Regulating the flow is more complicated than it looks

The proposed Murray-Darling Basin Plan has been one of the most controversial pieces of public policy in Australia’s recent history. There has been the predictable divide between irrigators calling for more water to be extracted from the river, and environmentalists, who say too much is coming out already. But between the two, many experts are looking at the nuances of the plan and saying it’s a lot more complex than farming versus nature.

This week, researchers around the Basin will give us their view of how their local area has fared in recent years and tell us whether the proposed plan will make things better or worse. Today, Susan Lawler, Head of the Department of Environmental Management & Ecology at La Trobe University, investigates the intricacies of river flows and finds “how much?” isn’t the most important question.

I live in Albury-Wodonga just below the Hume Dam, which is a major component in the regulation of flow in the Murray River. In normal years, Lake Hume fills over winter and empties over summer, although flows are highly variable. In 2007 the lake dropped to only 1% of capacity, while this year the lake remains at about 90% capacity well into autumn.

The impact of river regulation cannot be overstated. For human communities along the river it provides security of supply. For the fauna and flora of the river, however, it has altered their habitats in ways that have been largely detrimental. Floodplains, billabongs and wetlands are part of a complex, interconnected mosaic that relies on regular flooding events. The frequency of flooding has been drastically reduced. Too often water arrives at the wrong time of year and at the wrong temperature to support natural ecosystems.

Adjusting flows can therefore make a big difference to the health of ecosystems within the basin.

The plan

The proposed Murray Darling Basin plan has several noble objectives. It seeks to create a healthy basin that will simultaneously care for the environment, our communities and the economy. These goals may not be as diverse as originally thought, since healthy ecosystems can and do benefit people and the economy.

The Hume Dam controls how much water flows down the Murray, but do we know enough to control it? Geoff Edney

But to achieve these outcomes, people will have to adapt to using less water. In the current version of the plan, we will reduce human water consumption by 2750 gigalitres (GL) per year until we reach an environmentally sustainable level of flow. This will happen by setting sustainable diversion limits (SDL) for different catchments within the basin.

Some of these limits apply to surface waters, which are the rivers and lakes that we can see, and some limits apply to groundwater, the invisible aquifers that we tap into when we dig wells. Because surface waters and groundwater are connected, the difference between them may not be as clear as the plan assumes.

The problem

The Murray Darling Basin covers three-quarters of New South Wales, over half of Victoria, all of the Australian Capital Territory, and large parts of Queensland and South Australia. The basin contains thousands of water dependent ecosystems such as floodplains, wetlands and marshes, each of which contains plants and animals whose health depends on a supply of water at different times.

Unfortunately, our understanding of what constitutes a healthy ecosystem is limited. Targets for criteria which are easily measured - such as turbidity, phosphorus, nitrogen and pH - are easy to set and to monitor. But targets for the proper functioning of ecosystems are not yet available. Monitoring the frequency of inundation of critical wetlands is a start, but it must be met with assessments of the response organisms. Flora and fauna not only benefit from the river ecosystem, they are part of the system and can contribute to its health.

Trees stabilise soil and prevent erosion. They also create habitat for other plants and animals, and they may be an excellent indicator of ecosystem functioning. Trees have been removed from large parts of the basin, and river regulation has deprived many trees from the floods that keep them healthy and allow them to reproduce.

Red gums require flooding every two to three years, and black box trees need to be inundated every seven to ten years. Decades of flood reduction and unseasonal water delivery has stressed our native forests. Throughout the basin, trees are dying and recruitment is poor.

Native fish have fared no better. Fish stocks are only 10% of pre-European levels. Many native species are verging on extinction. Unfortunately, flows are not always going to help the situation. Floods can allow introduced species such as carp to dominate some systems.

Altering the timing of water releases can improve the situation for fish, crayfish, and other small creatures, because their breeding cycles are used to high flows in winter and spring. Currently, flows are high in summer and autumn, when cold water from reservoirs creates an unnatural condition. Without healthy invertebrates the fish do not thrive, and without fish and crayfish, larger species such as birds and mammals do not have enough food.

Many wetland-dependent birds have also had their breeding cycles disrupted by inverted flow patterns. Some birds migrate long distances and rely on wetlands being inundated at the right time.

Riverine ecosystems are a complex mosaic of interconnected components. The Murray Darling Basin cannot be understood at one scale. Hydrology and geomorphology are important drivers of the system, but so are the zooplankton and macroinvertebrate communities. These tiny creatures are the basis of the foodwebs that provide for healthy fish, bird and floodplain vegetation.

Outputs versus outcomes

Environmental outcomes can only be observed by long term research on many of these components. This will require proper resourcing of scientists studying many parts of the basin, and many aspects of each part. Ultimately, the challenge will be to make careful observations and follow these up with the right kind of questions.

The current version of the plan assumes that controlling outputs can protect vastly different types of water-dependent ecosystems. This can only happen if we keep our eyes wide open and are prepared to modify our views when more information is available. We need to develop an understanding of potential environmental outcomes, appreciate their impact on our own communities, and adapt accordingly.

In Albury-Wodonga, regulated flows from the Hume Weir currently prevent the natural cycles of wetting and drying out of local floodplains. It is hard to imagine changing this pattern without a significant reduction in human consumption. If flows can be reduced and managed in a way that benefits the environment, then people will benefit too.

References

Gawne B, Butcher R, Hale J, Kingsford R, Brandis K, Leo V, Wheaton L and Hladyz S (2011). A Review of River Ecosystem Condition in the Murray-Darling Basin. Final Report prepared for the Murray-Darling Basin Authority by the Murray Darling Freshwater Research Centre, MDFRC Publication 01/2011, June 289 pp.

Young WJ, Bond N, Brookes J, Gawne B and Jones GJ (2011) Science Review of the Estimation of an Environmentally Sustainable Level of Take for the Murray-Darling Basin. A report to the Murray-Darling Basin Authority from the CSIRO Water for a Healthy Country Flagship. 36 pp.

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