Climatic and agronomic conditions
Australian Crop Report: September edition
May to July rainfall was average to above average in most cropping regions in Victoria, Queensland and South Australia. In cropping regions in New South Wales and Western Australia, May to July rainfall ranged from average to extremely high (Map 1).
August rainfall was generally below average to average in many cropping regions (Map 2). Average August rainfall boosted soil moisture in some cropping regions in central and northern New South Wales. This moisture will support the final stages of winter crop development and the early planting of summer crops in these regions.
Map 3 shows the relative levels of modelled root zone (0 to ~1 metres) soil moisture for cropping zones across Australia on 29 August 2021. Soil moisture estimates are relative to the historical long-term average (1911 to 2016) and presented in percentiles.
Relative root zone soil moisture on 29 August 2021 was around average to above average for this time of year in most cropping regions. In contrast root zone soil moisture was below average to extremely low for this time of year in northern cropping regions in Queensland and Western Australia and the Mallee in South Australia (Map 3).
The rainfall outlook presented in Map 4 provides an indication of how favourable conditions for agricultural production are likely to be over spring. The latest three-month rainfall outlook (September to November), issued by the Bureau of Meteorology on 2 September 2021, suggests that spring rainfall is very likely to be above average in most Australian cropping regions in the eastern states and South Australia. However, in Western Australia there are roughly equal chances of higher or lower than average spring rainfall in most central cropping regions and below average spring rainfall is more likely in some western cropping regions in Western Australia. It is likely rainfall will not be evenly distributed in most cropping regions over spring with a lower chance of exceeding average rainfall in September compared to October and November.
There is a 75% chance of between 25 and 100 millimetres in most cropping regions in Australia during September and October (Map 5). It is highly likely spring rainfall totals will be enough to maintain average to above average crop yields through to harvest in regions where crops were in a good position at the end of winter.
However, it is more likely that most cropping regions in Western Australia and northern cropping regions in Queensland will receive less rainfall. There is a 75% chance of rainfall between 10 and 50 millimetres. In the small minority of cropping areas with below average levels of soil moisture at the start of spring, there is a risk that these probable September to October rainfall totals may result in lower yields than currently forecast.
The outlook for maximum and minimum temperatures for September to November 2021 indicates that night-time and daytime temperatures are likely to be around average in spring in most cropping regions. The current temperature outlook for spring will continue to benefit plant growth, particularly benefiting later sown crops. Mild spring temperatures also decrease the probability of a late frost potentially impacting yields at flowering, and heat stress during grain fill.
Potential crop yield is determined by soil moisture at planting and rainfall received during the growing season. Estimates of water availability over a growing season provide an indication of potential crop yield which can be used to inform crop production forecasts. Map 6 shows modelled water availability levels that have a high chance of occurring by the end of October 2021.
On average, the total water requirement to achieve the national 5-year average wheat yield of 2.0 tonnes/ha is 235 millimetres, based on a conversion rate of 16kg of wheat per millimetre of water and a standard soil evaporation loss factor of 110 millimetres. The total water requirement to achieve 3.0 tonnes/ha, 2.5 tonnes/ha, 1.5 tonnes/ha, 1.0 tonnes/ha and 0.5 tonnes/ha based on this same conversion rate have been estimated to be 295, 265, 205, 175 and 140 millimetres, respectively.
The crop yield associated with a specific level of water availability varies across regions with variations in soil characteristics. The implications for yield of the analysed threshold values of water availability will differ across regions, as responsiveness of crop yield to soil water availability depends on factors such as temperature, humidity, soil nutrition and the timing of rainfall.
ABARES estimated the winter cropping areas likely to achieve 295, 265, 235, 205, 175 and 140 millimetres of water availability. These indicative estimates are based on modelled plant available soil moisture as at 30 April 2021, recorded rainfall totals as at 21 August 2021, and an estimate of rainfall totals with a 75% chance of falling during September and October derived from the Bureau of Meteorology rainfall outlook released on 19 August 2021.
As at 21 August 2021, the analysis presented in Map 6 indicates average or above yield potential across much of New South Wales, southern Victoria, south-eastern Queensland, central and southern South Australia and much of Western Australia. This average or above yield potential is highly dependent on the timing and intensity of rainfall event during the remainder of the growing season, particularly in Western Australia where soils have a lower moisture holding capacity. Across parts of northern Victoria, northern Queensland and eastern South Australia, modelled water availability analysis indicates that below average yield potentials are more likely by the end of October 2021.