Rainfall deficiencies and water availability
Below average July rainfall for southern Australia sees little rainfall deficiency relief
Rainfall during July 2020 was below average for most of the southern half of Australia, although areas of eastern Victoria and coastal New South Wales had above average rainfall. Daytime temperatures during July were warmer than average for most of the country; the maximum temperature was the seventh-highest on record for July for Australia as a whole.
Below average July rainfall has seen rainfall deficiencies increase for the period April–July 2020 over the south-western half of Western Australia, and to a lesser extent over the same area for the period since April 2018.
Low pressure systems off the east coast of Australia brought periods of very heavy rain to the east coast at times during the month, resulting in a reduction of rainfall deficiencies in parts of eastern Victoria and south-east New South Wales at both timescales.
May–July rainfall has been below or very much below average over most of the southern half of Australia, except in Gippsland and eastern New South Wales where totals have been close to average.
The temperature gradient across the tropical Indian Ocean, with waters in the east being cooler than waters in the west, likely contributed to a suppression of moisture entering Australia from the north-west. This temperature pattern has dominated the Indian Ocean Basin over most of the past three years. Long-term rainfall trends are also likely to have played a role in low rainfall over the past three months. 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 which shows that parts of south-west, south-east 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.
The Bureau of Meteorology also monitors rainfall deficiencies, and impacts on water resources, on longer timescales such as the current severe multi-year drought affecting large parts of eastern Australia. This is discussed further below, and in more detail in our Special Climate Statements.
The Climate Outlook for September to November, issued 6 August, indicates an increased chance of wetter than average conditions across most of the eastern half of Australia. The Bureau will continue to closely watch and report on observed rainfall and seasonal outlooks over coming months.
4-month rainfall deficiencies
Most of southern Australia receives the bulk of its annual rainfall during the cooler months from April to November. As outlined in State of the Climate 2018, April–October rainfall has declined by 15 to 20% over southern Australia since the 1970s, and with below average rainfall for the southern wet season so far, 2020 is on track to become the fourth straight drier than average southern wet season (relative to 1961–1990). Consecutive poor wet seasons in agricultural regions can amplify the impact of rainfall deficiencies on economic and social systems.
Serious or severe rainfall deficiencies for the 4-month period April–July 2020 are in place in Western Australia across most of the south-western half of the State. Serious rainfall deficiencies are also in place across parts of south-west South Australia and some southern parts of the pastoral districts; parts of central southern and southeast Queensland; and pockets in the south of the Northern Territory.
In addition to deficiencies in Western Australia, large areas of South Australia, western New South Wales, north-western Victoria, and northern Tasmania are experiencing serious or severe rainfall deficiency at the 3-month period (May–July 2020); much of south-eastern Australia received above average rainfall for April, keeping these areas out of deficiency at the 4-month timescale, but have generally received below average rainfall since April.
The small area of serious rainfall deficiencies on the extreme south coast of New South Wales last month has been cleared by very much above average July rainfall.
28-month rainfall deficiencies
Rainfall deficiencies at the 28-month timescale, from April 2018 to July 2020, generally persist with little change, although very much above average July rainfall in eastern Victoria and south-east New South Wales has decreased the severity of deficiencies in these areas.
Serious to severe rainfall deficiencies are in place for the 28-month period across much of the south-western half of Western Australia, the northern Kimberley, and the east and south of the Interior District in that State. Deficiencies are also in place across much of the Northern Territory; nearly all of South Australia; the southeastern quarter of Queensland; much of New South Wales except areas of the central coast and a broad band through the central west from around Griffith to the Queensland border north of Bourke; and across eastern Victoria and the northern border regions of that State.
The area of lowest on record rainfall for the 28-month period has increased month-on-month. These include areas on the south coast and west coast of Western Australia; along the border of New South Wales and South Australia, extending into parts of the Lake Eyre / Kati Thanda catchment; and parts of northeast New South Wales and greater southeastern Queensland.
Extended dry conditions over eastern Australia
Rainfall deficiencies have affected much of the southeastern quarter of Australia since early 2017. Rainfall deficiencies for the period starting January 2017 are most extreme in the northern and western Murray–Darling Basin, and across much of eastern and central Gippsland in Victoria. This long-running dry period has previously been discussed in the last update on the long-running dry and for 2019 in the Annual Climate Statement.
The size of the rainfall deficit accumulated over annual and longer timescales prior to 2020 remains very large over much of Australia. Despite above average rainfall for much of southeastern Australia and parts of the inland northwest and Northern Territory during one or more months in 2020-to-date, serious or severe longer-term rainfall deficiencies persist over very large areas.
Additional 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 (see Water section of this Statement).
Water storages continue to fill in the southern Murray–Darling Basin but levels remain low in the north
Water storage levels increased slightly in most of the major storages in the Murray–Darling Basin during July. The total storage reached 47% at the end of July, an increase of 4% since last month and 8% since the same time last year.
