Rainfall deficiencies and water availability
Deficiencies continue in WA following below average October rainfall
Rainfall during October 2020 was above average across much of the country, including much of the south-east, South Australia, and the Northern Territory. However, rainfall was below to very much below average for the south-west of Western Australia.
Rainfall deficiencies at shorter timescales (e.g. the 7 months to October 2020) continue in Western Australia, although deficiencies have been removed from the far south-east of the state following above average October rainfall.
The Climate Outlook, issued 5 October, indicates December 2020 to February 2021 is likely to see above average rainfall across most of Australia, except west coast Tasmania.
Most of southern Australia receives the bulk of its annual rainfall during the cooler months of the year. As outlined in State of the Climate 2018, April–October rainfall has declined by 10 to 20% over southern Australia in recent decades. Rainfall during the past three southern cool seasons was below average, and the first half of the 2020 cool season was also drier than average (relative to 1961–1990). Consecutive poor wet seasons in agricultural regions can amplify the impact of rainfall deficiencies on economic and social systems.
7-month rainfall deficiencies
Serious or severe rainfall deficiencies for the 7-month period April–October 2020 are in place in Western Australia across most of the southern half of the State, excluding the western Gascoyne, southern parts of the South West Land Division, and the south-east of the state. Deficiencies remain similar to those for the preceding 6-month period, but have increased in severity in some parts of the west of the state while decreasing or being removed in the state's far south-east. Serious rainfall deficiencies also continue in scattered pockets in greater south-east Queensland, although have decreased in spatial extent compared to last month's Drought Statement.
Area-averaged rainfall for the 7-month period was the seventh-lowest on record for Western Australia's South West Land Division for April–October, and the second consecutive April–October with rainfall in decile 1.
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. For example, over half of the country is in severe rainfall deficiency for the 31-month period from April 2018 to October 2020. Regions affected include north-east New South Wales and the south-eastern quarter of Queensland, western New South Wales, north-western and eastern Victoria, much of South Australia, Western Australia, and the Northern Territory.
The size of the rainfall deficit accumulated over annual and longer timescales remains very large over much of Australia. Despite above average rainfall for much of south-eastern Australia and parts of the inland north-west and Northern Territory during one or more months this year, serious or severe longer-term rainfall deficiencies persist over very large areas.
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 storages are still 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 2018. Parts of south-west, south-east, and eastern Australia—including parts of south-east Queensland and southern and eastern New South Wales—have seen substantial declines in cool-season rainfall in recent decades.
Major water storage levels increased in October but are still low in the northern Murray–Darling Basin
The water storage levels in most of the major storages in the Murray–Darling Basin increased in October. The total storage reached 62% at the end of the month, an increase of 6% since last month and 22% since the same time last year.
Total water storage in the northern Basin remained at 25.8% of capacity (1,200 GL) at the end of October, 18.6% higher than the same time last year. With the exception of Burrendong, at 46% full, all of the larger storages in the northern Basin remain at less than 30%, with Split Rock in the Namoi valley still at only 5%.
The above average rainfall during the second half of October increased the soil moisture levels in the northern Basin water storage catchments. These wet soils provide a good base from which to generate runoff and inflows into the storages from further rainfall. The northern Basin does not have a defined 'winter filling season', unlike the southern Basin. Many of the northern storages are dominated by late spring and summer inflows, providing an opportunity for further filling in 2020.
The total storage in the southern Basin increased by 4.9% during October to 68.9% (14,200 GL), 21.7% higher than October 2019. The irrigation season has been slow to start in the southern Basin due to widespread early spring rainfall. Releases from many of the major water storages are currently being more than matched by inflows with increases in most major water storage. The three largest southern storages of Hume, Lake Eildon, and Dartmouth increased by 13%, 12%, and 6% respectively representing an increase of more than 841 GL.
Compared to last month, root-zone soil moisture (in the top 100 cm) has increased across much of South Australia, the Northern Territory, Victoria, Tasmania, and the northern Kimberley. Root-zone soil moisture has decreased across the west of Western Australia.
Soils were drier than average for October across much of the west and south-west of Western Australia; and pockets of the inland Kimberley and eastern Queensland. Soil moisture was above average for the month through much of South Australia, the Northern Territory, New South Wales, central to western Queensland, parts of eastern Western Australia, and some parts of Victoria and Tasmania.
