Activity to June 2007

Soil sampling to provide for a full physical and chemical analysis of these sites has been undertaken, and those analyses are being put into Yield Prophet to provide the model's yield predictions for futue releases of this Bulletin.

The Season to Date

Conditions pointing to an average to above average wheatbelt season have been developing this year. In March we moved from El Nino conditions to neutral. The weak sea surface temperature gradient in the Indian Ocean is continuing to gain strength to the west/northwest of WA after the cooling that resulted from late Summer cyclones. And a gradual, though erratic, trend towards below-normal air pressure over southern Australia has occurred.

Esperance appears to be the first to benefit from these improved conditions and to state the obvious has had a brilliant start to the season. In most locations rainfall is tracking at about decile 9 at the time of print. To the west at Mt Madden the situation is less favourable but is still tracking at about decile 5. What's a decile?

Seasonal Outlook

The May-June outlook has been released by the Department. Neutral ENSO conditions were observed in the Pacific in April, and were consistent with a high chance (70-90%) of a La Niña event developing later in the season. In our region, neutral conditions have favoured a wet year in most locations. A look at past years with the same conditions (1942, 1952, 1970, 1978 and 1988, called Analogue Years) indicate that at least average rainfall for WA for the May-October period is likely, with above average rainfall likely in more southern and south eastern regions. An enhanced sea surface temperature gradient is gradually developing to the west/northwest of Australia and this should contribute to a gradual improvement in northwest cloud bands and moisture inflow from the northwest. A gradual, though erratic, trend towards below-normal air pressure over southern Australia has occured and shoud be enhanced in winter if a La Niña develops. When this occurs, cold fronts and low pressure systems should be more effective in bringing rainfall to cropping areas in winter and spring.

Average to above average rainfall is also likely across the eastern Australian Wheatbelt for the same period. See www.agric.wa.gov.au/climate for more detail.

Growers should be cautious about the level of confidence they place in these predictions. The predictive value of analogue years based on ENSO modelling varies across regions, and in our region has been less reliable than elsewhere. Never the less, rainfall records over 50 years for Esperance suggest that years with a neutral El Nino generally are wetter than average.

These graphs show the cumulative rainfall since April (for each location) against longer term deciles.

	What's a decile? Rainfall deciles are a method of describing the distribution of historical rainfall over fixed periods of time. They are useful for comparing recent rainfall with past rainfall, to express the rarity (or otherwise) of a given amount of rain. Decile 9 means that 9 out of 10 years will have this amount or less, decile 5 means 5 out of 10 years will have at least this amount of rainfall and so on. Decile 5 is the median rainfall but for winter rainfall is close to the average.

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