In the northern Murray–Darling Basin, major storage levels remained low at only 20.6% of total capacity (959 GL accessible storage). Most of the northern storage levels increased at least slightly in July with the largest increases of 7% being seen in the Burrendong storage in the Macquarie Valley and the Chaffey storage in the Namoi. Unlike the significant and rapid recovery of these storages in 2010 and 2016, the current rate of recovery is slow and significant follow-up rainfall is needed to replenish these northern Basin water storages.
The total storage in the southern Basin increased by 4.4% during July to 54% (11,103 GL), 8% higher than July 2019. Lake Hume again had the most significant increase of 12%, bringing it to 49% of capacity. This is an encouraging start to the winter filling season given the past three filling seasons have had insufficient inflows to replenish the seasonal drawdown. The filling season in the southern Basin will generally continue until the end of September.
Some rivers in the northern Murray–Darling Basin see valuable increases in flow
The Namoi, Castlereagh, and Macquarie rivers received valuable inflows in July, increasing river levels to above average across most gauges. However, the monthly average river flows remain low through the far northern Basin, particularly in the Gwydir and Border Rivers where the significant coastal rain did not extend over the Great Dividing Range.
The majority of the southern Basin rivers are running at near average flows for July. There are large flows (greater than 10,000 ML/day) in the Murray River main channel downstream of Barmah but this is not unusual for the southern winter period. Flows are reaching the Lower Lakes in South Australia with more than 65 GL of water being released into the Coorong.
Minor flows are being maintained down the Lower Darling River with releases from the Menindee Lakes, but there are no significant flows in the Darling upstream of Menindee Lakes.
Storage and river levels in south-west Western Australia remain low
The total water storage levels in the South West Coast drainage division are at 41.1% of capacity. The Perth water supply system relies on desalinated water and groundwater extraction to supplement natural inflows to water storages. The addition of these climate-resilient water sources buffers the supply system against extended dry periods. The dry conditions are more accurately reflected in the Harvey rural supply system south of Perth. The water storage levels in this system have been declining since the start of the 2018 spring irrigation season with minimal recovery in the winter of 2019. There has been a good start to the 2020 winter filling season in both June and July, but a further 100,000 ML is needed to replenish the drawdown of the past two years.
The catchments of the storages of the Harvey system stretch out onto the Darling Plateau where the soil moisture is very low, resulting in limited runoff. July streamflow in the South West Coast drainage division reflects this limited runoff with most gauges recording below average to lowest on record flows. The very much below average streamflow in the Moore River and Gingin Brook north of Perth also reflect the long-term very dry soil conditions in this area.
After a drier than average July across most of the southern half of Australia, root-zone soil moisture (in the top 100 cm) remains below average over very large areas. Soil moisture have decreased across most of south-east Australia and nearly all of South Australia, compared to June.
Soils were drier than average for the month across the southwestern half of Western Australia; nearly all of South Australia, the south-eastern Northern Territory and Top End; most of western and central Queensland, extending through the Central Highlands and Coalfields District; western New South Wales and north-west Victoria.
Soil moisture was above average for July in a few areas, including west of the ranges in New South Wales around the Central Tablelands and areas of Queensland's tropical coast between Cairns and Townsville.
The influence of very low rainfall over longer timescales is still evident in the 12-month soil moisture for August 2019–July 2020, which was very much below average over very large areas of Australia.
Western Murray–Darling Basin yet to see winter soil moisture increases
The root-zone soil moisture in the western Murray–Darling Basin dropped to very much below average in July. This region of the Basin generally contains some of the driest soils on average, but experiences cool, slightly wetter winters which mean that the soil moisture can often recover in the winter season. The record-low rainfall in this area during July has limited this recovery in the 2020 winter season.
Rain in late July brought increases in soil moisture to the east of the Basin, but soil moisture levels mostly remained near average for July.
Long period of dry soils in south-west Western Australia continues
Consistent with the declining rainfall trend in south-west Western Australia, soil moisture levels continue to reach new record lows with the recent dry conditions.
Despite monthly fluctuations and local rain events, the region has been experiencing low soil moisture since April 2018. Root-zone soil moisture deciles for the 28-month period to July 2020 show a significant portion of the South West Coast drainage division has experienced record-low soil moisture over this period. Recent rainfall in the far south-west brought soil moisture levels for July up to average, but more broadly across the south-west monthly soil moisture was below average to lowest on record for this time of year.
- July rainfall was below average for most of southern Australia; above average for areas of eastern Victoria and coastal New South Wales
- Both short- and longer-term rainfall deficiencies have increased in the South West Land Division in Western Australia
- Heavy rainfall has decreased deficiencies in parts of eastern Victoria and south-east New South Wales
- 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 much of south-east Australia
- Soil moisture and streamflow in southwest Western Australia reflect an extended period of dry conditions
- Water storages continue to fill in the southern Murray–Darling Basin but levels remain low in the north
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 11 August, rainfall was recorded in all states and territories. Little or no rainfall was recorded in most of the eastern half of Western Australia, the northern two-thirds of the Northern Territory, and much of northern Queensland.
