Darling River’s Murray myth: It’s a flood plain harvesting blame game

A 20-year-old, one-off calculation is being used by opponents of flood plain harvesting to exaggerate the value of the Darling River’s flow to Murray River communities.

Floodplain harvesting by northern NSW irrigators is being blamed for a reduction in the Murray River’s flows and allocations — but what are the facts?

HOW important is the Darling River to the Murray River and its irrigation communities?

NSW Murray River irrigators, the Speak Up Campaign and media have been repeatedly quoted a 20-year-old Murray Darling Basin Authority report that states: “the Menindee Lakes supply part (approximately 39 per cent) of annual entitlement flows to South Australia”.

That 39 per cent has been used to highlight the importance of the Darling River’s contribution to the Murray, along with the argument the NSW Government is allowing its northern flood-plain harvesting irrigators to drain the Darling, creating a shortfall in the flow to SA that southern NSW and Victorian irrigators must meet.

It’s a powerful argument. But what are the facts?

Just how the 39 per cent was calculated by the original 2000 report’s author Martin Thoms is unknown, given he told the NSW Department of Planning, Industry and Environment that he was unable to recall why the statistic was included as one line in the 169-page report from 20 years ago.

It seems likely Thoms calculated the Darling’s contribution based on the assumption it delivered an annual average of 720GL to the Murray, which is 39 per cent of SA’s 1850GL annual entitlement.

The actual Darling River average annual flow is closer to 912GL, based on 113 years of modelled data at the Burtundy gauge (about 180km up upstream of the Wentworth Murray junction), which would lift Thoms calculation to 49 per cent of SA’s 1850GL entitlement.

But anyone who steps back and looks at the whole river system soon realises such calculations are irrelevant.

Arguing every drop of the Darling River’s 912GL average annual contribution goes towards SA’s 1850GL entitlement makes no sense, because that’s not how the river system is operated.

Each year the Murray River’s upper catchments — the Kiewa, Ovens, Goulburn-Broken, Campaspe, Loddon, Murrumbidgee and Darling — deliver about 7000GL of water to the South Australian border. The Darling River’s contribution averages out at just under 13 per cent.

The Murray and all its tributaries contribute to SA’s 1850GL entitlement flow, with the Murray Darling Basin Authority’s river operations team using a series of complex rules to allocate reserves and forecast flows to critical human water needs, conveyance losses, SA entitlement, environmental and state allocations.

Which water fills SA’s 1850GL bucket first is meaningless. But as always it’s the simple one line grab of 39 per cent that dominates the debate.

It’s also worth looking at the arguments Murray irrigators mount against the NSW Government’s decision to implement its 2008 flood plain harvesting policy that finally grants the state’s northern irrigators 300GL of entitlement.

Again, the arguments against handing out these floodplain harvesting entitlements are that it will entrench the loss of not just 300GL flowing down the Darling, but will allow northern irrigators to harvest up to five times that volume in any one year – so 1500GL in total.

But NSW northern floodplain harvesters can only take five times their entitlement in one season, if they have not used a drop in the previous four.

And if they take 1500GL in a flood year, they are restricted to just 300GL in the following season.

It’s also worth understanding that about half the water northern NSW irrigators harvest off the floodplain would never have returned to the Darling and its upper tributaries anyway.

But the big question that remains unanswered is what impact harvesting 1500GL in a flood year has on the Darling’s flows at Bourke and southern Murray irrigators allocations.

It’s clear when flows at Bourke are high, so are allocations to Murray irrigators.

In 2010 the flow at Bourke was 4886GL and Murray Irrigation Limited’s shareholders’ accumulated 101 per cent of their general security water in allocation and carryover.

In consequent wet years the Bourke flow and MIL allocations were:

2011 6193GL and 110 per cent.

2012 7407GL 110 per cent.

2016 2387GL 111 per cent.

It’s also worth mentioning that if you look at the proportion of inflows consumed within each system, you realise that southern irrigators are throwing stones in glass houses.

Murray irrigation communities consume 33 per cent of their valleys’ inflows, on the Goulburn 46 per cent and on the Murrumbidgee 42 per cent.

Compare that to the northern Basin valleys, which on average consume 23 per cent of their inflows.