Issued 6 May 2016
April rainfall reduces deficiencies in some areas
Rainfall in April was below average to well below average in eastern and northern Australia, apart from some small areas in northern and western New South Wales and Far North Queensland. Totals were above average in the southwest of Western Australia, causing a reduction in deficiencies in that area compared to the previous drought statement Some easing of shorter time scale deficiencies in some areas of northern Australia has occurred, but there was little effect on the deficiencies on the longest time scale.
Rainfall in eastern Australia, especially in the southeast and over large parts of Queensland, has generally been very much below average for the period since the conclusion of the last La Niña in autumn 2012. The past 48 months has also been the warmest such period on record, which is likely to have added to the impact of the rainfall deficiencies. Long-term deficiencies also exist in southeastern and southwestern Australia over the 16 years since 2000, which has also been the warmest such period on record.
12-month rainfall deficiencies
As a result of rainfall in April most areas of deficiencies in the southwest of Western Australia have seen a slight contraction of the area covered, but there have been small increases in the area of severe deficiency. Meanwhile, the total area of deficiencies northeast of Darwin has contracted from the south towards the coast but the area of severe deficiency has expanded.
The area of rainfall deficiency in the Gulf of Carpentaria that straddles the border of Queensland and the Northern Territory has shown a reduction in extent and the severity has eased. Areas in central Queensland and the southeast of the country have seen an increase in the area of deficiencies.
There has been an emergence of small areas of deficiency around the central Queensland interior, which will continue to be monitored. Central to western Victoria has seen a worsening of deficiencies, both in extent and severity.
This extends into southeast South Australia. South Australia's Mount Lofty Ranges, where a small area of deficiency has been in place, shows increased severity in this time-scale compared to the last drought statement and a new area of deficiency has emerged on the Yorke Peninsula.
Across Tasmania there has been an extension of serious to severe deficiencies, which now cover much of the State, with areas of lowest on record evident in the west. April was another dry month for this State.
22-month rainfall deficiencies
The rainfall in April had little effect on the deficiencies in the period starting July 2014 in northern and eastern Australia. Rainfall deficiencies in eastern Australia have in general worsened in both extent and severity. However, above average rainfall in the southwest of Western Australia in April has contributed to further contraction of the affected area there.
In the Northern Territory there has been expansion in all rainfall deficiencies away from the coast. In central Queensland, the area of deficiency has expanded, the severity has worsened, the area covered by severe deficiency has expanded and there is a small area of lowest on record deficiency that was not evident on the previous drought statement. Elsewhere in Queensland there has been an expansion of serious deficiency across southern Cape York and in the western border area of the Gulf of Carpentaria.
In Victoria the rainfall deficiencies in the southwest of the State have expanded. Almost the entire western half of the State is experiencing severe rainfall deficiency, with a large area of lowest on record rainfall observed for this time period. These deficiencies extend to the southeast of South Australia and some southern coastal regions of the State.
The rainfall deficiency in Tasmania has expanded to cover much of the State. In the southwest and far northwest of Tasmania the severe deficiency has worsened to lowest on record along the coast, while central parts of the State saw deficiencies both increase in extent and severity. Much of this area that was previously in serious deficiency is now in severe deficiency and there are some areas of lowest on record deficiencies.
43-month rainfall deficiencies
The period starting in October 2012, which now runs to 43 months, show serious to severe rainfall deficiencies have slightly increased in severity and slightly extended in Queensland over the last month.
Areas of serious to severe deficiency remain through inland Queensland and into northern New South Wales. Serious to severe deficiencies expanded westward over the Northern Slopes and Tablelands of New South Wales. Serious to severe deficiencies are also evident at the 43-month scale in a zone from southeastern South Australia, across the western half of Victoria, and in parts of western and southeast Tasmania.
Areas of serious deficiency have contracted but still remain in southwest Western Australia, and the Kimberley.