At the start of the week, a low pressure system moved across southern Western Australia, with localised showers and storms mainly along the south coast. A broad low pressure system developed in western South Australia with an associated trough stretching into central Australia, triggering a rainband over South Australia with embedded thunderstorms producing moderate falls.
Another low pressure system was located to the east of Tasmania, and triggered a cold outbreak over southeastern Australia, with snow falls down to 300 m to 400 m in Tasmania, Victoria and Alpine New South Wales. The low continued to generate showers in Tasmania, Victoria and parts of the New South Wales coast.
During the middle of the week, the low pressure system and trough moved east over South Australia and central Australia, with the associated rainband generating widespread moderate falls over South Australia, the far southern parts of the Northern Territory, southwest Queensland and western New South Wales.
The complex low pressure system, with a frontal system and trough, moved over western Queensland, then western and southern New South Wales. An upper trough over northwest New South Wales, and the associated cold front swept over southeast Queensland, producing showers and thunderstorms over the central Queensland coast, and widespread moderate falls were recorded from central to southeast Queensland, much of New South Wales, northern Victoria and southeast South Australia.
In the last part of the week, the complex low pressure system crossed the New South Wales coast near Gosford, with showers and rain feeding into the southern half of the coast, driven by a mid-level cold trough centred over central northern New South Wales. Moderate falls were recorded along the east coast of New South Wales with heavier falls reported about the Illawarra Coast, and three-day totals to 9am on Monday 10 August exceeding 300 mm around Nowra.
In the west, a strong cold front and associated cloudband tracked over the west coast of Western Australia. Widespread moderate falls were recorded in the western parts of the state from the Pilbara coast and Gascoyne districts, down to most of the South West Land Division in Western Australia. The front and rainband moved eastwards, and brought light to moderate falls to central Australia and southwest South Australia.
Rainfall totals in excess of 200 mm were recorded along the Illawarra and Hunter coasts of New South Wales, including the highest weekly total of 374 mm at Nowra Boat Shed (Shoalhaven River).
Rainfall totals in excess of 100 mm were recorded across most of the South Coast District of New South Wales.
Rainfall totals in excess of 50 mm were recorded along the west coast of Western Australia, in areas of central Australia, southwest and along the central coast of Queensland, pockets of the northwest and much of the southeast of New South Wales, small areas of central Victoria, and southeast Tasmania.
Rainfall totals between 10 mm and 50 mm were recorded from the Pilbara coast to the South West Land Division, along most of the south coast, and inland areas bordering the southwest Northern Territory; southern parts of the Northern Territory; most of South Australia, Victoria, Tasmania and remaining areas of New South Wales; and in western, southern and east coast Queensland.
Impact of recent rainfall on deficits
Rainfall deficits over Australia for the 4-month (April–July 2020) and the 28-month (April 2018–July 2020) periods are discussed in the Drought Statement, issued on 6 August 2020.
Rainfall deficit maps are available for these periods as well as for standard periods. The maps below show the percentage of mean rainfall that has been received for the rainfall deficit period for the 4-month and 28-month periods, extended to the week ending 4 August 2020.
Rainfall for the period 1 April to 11 August 2020
Serious or severe rainfall deficiencies for the 4-month period are in place in Western Australia across most of the southwestern half of the State. Serious rainfall deficiencies are also in place across parts of central southern and southeast Queensland. Other small areas experiencing serious rainfall deficiencies include the southwest of the Alice Springs District in the Northern Territory, and pockets of southwest South Australia.
Rainfall in the past week has eased deficiencies somewhat in affected areas.
Inland areas of Western Australia have received less than 40% of average for the period. Remaining areas in Western Australia, South Australia and parts of central Queensland have generally received less than 80% to 60% of their average for the period.
Rainfall for the period 1 April 2018 to 11 August 2020
Serious to severe rainfall deficiencies are in place for the 28-month period across the South West Land Division, much of the west of the Gascoyne and Pilbara, along the south coast of Western Australia, the northern tip of the Kimberley, the east of the Interior District, and pockets throughout the inland areas of the State. Deficiencies are also in place across much of the Northern Territory, nearly all of South Australia, the southeastern quarter of Queensland, much of western and northern inland parts of New South Wales, and across eastern Victoria and the northern border regions of that state.
Rainfall in the past week has eased deficiencies in parts of Central Australia, eastern South Australia, and western New South Wales.
Affected areas through the interior of Australia into western New South Wales, southern inland Queensland, northeastern South Australia and the southern parts of the Northern Territory have generally received less than 50% of average rainfall for this period. Remaining areas have generally received between 80% and 50% of average rainfall.
Product code: IDCKGRWAR0