Improving Seasonal forecasts for SWWA

Product Information

More than half of the variation in gross margins in wheat cropping in south-west Western Australia (SWWA) can be explained by growing season rainfall variability (May to October). Forecasting seasonal rainfall should therefore enable the adjustment of management practices to maximise returns from ‘good’ seasons and minimise losses from ‘bad’ seasons. Growing season rainfall forecasts from Australia’s seasonal climate forecast model POAMA 1.5 (Predictive Ocean Atmosphere Model for Australia) indicated some skill with a linear correlation of 0.3 to 0.4 with observed rainfall data in some parts of the south of SWWA, but less in the north of SWWA.

The percentage of correct forecasts in a two-category forecast (above and below average rainfall) reached up to 70% in the south of SWWA which was close to the highest values from eastern Australia. When benchmarking the POAMA forecast with other global circulation model forecasts and statistical forecasts for SWWA, POAMA showed significantly higher skill for the south of SWWA, but similar low or less skill for the north of SWWA.

The POAMA forecast was then applied to management decisions on N fertiliser applications in wheat in the south of SWWA. The forecast produced up to $A57/ha/year higher returns by varying N applications according to above and below average seasons compared to a constant fertiliser application. The current lack of forecast skill for the north of SWWA resulted in no value in using a forecast for N management decisions in this region. The benefits from using the POAMA forecast in the south of SWWA will vary with increasing wheat prices and cost of fertiliser and differs for soil types and initial soil water conditions.