Very long term deficiencies persist
The rainfall deficiencies in southern Australia at periods up to 43 months are also evident on much longer timescales. The map of rainfall deficiencies for the past 16 years shows record low rainfall over most of southwest Western Australia, with serious to severe deficiencies widespread in northern and central Victoria, neighbouring parts of South Australia and New South Wales, and northern and western Tasmania. Patchy deficiencies are also evident in Queensland, most notably in western parts of the Darling Downs.
The affected regions typically receive significant rainfall during the period from autumn through spring from cold fronts and low pressure systems. However, Australia has experienced a substantial decrease in this activity over recent decades, as high pressure systems have become more dominant. This suggests the tendency for recurrent dry conditions is less related to variations such as El Niño, increasingly due to climate change. Research suggests that long-term drying trends over southern Australia cannot be explained by natural variability alone.
These very long term deficiencies are most significant for long-term processes, though recurrent dry seasons may have direct agricultural impacts. These longer-term processes include recharge of surface and ground water storages, and the drying of forest fuels. It is noteworthy that rainfall in northern Australia has been very high during the same period, and particularly so in northwest Australia.
Soil moisture for April 2016 in the lower layer (from 10 cm to 100 cm deep) is very much below average, over almost all of Tasmania, in areas of the far northwest of the country, in southern parts of Cape York Peninsula and in patches along the Far North Queensland coast. The below average soil moisture in southeastern Queensland has extended into the State's central western interior and here new areas of very much below average have emerged in isolated patches. An area of very much below average soil moisture in northern New South Wales has shown some contraction.
Meanwhile soil moisture is below average over much of southeastern Australia and there are new areas of very much below average soil moisture that have emerged in isolated patches here. This is a marked change from last month when much of southeastern Australia had average soil moisture. In central Australia the eastern and western border regions, above average soil moisture areas continue to contract. About the Gascoyne coast, southern parts of the Pilbara and the adjacent interior, areas of very much below average soil moisture have expanded and areas of below average soil moisture have extended away from the coast. Soil moisture has remained below average for much of the Top End.
Conversely, soil moisture in much of the Kimberley has become average in the north and there is an area of above average soil moisture confined to the northern coast. There remains very much above average soil moisture in much of South Australia, northwestern Queensland and in the southeast and southwest of Western Australia. There are small areas of highest on record soil moisture in the southwest of Western Australia.
Soil moisture information presented here is from the Bureau's operational Australian Water Resources Assessment Landscape (AWRA-L) model.
Product Code IDCKGD0AR0
The Weekly Rainfall Update describes rainfall over the previous week. It includes a map and a summary table of the highest weekly totals. A discussion of the impact of recent rains on rainfall deficiencies is also presented.
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Australian Water Resources Assessment Landscape maps include maps of soil moisture and water fluxes contributing to changes in soil moisture (rainfall, transpiration, soil evaporation, surface runoff and deep drainage).
What is drought?
Drought is a prolonged, abnormally dry period when the amount of available water is insufficient to meet our normal use. Drought is not simply low rainfall; if it was, much of inland Australia would be in almost perpetual drought. Because people use water in so many different ways, there is no universal definition of drought. Meteorologists monitor the extent and severity of drought in terms of rainfall deficiencies. Agriculturalists rate the impact on primary industries, hydrologists compare ground water levels, and sociologists define it by social expectations and perceptions.
It is generally difficult to compare one drought to another, since each drought differs in the seasonality, location, spatial extent and duration of the associated rainfall deficiencies. Additionally, each drought is accompanied by varying temperatures and soil moisture deficits.
Rainfall averages, variability and trends
Lowest on record – lowest in the historical analysis, which runs from 1900.
Severe deficiency – rainfalls in the lowest 5% of historical totals.
Serious deficiency – rainfalls in the lowest 10% of historical totals, but not in the lowest 5%.
Very much below average – rainfalls in the lowest 10% of historical totals.
Below average – rainfalls in the lowest 30% of historical totals, but not in the lowest 10%.
Average – rainfalls in the middle 40% of historical totals.
Above average – rainfalls in the highest 30% of historical totals, but not in the highest 10%.
Very much above average – rainfalls in the highest 10% of historical totals.