8 Links between the significant ecological assets
8.1 Introduction
In its November 2003 Communiqué, the Murray-Darling Basin Ministerial Council's first of four stated high-level principles for The Living Murray is that `Action will be taken to achieve a healthy working river' (MDBMC, 2003b). This chapter deals with the issue of complementarity between the needs of ecological assets along the River Murray. Other documents (Jones, et al. 2002; SRP, 2003; Gippel, 2003) provide supporting information. It should be emphasised that detailed work on optimising the combinations of actions undertaken at the six significant ecological assets (SEAs) (perhaps to achieve the greatest synergy in ecological benefits) has not been undertaken. Concepts and details of the integration of management of the SEAs by water delivery are likely to evolve in the near future as The Living Murray aims to progress the areas of water accounting, modelling the ecological benefits of various flow regimes, and modelling rostering scenarios.
8.2 Distinctive features of each asset
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enlarged imageThe six identified SEAs have distinctive characteristics that are relevant to the way the hydrology is best managed in order to achieve the expected benefits; these are summarised in Table 8.1. Some environmental assets, such as the Murray Mouth, need low flows for extended periods. Some, such as the Chowilla Floodplain, need very high flows in spring every few years. Flooding at other times of year can be detrimental (as is the case with unseasonal `rain rejection' releases into the Barmah-Millewa Forest).
The environmental demand for increased flows at the three most upstream SEAs is in relatively wet years, particularly during spring. Despite the needs of these SEAs appearing to be aligned, the reality is more complex, due to factors such as changes in flows in the long distances between ecological assets (including attenuation), and the differences in `commence to flow' levels (Table 8.1).
Due to the significant reduction of flows in the lower reaches, increased flows to the Lower Lakes and Coorong at virtually any time of year would also yield environmental benefits. To highlight the disparity of flow needs of some different ecological attributes, some indicative thresholds corresponding to the maximum number of years between effective waterings before the health of an attribute declines, and the magnitude of the event required for watering, are illustrated on Figure 8.1. These are examples, with the specifications drawn from the information provided in the previous chapters. The use of these examples does not imply that these should be management targets under the First Step Decision. The maximum period between events is based on historical data.
Table 8.1 - Distinctive features of the significant ecological assets
8.3 Rostering to maximise and share ecological benefits between assets
The problem of how to share water throughout the River Murray system cannot be solved through a simple optimisation that always delivers flows where the average benefit is estimated by modelling to be greatest. Rather, the flows need to be shared, or rostered, over an appropriate management time frame, because ecological benefit varies according to many factors, including antecedent conditions (e.g., time since last natural watering, or since last environmental allocation).
Out-of-channel floods have the added benefit over environment water allocations that are delivered entirely within the channel of being able to achieve targets at multiple SEAs. For example, the 2000-01 flood along the River Murray was managed to achieve environmental responses in the Barmah-Millewa Forest, and downstream of the Darling River junction (this flood was enhanced by river operators). Ecological benefits also occurred in the Gunbower and Koondrook-Perricoota forests. The Lower Lakes and Coorong also benefited from the increased flows. Wet years present considerable opportunities to achieve multiple ecological gains.
Floodplain wetlands require regular watering; there is a maximum period of time between watering, which, if exceeded, leads to a reduction of ecological health. Based on modelling, under current conditions each of the maximum periods between effective watering would be exceeded at least several times per century, leading to a reduction of the health of each SEA. Providing that past hydrology is a good guide to the future, 500 GL of new environmental water, allocated to the River Murray each year on average accompanied with appropriate structural and operational measures as proposed in the Environmental Works and Measures Program can significantly reduce the incidence of these detrimental spells. This conclusion must be tempered by concerns that future flows in the River Murray may not be as high as in the past, for various reasons, including climate change and land use change in the upper catchment (Jones, R. et al., 2002) (see Box 8.1).
It is appropriate that environmental managers have flexibility to target the SEAs (and the elements of these assets) where the greatest benefit can be produced at that time, given the availability and location of environmental water allocations in the river system, and taking into account the history of delivery of these allocations between the assets. Any management of environmental water to SEAs should consider the ecological condition of sites.
Not all water allocated to an SEA is `consumed' there. `Losses' of water in wetlands has not been analysed rigorously in the past using hydrological models, as inundation of wetlands usually occurs during periods of surplus flows when these losses are not significant from a water conservation perspective.
In extended periods of dry conditions, emergency measures (such as dredging of the Mouth, and pumping of water to wetlands or areas of the Chowilla Floodplain and Lindsay-Wallpolla Islands) may be required to maintain the viability of SEAs until wetter climatic sequences recur. It is important also to note that the impact of a severe drought may take a greater toll on some components of SEAs under current conditions compared to natural conditions, as their resilience is reduced by flow changes and other pressures.
In addition to flows in spring in wet years, the Murray Mouth requires a baseflow to be maintained year round to reduce risk of closure and maintain the health of the Coorong, particularly during spring, summer and autumn. Use of such environmental water allocations may result in negative ecological effects at other locations:
Using environmental water allocations to achieve outcomes at some SEAs may result in reduced opportunities to achieve outcomes at others (e.g., there is an `ecological opportunity cost').
The Scientific Reference Panel (SRP, 2003) raised the idea of `rostering' the quantity of environmental water allocations available through time between environmental assets. The rostering would depend on both:
• the environmental demands (flow requirements to meet ecological objectives, time since effective watering); and
• supply (how much water is in environmental accounts and where it is located, how much unregulated flow is in tributaries).
The roster would have to be sufficiently flexible to cater for environmental `emergencies' (unexpected acute events such as a salinity spike that may require an allocation to avoid environmental damage), or unexpected opportunities that, with addition of some additional water, can be managed to deliver large environmental benefits.