During autumn 2018, rainfall was average to extremely low in cropping regions in Queensland and well below average in cropping regions in northern New South Wales, Victoria and South Australia. Rainfall was severely deficient to extremely low in many cropping areas in central and southern New South Wales and Western Australia (Map 1).
For the Australian cropping region, area-averaged rainfall during autumn 2018 was the second lowest on record and the lowest autumn since 2008 (Table 4). In cropping zones in New South Wales and Western Australia, area-average autumn rainfall totals were the sixth and third lowest on record.
Table 4 Area-average rainfall
|Lowest on record|
|May (mm) ||Rank|
|Lowest on record|
|New South Wales||38.8||6||25.6 (2005)||14||22||1.7 (1912)|
|Victoria ||48.8||15||16.6 (2005)||34.3||54||0.4 (1934)|
|Queensland ||60||15||20.4 (1922)||5.6||13||0.7 (1975)|
|South Australia||36.2||12||17.6 (1994)||25.4||37||5.7 (1934)|
|Western Australia||33.3||3||28 (1914)||18.6||12||4.7 (1948)|
|Australia ||43.9||2||36 (1902)||16.2||6||11.2 (2006)|
Notes: Rank ranges from 1 (lowest) to 119 (highest). The figure within brackets in the lowest on record columns refer to the year in which lowest on records area-average rainfall was recorded.
During May 2018, rainfall was generally average in cropping regions in southern New South Wales, Victoria, South Australia and across northern cropping regions in Queensland (Map 2). Rainfall was also close to average in the Geraldton region of Western Australia. However, rainfall in May 2018 was extremely low to well below average in other cropping regions in New South Wales, Queensland and Western Australia. While there were rainfall deficiencies evident in many cropping regions in Western Australia during May, at the monthly scale, rainfall totals of between 10 millimetres and 50 millimetres were recorded on 25 and 26 May in most of these regions.
Follow-up rainfall in June of between 5 millimetres and 50 millimetres is forecast in most Western Australian cropping areas. Similar rainfall totals are forecast for most cropping regions in Victoria, South Australia and southern and central New South Wales (Map 3). Little to no rainfall is forecast for cropping regions in northern New South Wales and Queensland.
Map 4 and Map 5 show the relative levels of modelled upper layer (~0.1 metres) and lower layer (~0.1 to ~1 metres) soil moisture for cropping zones across Australia for May 2018. Soil moisture estimates are relative to the historical long-term average (1911 to 2015) and presented in percentiles.
Upper layer soil moisture responds quickly to seasonal conditions and often shows a pattern that reflects rainfall and temperature events in the days leading up to the analysis date. Lower layer soil moisture is a larger, deeper store that is slower to respond to seasonal conditions and tends to reflect the accumulated effects of events that have occurred over longer periods.
Relative upper layer soil moisture on 30 May 2018 was predominantly average in cropping regions in South Australia and Western Australia (Map 4). It was generally above average to well above average in cropping regions in Victoria. In most cropping regions in New South Wales relative upper layer soil moisture was around average but was extremely low in the northern cropping regions. In southern and central cropping regions in Queensland, relative upper layer soil moisture was generally extremely low to well below average but mostly around average in northern cropping regions.
Relative soil moisture in the lower layer as at 30 May 2018 was generally extremely low to well below average in most cropping regions in New South Wales, variable in South Australia, Western Australia and Queensland and generally average for most cropping regions in Victoria, with the Mallee region being the main exception at well below average (Map 5). Crop establishment and development in areas of well below average lower layer soil moisture will be highly reliant on in crop rainfall during the remainder of the growing season.
The current rainfall outlook reflects the neutral state of major climate drivers, with no strong shift towards wetter or drier conditions across most of Australia. When broadscale drivers are neutral, the climate is often influenced by more local effects, such as the ocean temperatures immediately surrounding the continent. Below average pressure over the Tasman Sea, associated with warmer than average ocean temperatures, are likely to weaken the westerlies that bring rain systems to southeast Australia.
The rainfall outlook scenario for June 2018, indicates that cropping regions in Western Australia have a 50 per cent chance of recording median rainfall for the month, while cropping regions in New South Wales, Victoria, South Australia and Queensland have a 50 per cent chance of recording half their median rainfall. This means cropping areas in Western Australia have a 50 per cent chance of receiving between 25 millimetres in the east and 100 millimetres in the west in June (Map 6). In New South Wales, Victoria and South Australia it means cropping regions have a 50 per cent chance of receiving at least 10 millimetres and possibly up to 50 millimetres of rain during June 2018 and most cropping regions in Queensland have a 50 per cent chance of receiving at least 10 millimetres of rain.
The Bureau of Meteorology’s seasonal rainfall outlook for June to August 2018 indicates that a drier than average winter is likely for most Australian cropping regions (Map 7) The highest chance of exceeding average winter rainfall is in Western Australian cropping regions. Maximum temperatures are likely to be higher than average across southern and western Australia.
If seasonal conditions were to eventuate as forecast, this combination of warm and dry conditions during winter is likely to limit the level of soil moisture that is available for crop development during spring 2018.
Map 8 shows the shire-scale forecast of wheat yields obtained from the University of Queensland’s Queensland Alliance for Agriculture and Food Innovation (QAAFI). These forecasts are based on soil moisture conditions and the seasonal outlook, including the most recent trend in the Southern Oscillation Index (SOI).
At the beginning of June 2018, the probability of exceeding median wheat yields was generally below average to average in most cropping regions across northern and southern Victoria, southern New South Wales, South Australia, Queensland and some northern and southern cropping regions in Western Australia (between 20 per cent and 60 per cent). Central and eastern cropping regions in Western Australia, parts of northern and southern Queensland, central and northern New South Wales, central Victoria and scattered shires in South Australia show reduced chances of exceeding median yield (between 0 per cent and 20 per cent). This pattern is largely consistent with rainfall and soil moisture conditions as at the end of May 2018.
Table 5 Rainfall in major cropping districts, median and actual, March 2018 to May 2018
New South Wales|
|NW Plains (W)||52||34||26||25||21||27||1|
|NW Plains (E)||53||44||34||27||15||32||3|
|NW Slopes (N)||54||54||41||30||19||37||2|
|NW Slopes (S)||55||45||32||32||14||34||5|
|N Tablelands (N)||56||72||44||38||29||40||5|
|CW Plains (S)||50||26||4||23||7||29||14|
|CW Plains (N)||51||27||15||17||10||28||7|
|CW Slopes (N)||64||41||43||32||8||34||10|
|CW Slopes (S)||65||37||8||33||12||38||23|
|C Tablelands (N)||62||46||46||37||16||36||15|
|C Tablelands (S)||63||51||39||50||15||42||24|
|SW Slopes (N)||73||37||8||35||10||43||34|
|SW Slopes (S)||72||56||27||63||32||77||61|
|Lower North East||82||52||39||57||27||85||67|
|W Darling Downs||42||50||41||23||20||29||6|
|E Darling Downs||41||54||45||23||11||29||7|
|Moreton S Coast||40||98||83||56||34||48||39|
|Upper South East||25B||14||5||30||12||45||42|
Note: Median rainfall is calculated over the period 1900 to May 2018. Australian rainfall districts are shown in Map 9 of the Australian crop report.
Source: Bureau of Meteorology monthly district rainfall reports