Climate outlook for the coming growing season

Ground Cover asked lan Holton in South Australia to provide the weather prognosis for the southern states in winter-spring (see facing page). Meanwhile, the APSRU team refined its climate analysis based on the SOI and offered the following outlook for likely wheat yields across the Australian wheatbelt. We leave it to you, the reader, to take what you care to from both systems. It seems to us that they are largely complementary.

The 30-day SOI to mid-June was -27. The associated reduction in average rainfall is strongest in north-eastern Australia but also affects south-western Australia, the Eyre Peninsula and parts of Tasmania.

The probable effects of this rainfall pattern on wheat yields is shown in the table below.

Expected reduction in median yield compared to Chances of exceeding median yield
LocationEntire record1964-93Entire record1964-93
Wagga Wagga13%18%25%13%
Wongan Hills18%19%36%35%

The researchers used negative May SOI phases from the last 100 years of rainfall records to derive a 50 per cent (median or average) yield level and compared it with 1997. Eleven locations were chosen through the Australian wheat belt.

The 1960s and 1970s were characterised by above-average rainfall in many regions which will have influenced producers' perceptions about the underlying regional rainfall variability. The impact of these 'good years' (i.e.: above-average years) has influenced the average for the last 30 years. On our wheat yield table this shows up as an even greater reduction in expected yield when 1997 is compared with 1964-93.

This reduction ranges 12-43 per cent depending on location.

The table reflects probabilities. This means that, although the current outlook is for a high probability of below-average yields, there is still a low chance, hovering around 30 per cent, of exceeding the average yield.

Local factors such as stored soil moisture will make a considerable difference and dampen the negative effects of insufficient rainfall. Growers should carefully assess their soil moisture conditions when judging likely yields in the coming season.

Rainfall and decision-making when the crop is in the ground

The climatic outlook and yield probabilities indicate growers should take a cautious approach to estimating likely yield for forward selling. They should weigh up carefully the use of nitrogen fertiliser because the season could worsen and a more economical alternative to a poor milling wheat crop could be to use it as a fodder crop instead, considering that fodder might be quite valuable in a dry season.

The climate outlook also suggests careful planning for the whole rotation because a negative SOI phase by the end of May indicates a strong chance of below average rain until at least autumn the following year.

Based on information supplied by Dr Holger Meinke, Agricultural Production Systems Research Unit, Toowoomba.