Deficiencies continue in southeast Queensland and southwest WA

Rainfall deficiencies at shorter timescales such as the 9 months from April to December 2020 continue in southwest Western Australia and southeast Queensland following close to average December 2020 rainfall in these areas. It was a wet December in many other parts of Australia, including northeast New South Wales, which saw record December rainfall in some parts. It was Australia's fourth wettest December on record.

The Climate Outlook, issued 7 January, indicates February to April 2021 is likely to see above average rainfall across much of northern and eastern Australia, especially in Queensland.

The Annual Climate Statement 2020 confirms it was the fourth warmest year on record for Australia with annual total rainfall 4% above average. Rainfall during the year helped relieve some of the most extreme drought conditions in parts of eastern Australia but more rainfall is needed over an extended period to fully recover from the extended extreme dry conditions of 2017 to 2019.

9-month rainfall deficiencies

Serious or severe rainfall deficiencies for the 9-month period April–December 2020 are in place in southwest Western Australia across most of the southern half of the State, excluding the western Gascoyne and southern parts of the South West Land Division. Deficiencies are largely unchanged compared to the previous 8-month period with December typically climatologically drier in the WA deficiency area. Serious rainfall deficiencies have contracted in in southern parts of southeast Queensland and northern New South Wales following close to average to above average December rainfall in these areas.

Extended dry conditions over eastern Australia

Australia has experienced a prolonged period of below average rainfall spanning several years.

Rainfall deficiencies have affected much of Australia since early 2017. Multi-year rainfall deficiencies and their impact on the Murray–Darling Basin are discussed in Special Climate Statement 70 and the dry conditions over Eastern Australia for the period commencing January 2018 are described in Special Climate Statement 66. The strip along the west coast and south coast of Western Australia has also been affected by rainfall deficiencies for the periods commencing January 2017 and January 2018.

For periods longer than 24 months, the greatest impact of the prolonged below average rainfall has been in the cooler months of April to October. Large parts of the country remain in severe rainfall deficiency for the 33-month period from April 2018 to December 2020, though rainfall in 2020 has seen improvement across many areas. Above average December rainfall has eased much of the longer-term deficiencies in inland NT and northern WA.

Persistent, widespread, above average rainfall is needed to lift areas out of deficiency at annual and longer timescales and provide relief from the impacts of this long period of low rainfall (such as renewing water storages). The impact of the longer dry on water resources is still evident, especially in northern parts of the Murray–Darling Basin where total storage levels are low.

The role of climate change in rainfall reduction over southern Australia and along the Great Dividing Range is discussed in State of the Climate 2020. Parts of southwest, southeast, and eastern Australia—including parts of southeast Queensland and southern and eastern New South Wales—have seen substantial declines in cool-season rainfall in recent decades.

Soil moisture

Compared to last month, root-zone soil moisture (in the top 100 cm) Root-zone soil moisture has increased across northern Australia and northeast NSW following above average December rainfall in these parts but has decreased across areas of southeast and southwest Australia.

Soil moisture was above average for the month through southwest Western Australian, across areas of northern Australia and eastern and southern New South Wales into South Australia. Soils were drier than average for December across central parts of inland Western Australia; and areas across central and southern Queensland and the Northern Territory and western Tasmania.

The influence of very low rainfall over longer timescales is still evident in the 14-month soil moisture for October 2019 – December 2020, which was very much below average over large areas, predominantly in the west and south of Western Australia.

• Deficiencies continue in southeast Queensland and southwest Western Australia
• Accumulated rainfall deficits at multi-year timescales are significant in many parts of Australia, and may persist for some time
• Root-zone soil moisture has increased across northern Australia and northeast NSW but has decreased across parts of southeast and southwest Australia
• Northern Murray-Darling Basin storages remain low but are still significantly higher than same time last year
• Northern Australia water storages remain low despite an early start to the monsoon season

Major water storage levels remain low in the northern Murray–Darling Basin

The total water storage levels in the Murray–Darling Basin decreased slightly in December but remain in a significantly better position than the same time last year. The total storage fell to 58% at the end of the month, a decrease of 2.5% since last month but still an increase of 23.5% since the same time last year.

Total water storage in the northern Basin decreased by 1% to 23.6% of capacity (1,099 GL) at the end of December, 17.9% higher than the same time last year. Despite significant rainfall occurring over northeast NSW, there were only limited increases to the major water storages located on the west of the Great Dividing Range. Storages including Chinchilla, Chaffey, Split Rock and Keepit had some increases, with Chaffey and Keepit increasing to above 30% of capacity. However, with the exception of these two storages and Burrendong, at 40.6% full, all of the larger storages in the northern Basin remain lower than 30%.

Many of the rivers in the far northern Basin are only flowing due to releases from large storages such as Lake Kajarabie (Beardmore Dam) which decreased by 12.1% of capacity through releases this month. The rivers further south in the Border Rivers and Gwydir catchments are reaching very much below average levels but could expect inflows in the next few months with the coming of the wetter season.

The total storage in the southern Basin decreased in December by 3.2% to 65.8% (13,592 GL), still 24.4% higher than December 2019. The highest decreases in the major storages were for Lake Victoria by 23.2% and Hume by 8.2%. Other major storages- Blowering, Burrinjuck, Googong - also decreased slightly (2 to 5%) but remain near capacity (83-100% accessible storage) at the end of the month.

Northern Australia water storage levels remain low

The northern Australia monsoon season has started as expected in December but water storage levels in the north of Western Australia and the Northern Territory have increased very little. Water levels in Lake Argyle the largest water supply storage in Australia holding up to 10,400 GL (twice as much as any other water supply storage in Australia), have been decreasing since September 2017. Lake Argyle has consistently filled to capacity during the wet season every 1-2 years for the past 20 years. However, the past three wet seasons have not delivered significant inflows to the storage and the water level has continued to fall until November. This year's monsoon has brought some relief during December and the storage has increased by 1.8% to reach 26.8% of its accessible capacity.

The lack of wet season filling is also reflected in the Darwin and South East Queensland (SEQ). The storage levels in Wivenhoe, the largest storage in SEQ, have decreased significantly in the past three years to 39.4 % of capacity at the end of December. Darwin River storage has increased slightly, in response to the monsoon rainfall, to reach 51.6%.