configured below the storage, too allow for flow patterns to be replicated downstream. The assumption is that the reference flow is occurring at the closest storage (ie minimum travel time).  

Evaluate success and condition for translucency action events

Success is defined by the total volume delivered over the demand period (based on required flow duration and rate) and is expressed as a percentage.Translucency action success is fixed at 100% for every time step, and over the event 100% of timesteps much be succesful. This sets the time since last successful event For example, if the threshold is set at 80%,andeventthatdelivered80% success or more is flagged as successful. This allows for flexibility in the definition of success. Success will be calculated as the model runs and days since last success will be recorded for use in the calculation of condition.

The user can specify a condition function, but by default, the days since last success divided by the desired return interval will be used as a measure of condition. 

Life cycle of a spell

There are 9 states a spell can be in:

1. Dormant
2. Waiting for trigger
3. Rise Phase
4. Main spell phase
5. Finished ordering
6. Fall phase
7. Complete
8. Interrupted
9. Forcing
   
   

Dormant

Constraint Phase

Ordering Phase

Flow Phase

Waiting for trigger

When a spell is in the Waiting for trigger phase, it is waiting for the user-defined start threshold to be met. It should be noted that a spell only considers constraints and orders external to the node for triggering. This means that spells cannot be triggered by other spells within the node.

Constraint Phase

Ordering Phase

Flow Phase

Rise Phase

During the rise phase, orders are placed to try to transition between the current flow at the node and the target volume. If there is not rise defined, then the spell will transition into the main spell phase.

Regardless of what rise method is used, a rise phase can be broken down into how many timesteps are required, and what volume is required to order. The spell will transition to the Main Spell Phase once this spell has been in this state for the calculated number of timesteps. It does not matter if the rise phase orders are successful or not.

Constraint Phase

Ordering Phase

Flow Phase

Main Spell Phase

During this phase, demands are calculated to be equal to the target value. Demand values will be generated for at least the minimum spell duration.  If the extend spell option is used, demands will continue to be placed until the value evaluates to false.

Constraint Phase

Ordering Phase

Flow Phase

Ordering

Without a flow manager:

The order placed in the system without a flow manager depends on min upstream constraint or downstream orders.

If the minimum constraint is greater than the action with the maximum demand then the order placed in the system will be zero.

Otherwise, the order will be the maximum demand minus the downstream order.

With a flow manager:

The order placed in the system with a flow manager depends on min upstream constraint or downstream orders and other actions at the node with a higher priority (specified in the flow manager).

If the minimum constraint is greater than the action with the maximum demand then the order placed in the system will be zero.

Otherwise, the order will be the maximum demand minus the downstream order and the order of any other action with a higher priority.

This ensures that actions in higher priority groups will debit their accounts before lower priority actions and that the total order does not exceed the action with the largest demand.

Input Data

Refer to the Source User Guide for detailed data requirements and formats.

Output data

The model outputs daily surface and base flow. This may be saved in ML/d, m³/s or mm/d. See environmental flow recorders for r\descriptions