Rainfall deficiencies and water availability
Deficiencies ease in many affected areas following March rainfall
Above average rainfall for March 2021 resulted in a substantial lessening of deficiencies across much of Western Australia, and the southeast quarter of Queensland. However some pockets of rainfall deficiencies remain in southwest Western Australia and in some coastal areas in southern Queensland.
Rainfall in the past month has contributed further to gradual recovery from longer term multi-year rainfall deficits in parts of Western Australia, while in Queensland, shorter term deficits have eased but multi-year rainfall deficits persist. Deficiencies for the periods January 2017 to present and January 2018 to present still exist over very large parts of the country. More rainfall is needed over an extended period to continue the recovery from the extended dry conditions of 2017 to 2019.
The Climate Outlook, issued 8 April, indicates rainfall during May to July is likely to be below average across parts of northern Australia, though we are coming into the drier time of the year in that region. Some parts of the eastern Australia quarter are showing increased liklihood of below average rainfall.
12-month rainfall deficiencies
In Western Australia, areas of rainfall deficiencies have substantially reduced to small pockets compared to last month's Drought Statement following above average March rainfall. Serious or severe rainfall deficiencies for the 12-month period April 2020 to March 2021 remain in pockets of the Central West, Great Southern and Goldfields districts.
In Queensland, following above average March rainfall in the state's south, the areas and intensity of rainfall deficiencies have also lessened substantially. Deficiencies still remain in the Capricornia and Wide Bay and Burnett districts.
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 January 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 and south coasts 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, though rainfall in 2020and in early 2021 has seen improvement across many areas. Above-average February and March rainfall has eased longer-term deficiencies in the inland west of Western Australia. In Queensland and New South Wales into South Australia the severity of multi-year rainfall deficiencies has also eased with March rainfall easing shorter term deficiencies in northern parts of the Murray–Darling Basin and providing increases to water storages in that region, though storage levels in south-east Queensland remain low.
Persistent, widespread, above average rainfall is needed to further lift areas out of deficiency at the multi-year timescales and provide relief from the impacts of this long period of low rainfall (such as further renewing water storages). Recent improvement to water storages in northern parts of the Murray–Darling Basin, have provided some recovery. With saturated catchments further rainfall will continue to assist in replenishing water storages.
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 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.
Compared to last month, root-zone soil moisture (in the top 100 cm) has decreased across south much of Victoria, southern parts of South Australia, the Cape York Peninsula in Queensland, the Top End in the Northern Territory, and the Pilbara and Gascoyne districts in Western Australia.
Soil moisture was below average for part of eastern Queensland extending from around Mackay into the Wide Bay and Burnett region. Below average soil moisture emerged in western Victoria. Soils were wetter than average for the month across much of New South Wales except in the south-west, southern parts of Queensland, the northern parts of South Australia, the south and west of the Northern Territory, and parts of west and east of Western Australia.
The influence of very low rainfall over longer timescales is still evident in the 18-month soil moisture for October 2019 to March 2021, which was very much below average over large areas, predominantly in the southern half of Western Australia, but also in parts of Queensland extending from the east coast through central and northern Queensland to the Northern Territory border. In areas
- Rainfall deficiencies have eased in Queensland, and are now confined to coastal areas of the south
- Deficiencies have also eased further in parts of south-western to central regions of Western Australia, with only small pockets remaining
- Accumulated rainfall deficits at multi-year timescales remain significant in many parts of Australia, and may persist for some time
- Water storage levels in the northern Murray–Darling Basin increased significantly while in northern Australia they reached above 80% of accessible capacity
- South-east Queensland storages remain low while Sydney's major storage spills following March rainfall
Storage levels in the northern Murray–Darling Basin increased significantly
Significant amounts of rainfall which occurred in the second half of March have greatly increased soil moisture levels to very much above to above average in most of the Murray–Darling Basin, especially in the north. This has brought relief to drought affected catchments including the Border Rivers, Namoi and Gwydir in New South Wales. Saturated soil moisture conditions facilitated runoff and inflows to the northern storages.
The total water storage in the Murray–Darling Basin increased in March to 57% of capacity at the end of the month, an increase of 3.8% of capacity since last month. Compared to the same time last year, when capacity was only 33%, the total storage volume remains in a significantly better position.
During March, the highest and third highest daily increases in combined storage volumes were experienced in the northern Basin since 1993. Total water storage in the northern Basin increased significantly by 21% to 47.1% of capacity (2,179 GL) at the end of March, which is much higher than the same time last year when it was only 13.6%. In response to the above to very much above average rainfall occurring over much of the northern Basin in March, all the storages increased. The greatest increase was for Chinchilla which had fallen to zero accessible storage and rose to 91% of accessible storage. Coolmunda (+84%) and Lake Kajarabie (Beardmore Dam) (+51%) have reached their full accessible capacity. Other storages observing significant increases in volume are Pindari (+36%), Glenlyon (+35%), Keepit (+23%), Copeton (+22%), Leslie (+17%), Burrendong (+16%), Split Rock (+15%), and Chaffey (+15%).
