Do we have the capacity to sustain growth below the Choke?
23 December 2020
Delivery risks have always existed in the River Murray, however there has been growing concern in recent years that these risks have been increasing. The Murray-Darling Basin Authority and Basin states have jointly developed a paper exploring these risks and how they have changed over time. This paper is now available on the MDBA website here. The paper concludes that the risk of shortfall in the River Murray downstream of the Barmah Choke is increasing, and considers options for reducing or mitigating this risk.
The nature of the River Murray system, where it takes three weeks to deliver water from Hume Dam to Sunraysia, means the risk of a shortfall (of not being able to deliver water allocations to water users in the Lower Murray when they need them) is always present.
There are two different types of shortfall which may occur: Delivery shortfalls and System shortfalls. Delivery shortfalls occur when actual water use is higher than it was forecast to be when water was released from the Hume and Dartmouth storages, weeks earlier, to meet the forecast needs for irrigation and environmental water. With accurate weather forecasts only available for 7 days in advance, while releases must be made from upstream storages 3 weeks or more in advance, the risk of a delivery shortfall is always present if demands turn out to be much higher than anticipated when releases were made, for example an unexpected heatwave. To date, a delivery shortfall has occurred only once, in 2002.
A system shortfall occurs when the combined capacity of the system is unable to supply all downstream requirements over the full season. As a result, storage levels in Lake Victoria may fall too low to meet requirements at the South Australian border, at the same time as all upstream requirements.
The Barmah Choke is a natural constraint in the river which limits the amount of water which can flow downstream. The capacity of the Choke has fallen from 11,500 ML / day in the 1980s, to 10,500 ML / day in the mid-1990s to 9,200 ML / day in 2019. In addition, for much of the last 20 years the Darling River has been in drought, and unable to contribute significant volumes of water to meet demands in the River Murray system.
As a result, river operators have had to run the Choke at or near channel capacity for long periods, which has led to erosion of the banks. Some water can be transferred around the Choke through the Edward-Wakool system, but this is now also causing degradation to this system.
A recent investigation has indicated that there is a large volume of sand within the choke which is reducing its capacity. This sand is likely derived from upstream sources and may be an artefact of sluicing of gold in the tributaries during the goldrush. Further work is underway investigating the source and movement of sand through the Choke.
While annual irrigation use has increased in the reach between Wakool Junction and the South Australian border, this has been offset by a reduction in the reach between Barmah and Wakool Junction. Overall, the irrigation use between Barmah and the South Australian border has been relatively steady, both annually and over the peak irrigation period between January and April.
While irrigation use in this reach has not changed materially, recovery of water for the environment has meant that there is less water available to meet demands. Given that trade rules prohibit net trade in the Murray from above to below the Choke, additional water to meet demands can not come from above the Choke in the Murray. Instead, this additional water is being obtained through Inter-Valley Trade (IVT) accounts from tributaries such as the Goulburn and Murrumbidgee. This is putting additional pressure on these rivers, which are also suffering degradation as a result of higher sustained flows over summer.
When all these factors are considered together, more water needs to be delivered to the lower Murray to meet irrigation and environmental demand, but with less system capacity to do so. This leads to the conclusion that the risk of shortfall is increasing.
The MDBA and states are working together to address this increasing risk. The paper includes a roadmap of the planned and completed work. Tasks completed to date include the development of a system-wide Shortfall Response Plan, which complements and coordinates the individual plans of each jurisdiction. These plans were tested in an exercise earlier this year. Other work underway includes hydrological analyses of the Murray and major tributaries to understand how they are being impacted, further investigations of the sand in the Choke to understand its origin and movement, and investigation of options for reinstating capacity at the Choke.
Delivery risks in the River Murray are complex and the delivery risk paper aims to set out our understanding of the various components at this point in time. If you have any questions or concerns, please feel free to contact me on (02) 6279 0645 or tyson.milne@mdba.gov.au.
Tyson Milne
MDBA
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