Optimizing environmental flow rules – a conceptual model

Multi-objective optimization (MOO) is an effective and efficient method which can identify the trade-offs between different river system management options. However, a number of barriers remain between using optimization in research, and implementing it in practice. To address such limitations, this study developed a simplified conceptual model consisting of hydrologic, ecological and optimization components to explore the trade-offs between ecological and human water needs. A hypothetical catchment was used as case study, which drew on data from the Lachlan catchment in the Murray Darling Basin, Australia. The Lachlan River is highly regulated, and supports a range of conflicting water users including agriculture, urban areas and wetlands of high conservation significance. In order to capture some of the major components of the Lachlan catchment, the hypothetical case study incorporated two major headwater storages; water users representing a spatial distribution of demands; and a 16,000 ha terminal wetland, represented in terms of wetland vegetation response (using River Red Gum (Eucalyptus camaldulensis) as an indicator species). The influence of groundwater/surface water (GW/SW) interactions on ecosystem response and optimization outcomes was examined for dry versus wet periods. Decision variables in the optimization procedure were based on environmental flow rules and licence volumes for other water users, to explore which sets of rules achieved optimal outcomes. The objective functions consisted of maximising an ecological health score for the terminal wetland, and maximising the licence value and hence the water delivered to irrigators. A simple ecosystem response model of the wetland was developed for the purpose of this study, and consisted of a set of relationships between flow and a health score for River Red Gum. The methodology developed demonstrates the feasibility and effectiveness of using MOO as a transparent process to explore the trade-offs between different objectives for river system planning. The conceptual model incorporates the major system complexities of conflicting water users, multiple supply storages, and GW/SW interactions. This model is therefore adaptable to an actual representation of the Lachlan catchment, as well as to other areas. These outcomes aid in bridging the gap between use of optimization for water resource decision-making in research and in practice.