Impact of floodplain harvesting growth in the northern Basin

The NSW Government remains fully committed to addressing any growth in floodplain harvesting through implementation of the NSW Floodplain Harvesting Policy PDF, 410.45 KB in the northern Basin from July 2021.

In most valleys in northern NSW, revised modelling is showing that floodplain harvesting has grown above the legal limits described in NSW water sharing plans and the Australian Government Basin Plan. This growth in floodplain harvesting is reducing the volume of water that remains on floodplains and re-enters rivers and creeks.

The NSW Government is bringing floodplain harvesting into an enforceable regulatory framework, by controlling floodplain harvesting through licences and water supply works approvals. The process for this includes evaluating the environmental and downstream benefits of reducing floodplain harvesting to legal limits.

The first of these environmental and downstream benefits assessments, relating to the Border Rivers, Gwydir, Macquarie are already available. Assessments relating to the Namoi and Barwon-Darling valleys will be released in 2021.

In addition to assessing the downstream benefits of restricting floodplain harvesting in line with legal limits on a valley-by-valley basis, these benefits are also being assessed on a cumulative basis across the northern Basin. A report describing the cumulative downstream benefits of reducing floodplain harvesting to legal limits across the Border, Gwydir, Namoi, Macquarie and Barwon-Darling valleys will be released in 2021. This report will evaluate the potential range in benefits as increased flows move through the Barwon and Darling Rivers and into the River Murray.

Altogether, these assessments will allow for informed public comment about the impacts of floodplain harvesting growth and, in turn, the benefits of implementing the NSW Floodplain Harvesting Policy PDF, 410.45 KB.

It should be noted that each of these technical assessments will be subjected to external peer review.

Common misconceptions

Legal limits specify volumes that are not subject to revisions

This is false. Legal limits in both water sharing plans and the Basin Plan are defined as long-term volumes under a certain set of development and management conditions. These limits are described as formulae and are estimated using models. They reflect the best available information that was available at the time of modelling. Volumetric estimates can be updated through formal approval processes when there is better information about the development and management conditions that they reflect.

A commonly quoted 210 GL/yr estimate of floodplain harvesting was an estimate across the northern Basin (both QLD and NSW) made in 2012. The information that this estimate relied on at that time was poor, and it used river system models that were not built for this purpose.

Accordingly, the NSW Government expects that this estimate will change significantly to reflect better information and the use of better modelling tools. For NSW, valley-specific, peer-reviewed technical reports which describe the modelling process and the data relied upon to re-estimate these legal limits, are being published for transparency. The first of these reports, for the Border Rivers valley are now available on the website.

The Darling River contributes 39% of the supply to South Australia each year

This is false. This figure comes from Assessment of environmental water requirements for the proposed Basin Plan: Lower Darling River System published by MDBA in 2012. This report states ‘The Menindee Lakes Scheme delivers water to South Australia to meet part of its entitlement (39% on average)’.

The quoted figure reflects a long-term average contribution from the Lower Darling River, not a fixed annual contribution. The quoted figure does not account for year-to-year variation and the timing of inflows from the Lower Darling River into the River Murray. The figure also assumed that the Lower Darling River is always the “first” source of water to supply the SA entitlement. However, the Lower Darling inflows are a shared water resource and all inflows are managed to meet agreed priority demands in accordance with Murray-Darling Basin Agreement. Best available information is in the Murray-Darling Basin Authority technical report 2010/20, which suggested an average 86% of Murray inflows came from sources other than the Darling River over the climatic period from 1895 to 2009.

Growth in floodplain harvesting is having a significant impact on Murray River allocations

This is false. Inflows from the Lower Darling River represent, on average, 14% of the total inflows into the River Murray. This means that major changes to inflows from the northern Basin have only minor impacts on total Murray inflows. For example, a 10% reduction in inflows from the Lower Darling would result in only a 1.4% reduction in total inflows to the River Murray.

A report on the cumulative downstream benefits that result from returning floodplain harvesting to legal limits will be published in early 2021. Although there has been significant growth, reducing floodplain harvesting back to legal limits in the northern Basin is expected to result in only minor improvements in Murray allocations because of the natural processes that attenuate flood flows as they pass down through the northern Basin.

The most significant impacts on Murray allocations result from changed inflows upstream of Albury, which have decreased by more than a third in the last 20 years relative to the past century.

For more details, please refer to the Impact of lower inflows on state shares under the Murray-Darling Basin Agreement.