Either side of those years are the floods when there’s more than enough water around, or droughts when all water users have to cope with less. That’s the nature of the Murray-Darling Basin, extreme variability that is notoriously difficult to predict.

A hundred years ago, Victoria, NSW and South Australia negotiated a way to manage the variability together, by giving each state a share of water that would support towns to grow and industries to develop.

Today the effect of a variable climate has been smoothed with the Hume and Dartmouth dams, which collect and store water from both the NSW and Victorian side of the high country.

It has been smoothed by modifications to store water at the Menindee Lakes on the Darling River and at Lake Victoria in the mid-Murray.

Combined with major weirs, canals and pipelines built over the century, these works allow governments to determine how much, when, for how long and to whom the Murray’s water flows. 

 Managing the three-way split

A key part of the  Murray-Darling Basin Authority’s responsibility is to determine and regularly update how much of the total water in the Murray system belongs to each state.

Working out the volume of water that can be shared means calculating how much has to be set aside first to meet the future needs of the system, such as critical human water needs, a reserve to cover evaporation losses and flows to physically deliver water throughout the system.

The water remaining can then be shared between NSW and Victoria, who in turn provide equally for South Australia’s share as agreed in the Murray-Darling Basin Agreement.

That doesn’t mean the water in the Murray system is split into three equal shares.

Water resources in Hume and Dartmouth dams are divided 50-50 between NSW and Victoria, as are Menindee Lakes when they hold more than 480 gigalitres (any less and the water is controlled by NSW alone).

Flows from the rivers that feed the Murray downstream of Albury, however, such as the Murrumbidgee and the Goulburn, are owned by their respective state.

That means the volume of River Murray water available to Victoria usually exceeds that of NSW, by virtue of the states’ different rainfall and river networks.

Over the long-term, Victorian tributaries (3317 gigalitres) on average deliver to the Murray more than double that of the NSW tributaries (1543 gigalitres).

Since June 1 this year, the difference is fourfold. Victoria’s tributaries have delivered 700 gigalitres of water compared with 176 gigalitres in NSW.

South Australia’s total share is delivered according to a monthly schedule that adds up to 1850 gigalitres a year. As a result South Australia’s share of water is potentially less than that of the other states, however it is a reliable quantum.

In effect South Australia’s system prioritises reliability of supply during times of low water availability, as is the case now, over receiving high volumes when water is plentiful.  

Allocating the resource

Once a state knows how much water it has, it can allocate the water among those who have a licence to use it. The volume of available water at the start of the water year can and does change as the year progresses, usually as a result of rain boosting the storage levels.

Each state has developed its own set of licences and rules around how to allocate the state’s share of water to their licence holders. This means allocations in one state will be different those in another.

In some years that difference can be significant.

This is one such year—the licences of many growers in NSW have not yet had any water allocated by their government, while Victorian licence holders for the large part have received reasonable allocations and the South Australian government has allocated 100 per cent to entitlements.

Planning ahead for all scenarios

Regardless of the state, licence holders also have the facility of carrying allocated water into the following year as their own buffer against drought.

A great deal of the water in the dams and flowing in the river at the moment has been carried over by licence holders from last year. This is a choice available to those with an allocation this year.

There’s no right or wrong about how water is allocated.

Each state’s system of distributing the resource has evolved around the needs of its communities and supported an incredible growth in diversity and value in food and fibre production.

It is a challenge to manage in times of drought – no matter how carefully the system is managed we can never drought proof a climate like ours. Rain is needed to provide surety to all water users.

Everyone who uses water can be planning ahead for all scenarios, including the possibility that allocations do not improve and conditions stay dry.

Andrew Reynolds, Executive Director River Management, Murray–Darling Basin Authority