Environmental demand

Introduction

The Environmental Demand Model (EDM) in Source operates on a daily basis generating demands and extracting water to meet these demands using the environmental demand and supply point node. The model can be applied in both regulated and unregulated systems.

Table 1 shows the different objectives that can be achieved using the EDM using a set of flow rules. These rules can be grouped and prioritised to ensure that the required environmental needs are met. The Commonwealth Environmental Water Holder Framework (shown in Table 1), which determines environmental watering actions whereby watering options for a specific asset are a function of water availability.

	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 deiverse 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

One example where you would use environmental demand is say, for example, a river with a flow regime that used to be dominated by high flows in late winter and early spring. Due to irrigation demands, the flow is now high in late spring – summer. For the rest of the year there has been a large reduction in flow as the city water supply is extracted at the reservoir and flows via pipe to the city. In this case, environmental flow rules need to be incorporated, so that the environmental demands can be met, even in periods where flows are low.

Assumptions

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

Water requirements are not additive: environmental water is not consumed and as such every individual flow rule water requirement can use the same water in accounting for the success of a flow rule being met;
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 this day, the EDM checks to see if the total water required to complete the rule is available; and
The highest priority water demand is for flow rules which have commenced but not yet completed. If a 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 for a single supply point. 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.

These rules are configured in the environmental demand node's feature editor. For details on what to configure each of these rules in Source, refer to Environmental Demand node.

To combine the flow rules into a collective environmental flow requirement, rules can be grouped so that they are either always considered, or considered as a subset of the flow requirements. You can define the priority order of rules, and where there is a conflict in being able to meet the water demand, ( ie. insufficient availability of water) the additional water demands for the lowest priority rule is removed until the demand matches, or is less than the available water.

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 order 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. The EDM allows you to define the ‘look ahead’ period, which is the period that can be accurately forecasted. You can use the Expression Editor to define the method of forecasting. The recommended methods for determining the forecast period are to refer to an upstream node with a known travel time.