The Murray-Darling Basin Plan is fundamentally a set of rules that decide how water is shared between farmers, towns and the environment.
It has generally worked pretty well, but flood plain harvesting hasn't been properly accounted for in the plan, and that has generated a lot of concern.
To understand why, we need to look at the Murray-Darling Basin itself.
What is the Basin all about?
The Murray-Darling Basin is divided into a Northern Basin, covering south-west Queensland and north-west NSW, and a Southern Basin which stretches down the Darling River to the Murray and Murrumbidgee catchments, all the way to the mouth of the Murray in South Australia.
Water from the Northern Basin is supposed to flow south down the rivers and across flood plains all the way to South Australia, but some of it is captured along the way.
The question is, how much?
Disputes over the size of the water take in the north have caused a lot of friction between irrigators from different areas, environmentalist and state governments, and a lot of the concern is about flood plain harvesting.
Many people downstream in the Murray-Darling Basin system believe that flood plain harvesting is causing the Darling River to dry up, so when there is a drought or a flood, tempers flare.
What is flood plain harvesting?
Most irrigation systems rely on water stored in big dams, filled by spring rains and snowmelt in the mountains and delivered to farms via rivers and canals like the Murray and Murrumbidgee.
You see them all around the Riverina and in many parts of the southern Murray-Darling Basin.
But flood plain harvesting in the Northern Basin is very different.
It is a highly variable system, and it can be dramatically affected by drought, which we've seen in the last three years when many irrigators in the north haven't had enough water to plant a crop.
There are few big rivers or major storages, so farmers have invested heavily to build private dams over the last 70 years.
The mix of water supplies is different too.
Just one-third of the irrigation water in the north is held in government dams. A third comes from rainfall events that swell the rivers below the dams (called supplementary water) and a third comes from flood plain harvesting.
When there is a flood, the water flows across wide open plains and farmers capture it using levy banks and divert or pump it in their own dams to use in the following years to produce a crop, mostly cotton.
How much can be sustainably taken from the Northern Basin?
The NSW Government has been reviewing the water take in the Northern Basin.
It is the last substantial parcel of water to be incorporated into the Murray-Darling Basin Plan, and if the water take is a large amount it could affect the sustainability of the whole plan?
Working out how much water is taken is difficult.
In the Southern Basin, you can measure the amount of water in the big government-owned dams, and you can also measure river heights, but in the north most of the flow is from 'unregulated' systems (those without any government dams).
Most of the water harvested in this way is unmetered, and water extraction is hard to police.
Trying to find the answers
In the last year, the NSW Government has been clamping down on illegal activity and counting the number of dams in the Northern Basin to measure how much water they can store.
It has conducted farm surveys, on-ground mapping of all flood plain harvesting works, and used satellite imagery and remote sensing including LiDAR imagery which is accurate to within a few centimetres.
Those results were published in the Healthy Floodplains Project last year and the figures raised in the NSW Parliament.
They identified 1,386 dams with an estimated holding capacity of 1450 GL, but that figure doesn't necessarily represent the amount of water taken by flood irrigation or the total for the year.
More or less?
It could be less; given those dams are also used to manage different kinds of water from licensed sources — such as general security from government dams, supplementary water that falls below the dams, water flowing onto farms as well as unregulated water taken in flood plain harvesting.
Or it could be more; given those dams can be re-filled during the water year if there is water available and they also capture run-off from irrigation fields.
So what is the take from Northern Basin overall?
That is a very contentious question.
The total amount of run-off in the Basin is about 32,500 GL/year.
About 12,500 GL is used in the Basin, mainly for agriculture. [Murray-Darling Basin Water Compliance Review 2017 p.41].
About 2,000 GL has been returned to the environment so far through the plan.
The annual take for NSW is 6,300 GL.
Unlike other forms of irrigation, flood plain harvesting is not licensed or measured, so estimates vary wildly.
Dr Peta Derham from the Murray-Darling Basin Authority said a certain amount of flood plain harvesting was accounted for in the Basin plan.
A baseline limit of 250G was estimated (schedule 3 of the Basin Plan) which was a number based on the best information they had at the time.
"We know that figure will change as both Queensland and NSW continue to roll out licensing over the next couple of years," Dr Derham said.
The question is, by how much?
Some people believe the take from the Northern Basin, before it even gets into the rivers, could be as high as 4,000 GL.
That is the case made by Darcy Hare, chairman of Southern Riverina Irrigators.
He wants tough restrictions on the Northern Basin irrigators to ensure more water gets into the river system to enable farmers in his area to have a bigger allocation.
He points to a 2012 report by the Natural Resources Commission [Review of the Water Sharing Planning for the Barwon-Darling Unregulated and Alluvial Water Sources, page 45, section 3.2] which said "off-river harvesting and storage is significant."
"In 2007, the estimated volume in ring tanks in the Barwon-Darling itself was 289 GL, with another 4,039 GL in ring tanks and hillside dams in the upstream catchments."
Total capacity is the question
The NSW Department of Planning and Environment said the 4,039 GL figure was an estimate of the total capacity of all on-farm dams most of which were constructed for stock and domestic purposes.
"Every five years, on average, they are used for flood plain harvesting but at other times they are used to store water taken under unregulated, supplementary and general security as well as to capture contaminated runoff," a spokesperson said.
The NSW Government is conducting a review called the Healthy Floodplains project , which will determine what the historical take has been, what licences should be issued and what regulation will be required.
In the end, the most important number that has to be decided is the sustainable diversion limit for the Northern Basin.
If that number is too high the Murray-Darling Basin Plan may not be sustainable and downstream environments, and communities will suffer, but if it is too low irrigators and communities in the north will face a difficult adjustment.
Early figures give an indication of the water take
For the first time, the NSW Government has measured the amount of water being taken from the flood plains by irrigators in the Northern Basin.
A historic embargo was put in place earlier this year as floodwaters flowed from the Border Rivers region in Queensland into northern NSW, but it was lifted for three days, triggering angry protests.
Four months later, using LiDAR remote sensing and satellite technology, the NSW Department of Industry and Investment estimated that 30GL, equivalent to 400 Olympic-sized pools, was taken during the lifting of the embargo.
The amount taken overall from that flood event, once the embargo was over, was 350GL.
By comparison, Sydney Harbour holds about 500GL.
A significant amount of water was left to flow down to the Southern Basin, according to the NSW Water Minister Melinda Pavey.
"Over 380 GL has made it to the Menindee Lakes, reconnecting the Lower Darling to the Murray River for the first time since 2016."
Irrigators in the Macquarie Valley kept their own figures during that period, and while they won't release them yet, they say the water take by irrigators was about 20 per cent of the flow.
"Eighty per cent of that water has gone down the creeks, including the Gunningbar and the Lower Bogan right through to the Darling." Tony Quigley, chairman of Macquarie Food and Fibre said.
