Expert review of return flows gives greater assurance to Basin Plan rollout

Changes in return flows will not undermine the outcomes we can achieve through the Murray-Darling Basin Plan, says Phillip Glyde.

The Murray-Darling Basin Authority is implementing the Basin Plan guided by the best available science, as was intended when the Basin Plan was drafted.

The Basin Plan is an internationally significant water reform that traverses a million square kilometres and is a long-term collaboration between six jurisdictions that aims to share water fairly between all users to achieve a sustainable future for communities, industries and the environment.

We are improving our knowledge and tools all the time to deliver on that sustainability aim. Most recently, the MDBA commissioned independent experts, led by the University of Melbourne, to review the evidence relating to ‘return flows’.

Return flows is the term used to describe the volume of water that returns to rivers after it has been diverted and used, usually by irrigation. Return flows can run off the land straight into rivers and creeks or they can seep through the earth into groundwater systems and return to rivers that way.

Some stakeholders raised concerns that reductions in return flows were undermining water recovery efforts — efforts that are key to rebalancing the use of water resources across the Basin to support a healthy environment.

The Basin Plan sets out to manage all water in the system for the first time. That includes the water in the rivers as well as groundwater (water in aquifers), flood water (when it occurs) and water in all dams, on farm and off. In a system where inflows can be as low as 7000 gigalitres (GL) and as high as 118,000 GL, this is a complex undertaking.

At the time the Basin Plan was drafted, the MDBA had a reasonable understanding of return flows from the decades of work to manage salinity and water quality. We used this knowledge to develop the Basin Plan.

Independent expert review examines risk

Recently, concerns have been expressed about possible reductions in return flows because of increased irrigation efficiency. Given more information is available about how water for the environment can be recovered, the time was right for an independent expert review of this risk to the Basin’s water resources.

We’ve always recognised that improvements in irrigation efficiency can recover water for the environment and also reduce return flows. The latest review estimates the long run impact might be around 120 GL a year. I understand that this is still a significant volume of water but, to put it into perspective, it represents about one per cent of the total volume of water that can be taken from the rivers for consumptive uses in an average year.

I’m confident that changes in return flows of this quantum will not undermine the outcomes we can achieve through the Murray–Darling Basin Plan.

Firstly, we already account for about 60 per cent of surface water return flows in the water accounts used to monitor diversions from our rivers. This lets us measure diversions in net terms. So if surface water return flows change, our measure of diversions will also change. And, diversions will need to stay within the new sustainable diversion limits.

As the managers of the Basin’s water resources, we will continue to improve our collective knowledge and continue to monitor for any effect that changes to return flows might have on our rivers.

Changes in return flows take decades

Secondly, changes in return flows do not happen overnight — in fact they may take years, decades or longer to have any impact on rivers. This is because the strength of connections between groundwater systems and rivers is highly variable – sometimes they aren’t connected at all. Eighty per cent of the 120 GL impact relates to return flows from groundwater systems.

Thirdly, the irrigation efficiency programs that the Australian Government have invested in have returned water savings in the form of water entitlements that can be used when and where they will have the most benefit for the environment. Active management of this water portfolio means communities can have confidence that the investment in recovering water for the environment is worthwhile.

The independent reviewers looked at another part of the water balance that is hard to measure and subject to a degree of uncertainty: the effect that increased groundwater extraction might have on river flows.

This is a more difficult subject area to investigate — the water is underground, some aquifers are connected to the rivers and some are not, and on top of that, depending on the geology it can take years, decades or even centuries for water to find its way back to the river.

Currently, much less water is taken direct from groundwater areas than is allowed for under the Basin Plan.

The review found that should groundwater use increase by two percent per year over the next 40 years, then flows from groundwater aquifers into the Basin’s rivers would be reduced by around 170 GL. This is slightly lower than previous MDBA estimates of around 190 GL.

Although different assumptions are used, the two estimates are broadly consistent. Key assumptions which differ between the two studies are about the degree of connection between different groundwater areas and the Basin’s rivers, and the assumptions on the future growth in use of groundwater resources.

Currently, there don’t appear to be any real economic drivers that would increase groundwater extraction substantially beyond existing levels. This may be because the water is often highly saline so has limited use.  In these situations, growth in water use from saline aquifers would lead to improvements in river salinity.

It is also important to point out that the reviewers used actual Murray-Darling Basin catchment data and examples to draw conclusions. Other analysis publicised in recent months has used generic assumptions that don’t reflect the Murray-Darling Basin context — our flat landscape with groundwater systems that take many years to change.

I acknowledge that much of our work is technical and complex and needs to be thoroughly and regularly tested. This assessment of the impacts of the Basin Plan on return flows is a good example of our ongoing commitment to do this. The report is published on our website and I encourage those interested to read it.

Top image:  The All Saints vineyard in Rutherglen, Victoria.

Phillip Glyde is the Executive Director of the Murray–Darling Basin Authority. Glyde’s 38-year public service career includes several Australian government departments and agencies including Agriculture, Prime Minister and Cabinet, Environment, and Resources and Energy, as well as the resource Assessment Commission and CSIRO. Internationally, Glyde has worked with the Environment Directorate of the OECD in Paris and the Department of Environment, Food and Rural Affairs in the United Kingdom.