Despite above to very much above average rainfall across the New South Wales part of the southern Basin, the total storage in the southern Basin remains almost the same as last month; at 59% (12,149 GL) of capacity which is still significantly higher than March 2020 when it was only 37.1%. The highest percentage increase in the volumes of the major storages in March were for Burrinjuck (19%), and Wyangala (10%).
Further detail on individual Murray–Darling Basin catchments can be found in the fortnightly Water Reporting Summaries for MDB Catchments.
Northern Australia water storage levels continued to increase significantly and reached above 80% of accessible capacity
The northern Australian monsoon season commenced in December and since then above average rainfall has occurred. In response, the volume of water in two of the major water storages in northern Australia has increased significantly. Water levels in Lake Argyle, the largest water supply storage in Australia (10,400 GL) increased significantly this year after the past three failed wet seasons. This year's monsoon started to bring relief and the gradual wet season filling escalated in March to increase Lake Argyle's storage volume by 26.6% to reach 82.1% of accessible capacity. In a similar way, the Darwin River storage increased by 10.4% in March to reach 90.5% of its capacity.
South East Queensland storages remain low
Despite good rainfall across most of the continent, the storages in south eastern Queensland did not benefit from the rainfall in March. The volume of water in the largest storage in the area, Wivenhoe, had a very minor increase to reach 36.9% of capacity at the end of the month. This is the opposite to Sydney where storage volumes reached close to full capacity with the major storage for the city - Warragamba Dam - spilling for a few days.
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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 13 April 2021, rainfall was recorded in the western half of Western Australia including the west Pilbara, west and south Gascoyne, South West Land Division, and southern Goldfields District. Rainfall was also recorded in the coastal Top End in the Northern Territory, the Cape York Peninsula and eastern Queensland, along the coast in New South Wales, the southern half and north-east of Victoria, and most of Tasmania. The far south-east corner of South Australia recorded some falls, though the rest of the state saw little or no rainfall.
At the start of the week, a trough and associated low pressure system off the south-east coast in Queensland moved further offshore. Onshore winds led to moderate rainfall in Cape York Peninsula, the north tropical coast, inland central and south-east Queensland, and along the central and north coast of New South Wales.
Tropical cyclone Seroja was located north of the west Pilbara coast early in the week, with a deepening tropical low not far away to its west. A cloudband extended from the system south-east to the Great Australian Bight, generating light to moderate falls along the southern coastlines in Western Australia.
From the mid-week onward, Seroja interacted with the tropical low (this low later became the shorted-lived tropical cyclone Odette) and intensified to severe (category 3 strength) before tracking south-east towards the west coast of Western Australia. Severe tropical cyclone Seroja crossed the coast just south of Kalbarri in the Central west Coast District on the 11th, then tracked rapidly across eastern parts of the South West Land Division before moving into the Bight as a low pressure system. Moderate rainfall was recorded in the west Pilbara prior to the landfall. Widespread moderate daily rainfall was recorded in the west and south Gascoyne District, most of the South West Land Division, and southern Goldfields District on the 12th. Heavier daily falls between 50 and 100 mm were observed near Seroja and on its south-western side.
In the country's south-east, a number of cold fronts produced a number of days of moderate rainfall in western Tasmania. Light falls were reported in eastern Tasmania, and light to moderate falls in southern, central, and eastern Victoria. In the south-east this was likely the most significant cold outbreak in the first half of April since 1995. Snow fell across the southern Alps, with light flurries in some lower elevations including the Dandenong Ranges.
In the tropics, rainfall was mostly confined to the north-west Top End in the northern Territory and Cape York Peninsula in Queensland.
Rainfall totals between 50 mm and 100 mm were recorded in the west Pilbara, the Central West, South West and Great Southern districts in Western Australia, spots in the north-western Northern Territory, the Cape York Peninsula, the north tropical coast and south-east coast of Queensland, the central and northern coasts of New South Wales, the central and alpine regions of Victoria, and western Tasmania. The highest weekly rainfall total was 114.2 mm at Worsley Downs in the South West District in Western Australia.
Rainfall totals between 10 mm and 50 mm were recorded in the western half of Western Australia, along the coast in the Northern Territory, in the far north and parts of eastern Queensland, along the central and northern coasts of New South Wales, in southern, central and north-eastern Victoria, and the west and south of Tasmania. The far south-eastern corner of South Australia recorded a little more than 10 mm for the week.
Impact of recent rainfall on deficits
Rainfall deficits over Australia for the 12-month (April 2020–March 2021) period are discussed in the Drought Statement, issued on 8 April 2021.
Rainfall deficit maps are available for this period as well as for standard periods. The map below shows the percentage of mean rainfall that has been received for the rainfall deficit period starting April 2020 and extended to the week ending 13 April 2021.
Rainfall for the period 1 April 2020 to 13 April 2021
Serious or severe rainfall deficiencies for the 12-month period April 2020 to March 2021 remain in pockets of the Central West, Great Southern and Goldfields districts in Western Australia. Deficiencies still remain in the Capricornia and Wide Bay and Burnett districts in Queensland.
Rainfall in the past week eased rainfall deficiencies in the Central West and Great Southern districts in Western Australia.
Affected areas in the Goldfields in Western Australia have received less than 50% of their average for the period. Queensland's Capricornia and Wide Bay and Burnett districts have generally received less than 70% of their average for the period.
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