SOI Forecasts: This could be the big one

Diagram showing the probability of rainfall in areas of Australia exceeding their annual median rainfall for June through to August.

This year is shaping up to be one of the few years when the majority of Australian graingrowers will have their best chance to make a good profit.

During the last decade climate predictions based on the Southern Oscillation Index (SOI) have mostly been warnings of lower-than-average rainfall in back-to-back El Niños 1991-95 and another in 1997 into the early part of this year.

All that may have changed with a significant rise in the SOI between April and May, signalling that the 1998-99 cropping year has a good chance of being excellent.

The El Nino pattern of the past year appears to be coming to an end, following the rapid rise of the SOI in May and changes in key sea-surface temperature.

The El Niño pattern is known to endure from autumn to autumn, so May is a crucial month for predictive purposes if a change is in the offing.

The rapidly rising SOI pattern through April-May is likely to produce above-average rainfall for many areas during winter. The pattern is similar to SOI patterns in the autumns of 1978, 1983 and 1988, which were all high-yield years.

On the basis of pasture and cropping models, it appears all grazing and cropping areas of Australia will have a good chance of average or above-average rainfall through winter, spring and summer.

Record shows chance of SOI slipping back is unlikely

Since April, the SOI has risen from deeply negative values to around zero. This is the best pattern we could hope for through autumn. Based on the historical record, there is only a 10 per cent chance that the SOI will drop back into negative values.

Of the 29 times in the last 100 years when the SOI has risen rapidly in May, it has never rapidly fallen again in September. Based on the record, low rainfall in spring is unlikely.

However, there are no guarantees in this business. Growers should be aware that all climate predictions are based on probabilities. There are a few years in the climatic record when a rapidly rising SOI in April-May resulted in below-average rainfall.

Growers should continue monitoring the SOI and updated forecasts through the season.

On the other hand...

Above-average rainfall may also mean waterlogging and the risk of rain at harvest time. Growers may want to consider harvesting early and drying grain this year. Taking into account the possible downsides of above-average rainfall, probabilities are as good as they get that this will be a bumper year for graingrowers. Grain farmers should prepare for the best possible crop yields as the probability grows for the best winter, spring and summer rainfall seasons in a decade.

Analysis of past weather records shows that a rapidly rising SOI through April-May has the potential to produce national wheat yields 22 per cent above average, an increase of about three million tonnes, worth around $450 million.

Growers should make sure their nitrogen applications and other crop management strategies are appropriate for wetter years with high yields. That's the advice from systems analyst Holger Meinke, who collaborates with the climatologists and crop physiologists at the Agricultural Production Systems Research Unit (APSRU) and the Queensland Centre for Climate Applications (OCCA).*

"The costs you may incur if you don't get the expected rain are much less than the profit that you would lose if you don't manage the crop well in potentially good years," said Dr Meinke.

Based on rainfall probabilities, he said growers could manage for a record crop, an opportunity that doesn't happen very often.

Applying more fertiliser is a key to management in wetter years, improving growers' chances of achieving top yield and protein goals.

Growers could also consider cropping larger areas and changing crop rotations to take advantage of any opportunities for double-cropping.

"Increasing cropping intensity is one of the best ways to prevent the excess run-off and erosion that can occur in wetter years," Dr Meinke said.

Once an SOI pattern is established by early winter, it usually persists until the following autumn. Chances are that planting rains in summer-cropping regions might start a few weeks earlier than normal and follow-up rains could be more frequent and reliable.

"This could provide an opportunity for shortening the fallow and planting higher-value summer crops."

The downside

Growers should be aware of the potential for waterlogging, especially in southern NSW and on duplex soils in Western Australia. Above-average rain at harvest could also lead to problems in some regions and growers might want to consider harvesting early and drying their crops.

APSRU crop physiologist Graeme Hammer said weed, pest and disease control become more important in wetter years. Growers should monitor carefully, plant resistant varieties (for summer crops now), and factor in the cost of pest and disease management.

This can be especially important in areas with winter and summer cropping where pest populations can build up rapidly.

What it means for frost

When an SOI enters a rapidly rising phase, frost risk is lower. Early frosts are likely to begin later and late frosts are likely to end earlier than the average date of the last frost.

For more detailed information on regional management decisions for growers, APSRU recommends the Wheatman computer program, available from Queensland Department of Primary Industries.

Region National, North, South, West