The influence of very low rainfall over longer timescales is still evident in the 13-month soil moisture for October 2019–October 2020, which was very much below average over large areas, predominantly in the west and south of Western Australia.
- October rainfall below average in south-west Western Australia, despite being above average for much of the country
- Accumulated rainfall deficits at multi-year timescales are significant in many parts of Australia, and may persist for some time
- Root-zone soil moisture decreased across the west of Western Australia, but increased in South Australia, the Northern Territory, Victoria, and Tasmania
- Despite increases in soil moisture in the water storage catchments of the northern Murray–Darling Basin, storage levels are yet to rise
- Major water storage levels continue to increase through the southern Murray–Darling Basin
Product code: IDCKGD0AR0
A very dry month for the southeastern mainland increases rainfall deficiencies
Soil moisture details are reported when there are periods of significant rainfall deficits.
Soil moisture data is from the Bureau's Australian Water Resources Assessment Landscape (AWRA-L) model, developed through the Water Information Research and Development Alliance between the Bureau and CSIRO.
See: Australian Landscape Water Balance.
For the week to 24 November 2020, rainfall was recorded in the northern and central pockets of Western Australia, much of the western Top End in the Northern Territory, parts of the east coast of Queensland, south-east and inland north-east New South Wales and large parts of southern and eastern Victoria.
At the start of the week, a surface trough was located across the Kimberley and the Top End , and from the Gulf Country to the south-east of Queensland. Showers and thunderstorms developed over the Kimberley, western parts of the Top End in the Northern Territory, and in the south-east coast of Queensland. Another surface trough extended from the Pilbara, through to central and south-east Western Australia, with isolated thunderstorms developing over parts of the Southern Interior District, and in areas of south-west South Australia. Onshore flow brought showers to the east coast of Queensland, and moderate falls were recorded over the tip of the Cape York Peninsula east coast, and about the north tropical Queensland coast.
From the middle of the week, broad surface troughs were located in northern and eastern Western Australia, the Northern Territory, through South Australia and towards eastern New South Wales. The troughs and an upper-level disturbance produced isolated showers, thunderstorms and patchy cloud across the north-west, central and inland eastern New South Wales. Widespread moderate falls were reported in the Kimberley in Western Australia and the north-west Top End in the Northern Territory. Widespread moderate falls were also reported over south-east New South Wales and much of southern and eastern Victoria, with the light falls reported over remaining areas of Victoria and large parts of southern New South Wales.
Rainfall totals between 50 mm and 100 mm were reported in about the Kimberley coast in Western Australia; north-west parts of the Top End in the Northern Territory, and the Central, the West and South Gippsland districts in Victoria. Isolated spots in the north tropical coast in Queensland and the Australian Capital Territory also recorded weekly rainfall totals in excess of 50 mm. The highest weekly total was 127 mm at Truscott in the north Kimberley coast in Western Australia.
Rainfall totals between 10 mm and 50 mm were reported in the Kimberley and a small area in central Western Australia, the north-west Top End and central parts of the Northern Territory, parts of east coast Queensland, an area of north-east and south-east New South Wales, and southern and eastern half of Victoria.
Impact of recent rainfall on deficits
Rainfall deficits over Australia for the 7-month (April–October 2020) period is discussed in the Drought Statement, which was issued on 6 November 2020.
Rainfall deficit maps are available for this period as well as for standard periods. The map below show the percentage of mean rainfall that has been received for the rainfall deficit period for the 7-month period, extended to the week ending 24 November 2020.
Rainfall for the period 1 April to 24 November 2020
Serious or severe rainfall deficiencies for the 7-month period are in place in Western Australia across most of the southern half of the State, excluding parts of the coast in the west Gascoyne, southern parts of the SWLD, and the south-east of the state. Deficiencies have increased in severity some parts of the west of Western Australia, while decreasing or being removed in the state's south-east. Serious rainfall deficiencies are also affecting scattered pockets in greater south-east Queensland.
Inland areas of Western Australia have generally received less than 20% their average for the period, while remaining areas in Western Australia and areas in south-east Queensland and have generally received less than 80 % to 40% of their average for the period.
Product code: IDCKGRWAR0