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Highlights in this update

  • The entire Basin received rainfall during October
  • Southern Basin storage levels continue to increase; northern catchments remain steady
  • Threats to water quality remain, despite forecast increased rainfall
  • Spotlight story: Flooding in the southern Basin
Rainfall deciles for previous 36 month period, highlighting the difference between northern and southern Basins

Drought

Persistent rainfall across the Basin in recent months has largely belied the ongoing impact of long-term drought in the Murray–Darling Basin, as demonstrated by the rainfall deficiency map of the past 3 years. This map clearly highlights a stark difference between northern and southern Basins in terms of ongoing drought impacts.

Following significant rainfall in the past 12 months most of the southern Basin has returned to average, with increased soil moisture and significantly replenished storage levels. In contrast, despite rainfall in the north over the same period creating notable streamflow events, it has not been enough to turn around long-term rainfall deficiencies or increase levels in critical storages.

While La Niña conditions are a welcome reprieve for many areas that have suffered from years of drought, forecast above-average rainfall may not be sufficient for some regions to fully recover.

Rainfall across the Basin for October 2020. The entire Basin received rainfall during the month.

Rainfall

During October, rainfall was recorded throughout the entire Murray–Darling Basin, although variability was high between northern and southern catchments.

The highest rainfall was concentrated around the southern ranges, particularly alpine areas of New South Wales and Victoria where more than 200mm fell over the month. The upper Paroo/Warrego catchments recorded comparatively little rain — as little as 1 mm.

Thunderstorms in the second half of the month were largely responsible for above 50mm of rainfall across the New South Wales tablelands, stretching down into the Victorian Mallee region.

Rainfall deciles for October reflect the total rainfall map, highlighting that while southern catchments are experiencing above average rainfall, much of the northern Basin is average to very much below average.

Water storage levels across the Basin for October 2020

Water storages and streamflow

Water storages across the Basin increased slightly during October, improving by 4% overall to 13,048 GL (59% of capacity) as at 28 October 2020. This was a 3% increase over August.

Storage levels in the northern Basin decreased marginally (by 18 GL) to 1,256 GL, remaining at 27% of active capacity overall. Although some storage levels have increased slightly, others remain below 20% of capacity.

Southern Basin storage levels continue to trend upwards due to persistent rainfall, and hold 11,002 GL (68% capacity), at 28 October 2020. This was an increase of 6% on September’s figure.

In the Murray storage levels at Hume and Dartmouth continued to steadily rise, while Lake Victoria began to fall from effective full supply level. In late October, the Lake Victoria storage level began to rise as inflows from tributaries below Hume Dam were captured. Demand for irrigation water varied in reaction to rainfall during the month, and diversions from the River Murray were adjusted in response.

During October, a component of water being released and regulated in river systems was water for the environment, targeting sites including Barmah–Millewa Forest, Gunbower Creek, the lower Darling River and the Lower Lakes/Coorong. This water helps support ecosystem health and is tracked and debited from water for the environment water holder accounts.

Climate outlook - Summer 2020–21

Information from the Bureau of Meteorology confirms that La Niña is now established in the Pacific Ocean to the east of Australia and will likely continue throughout summer. Associated with La Niña conditions, the Southern Annular Mode on the east coast has shifted positive, and as weather systems move further south this will bring higher winds and increased humidity to south-eastern Australia. On the west of the continent, the Indian Ocean Dipole is likely to remain neutral for the coming months, which is also an indicator of increased rainfall for Australia.

All of this points towards a wetter than average summer for south-eastern Australia, including much of the Murray–Darling Basin. However due to low soil moisture in the northern Basin, streamflow in many upper eastern catchments is expected to be low. Near median flows are forecast for much of the southern Basin, with only a few catchments expected to have high flows.

In the south, the combination of high soil moisture and higher than average rainfall has the potential to generate flooding in some areas. More detail on flooding is provided in the spotlight story below.

Summary of threats to water quality in the Basin as at November 2020

Water quality and salinity

The threat of water quality issues remains as temperatures increase towards summer, despite the forecast of increased rainfall. Rain can wash organic matter and nutrients into waterways, triggering events such as algal blooms and hypoxic blackwater. There is a significant risk of hypoxic blackwater events occurring, particularly in New South Wales. The MDBA and state authorities continue to monitor water quality across the Basin.

For more information on water quality, and a map of threats, see the water quality page of our website.

Blue-green algae red/high alerts

New South Wales sites on RED alert for blue-green algae:

  • Lake Wyangan North
  • Pindari Dam
  • Copeton Dam
  • Keepit Dam
  • Chifley Dam

Victorian sites on HIGH alert for blue-green algae:

  • Lake Eildon
  • Tullaroop Reservoir

Bushfire water quality risks

Risk of contamination continues to decrease as catchments recover from severe bushfires.

Blackwater

Warmer temperatures and other factors can combine to create a risk of hypoxic blackwater. Current watchpoints being monitored for potential blackwater events in NSW include:

  • Lachlan River downstream of Forbes
  • Murray River communities from Tocumwal to the South Australian border,
  • Murrumbidgee River downstream of Narrandera, including the Lowbidgee floodplain from Maude to Balranald
  • Edward Wakool system in the region of Deniliquin and Moulamein

Salinity

Salinity levels throughout the southern Basin were below target throughout October. Flows in the River Murray continued to keep salinity levels in the Lower Lakes below averages recorded in the past 12 months, aided by releases through the barrages allowing water to flow into the Coorong.

Spotlight: Flooding and Hume Dam

After 3 dry years, with the last 12 months punctuated by severe drought, southern Basin storages are filling. The Bureau’s outlook for a wet summer due to La Niña means that storages may fill and this increases the likelihood of flooding.

Recent rain has increased the Hume Dam storage level, which surpassed 80 per cent capacity during November 2020. This is good news for irrigators and communities as Hume Dam’s primary purpose is to store water for downstream communities and the environment along the River Murray. Very wet conditions are required before flooding downstream of Hume is likely to occur.

Dams exist in many catchments across the Basin to capture water for communities, industries and the environment, so that it can be used later. These dams also provide an opportunity to mitigate floods, by storing some of the flood water that flows in. Some dams have designated storage space for this purpose, while others do not. Dams without this storage space have limited capacity to mitigate flood events once they are close to full.

Hume Dam was designed for the primary purpose of maximising water storage, not mitigating floods. In wetter years, Hume’s storage level will therefore increase and reach close to full, if enough rain falls. Hume Dam also has no dedicated storage space for floods, and water is not stored above the Full Supply Level during flooding. This is to ensure the safety of the dam and means there is often less scope to mitigate floods at Hume compared with many other dams in the Basin.

Communities should not be complacent about floods downstream of Hume Dam.

There are three things you can do now to prepare for a flood.

1. develop your personalised flood emergency plan for your home and property. Log onto www.ses.nsw.gov.au or www.ses.vic.gov.au

2. sign up for the Early Warning Network to be notified by email and SMS about dam and supply activities during periods of flooding or high releases.

3. check the BoM to receive the latest weather information and warnings at www.bom.gov.au/australia/warnings.

During an emergency event listen to your local ABC Radio station and check the BoM to receive the latest weather and flood warnings.

The Bureau of Meteorology is responsible for issuing flood warnings to the general public. Check www.bom.gov.au/australia/warnings for up-to-date flood warnings in your area.

The Murray–Darling Basin Authority does not provide flood warnings or evacuation advice.


Image: Hume Dam in 2010 spilling 40,000 megalitres per day. Credit: Tony Crawford