Environmental demand

Introduction

The Environmental Demand Model (EDM) is designed to simulate environmental flow requirements according to a set of rules. Environmental flow requirements may generally be classified as either in-stream (refer to In-stream requirement) or floodplain requirements (refer to Floodplain), where the former creates flow conditions that remain within the river channel, and the latter creates flow conditions that spill over bank. How these situations are configured in Source is largely dependent on the conceptualisation used to model the interaction between the channel and the floodplain.

In Source, environmental demand is modelled using the environmental demand node. Refer to Environmental Demand node for details on configuring the node.

Table 1 shows the Commonwealth Environmental Water Holder Framework, which determines environmental watering actions whereby watering options for a specific asset are a function of water availability.

Table 1. Commonwealth Environmental Water Holder Framework for determining environmental watering actions 2009

	Extreme dry	Dry	Median	Wet
Ecological watering objectives	Avoid damage to key environmental assets	Ensure ecological capacity for recovery	Maintain ecological health and resilience	Improve and extend healthy and resilient aquatic ecosystems
Management objectives	Avoid critical loss of threatened species Maintain key refuges Avoid irretrievable damage or catastrophic events	Support the survival and growth of threatened species and communities including limited small scale recruitment Maintain diverse habitats Maintain low flow river and floodplain functional processes in sites and reaches of priority assets	Enable growth, reproduction and small-scale recruitment for a diverse range of flora and fauna Promote low-lying floodplain-river connectivity Support medium flow river and floodplain functional processes	Enable growth, reproduction and large-scale recruitment for a diverse range of flora and fauna Promote higher floodplain-river connectivity Support high flow river and floodplain functional processes
Management actions	Water refugia and sites supporting threatened species and communities Undertake emergency watering at specific sites of priority assets Use carryover volumes to maintain critical needs	Water refugia and sites supporting threatened species and communities Provide low flow and freshes in sites and reaches of priority assets Use carryover volumes to maintain follow-up watering	Prolong flood/high-flow duration at key sites and reaches of priority assets Contribute to the full-range of in-channel flows Use carryover to provide optimal seasonal flow patterns in subsequent years	Increase flood/high-flow duration and extent across priority assets Contribute to the full range of flows including over-bank Use carryover to provide optimal seasonal flow patterns in subsequent years
Key goal	Damage avoidance	Capacity for recovery	Maintained health and resilience	Improved health and resilience

Consider a river with a historic flow regime dominated by high flows in late winter and early spring. Due to irrigation demands,the flow regime has changed to having high flows in late spring to summer. There is also a large reduction in flow during the rest of the year as a result of urban water requirements. In this case, environmental flow rules need to be incorporated to meet environmental demands.

Assumptions

The following assumptions are made when an EDM is configured in Source:

Water requirements are not additive - A basic assumption used in the EDM is that environmental water is not consumed and, as such, flow rule water requirements for more than one environmental asset can potentially use the same water in accounting for the success of their flow rules being met. For example the minimum flow requirement for fish passage is not necessarily a separate parcel of water from the minimum flow required to prevent an algal bloom;
Flow rules can be co-dependent: A flow rule can be conditionally contingent on another flow rule also being met;
Flow rules should only be attempted if their requirements are likely to be met: the EDM determines the daily demand, however before passing the demand for a day, the EDM checks to see if the total water required to complete the rule is available; and
The highest priority environmental water demand is for environmental flow rules which have commenced but not yet completed. If an environmental flow rule has started to be met, then the continuation of meeting this flow rule requirement has precedence over commencing water ordering to meet a new flow rule.

Flow rules

The EDM provides a means of capturing prescriptive descriptions of water patterns that the environment requires. These definitions of watering patterns are captured as ‘flow rules’ within the EDM and many combinations of flow rules can be prescribed. The four types of flow rules presented in the EDM have been designed to capture the most commonly defined environmental flow requirements specified in environmental flow studies and water regulations. These environmental demand rules allow you to construct a collective environmental water requirement by using combinations of environmental demand rules. These are:

Flood/Fresh - specifies a flood fresh, usually associated with a recruitment event such as to trigger fish movement, water floodplain vegetation;
Flow pattern - specifies a pattern of flow, used to define multi-peak events;
Minimum - specifies 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.

A combination of these rules can be used to meet specific environmental outcomes. They are configured in the environmental demand node's feature editor. For details on configuring each of these rules in Source, refer to Environmental Demand node.

For each flow rule in the EDM, you can configure several flow characteristics that can be altered. A common feature across the flow rule types is the concept that they can be applied to a specific time of the year. This is termed the ‘season’ of the rule and is defined by a start and end day and month value.

Rule sets and priorities

Flow rules can be grouped and prioritised. The first (default) group allows consideration of every rule in the default group for every day of the simulation. For all other groups of rules, only one rule from the group is considered at any time-step. For any group of rules, the rules are prioritised according to the order in which they appear in Source.

Rules can be made active or inactive during a modelling run. In order to control rules throughout a model run, each rule can have a defined condition threshold. This condition threshold is compared to a ‘condition’ time series (which is entered using the Function Editor)[for each time step and the flow rule turned on or off for that time step as required. This functionality is specifically designed to allow the construction of asset based rule sets that vary according to water availability.

Rules can be made active or inactive during a modelling run using Disable in the rule's contextual menu.

Forecasting

The basic approach to determining the water ordered for environmental demand is to consider the collection of flow rules and determine how much water is required for a given day to meet these rules. As water is not consumed by the EDM, the water order is simply the difference between the calculated water required and the forecast flow at the extraction point on the future delivery day.The EDM allows two forecasting periods, a short term and a medium term. This approach is to reflect real world water management decision making, whereby, depending on the size of the catchment, river operators can generally make a very accurate prediction of the likely flow for the next few days to weeks. After that time the prediction is less accurate. In this way, the EDM allows you to define the ‘look ahead’ period, which is the period that can be accurately forecasted.