AgMemo Northern Agricultural Region

Below average winter rainfall forecast

Soil water map of SW western Australia representing April 2017
Plant available soil water at 24 April 2017, estimated using a simple two-layer model and assuming fallow conditions. The model uses daily rain from BoM and DAFWA stations since November 2016.

Western Australian grain growers have been advised to plan their 2017 cropping program carefully, with below average rainfall forecast during the growing season.

There is consistency between 10 out of 11 Australian and international climate forecasts, with none expecting wetter than normal conditions for April to June, suggesting greater confidence in the outlook.

Department of Agriculture and Food, Western Australia (DAFWA) Research Officer Meredith Guthrie said the strong agreement between models was unusual, especially early in the season.

The forecast is driven by the combined effect of a predicted El Nino event and the emergence of a positive Indian Ocean Dipole (IOD) by August, which is historically associated with below average rainfall.

In past years where an El Nino and a positive IOD have occurred together, the negative impact on rainfall became stronger in the South West and southern parts of Western Australia.

Department Research Officer David Ferris said the outlook for drier conditions, combined with varying amounts of sub-soil moisture across the regions due to widespread summer rainfall, could make it tricky for growers to roll out their cropping programs.

Currently, plant available soil moisture is less in the Northern Agricultural Region compared with the Central and South Coast Regions, where soil moisture levels are high.

Those areas with good soil moisture levels can still anticipate above average yields, in spite of the projected forecast for below average rainfall – provided crops emerge early or are sown into moist soils in May.

Growers who took the opportunity to deep rip compacted soils after summer rain will also benefit from crop roots accessing more of the soil profile this season.

Early, wet-seeding opportunities for canola have largely passed, as the soil surface has dried out, except where storms have occurred.

Canola growers should not to be tempted to chase soil moisture by seeding deeper than one centimetre, as results from early sown canola trials have consistently shown less emergence and lower yields with increased seeding depth.

With a drier than average outlook for April to June, growers in areas with limited stored moisture should manage their economic risk by delaying the bulk of nitrogen fertiliser applications until the season’s potential becomes clearer. 

The department’s Statistical Seasonal Forecast (SSF) system is indicating a 30-60% chance of exceeding average rainfall across the grainbelt for the three months April to June.

The SSF indicates rainfall is most likely to be in the 2-3 decile range, the second lowest, based on a poor to good predictive skill (confidence in the forecast).

The Bureau of Meteorology’s current seasonal outlook is consistent, which predicts a 20-35% chance of exceeding average rainfall from April to June, based on a predictive skill of moderate to good.

The Bureau expects early season temperatures to be warmer than normal, with a 60-70% chance of above normal day-time maxima, based on moderate to good predictive skill.

The department has several free online cropping decision aids located on its Climate and weather page.

These include links to weather stations, rainfall to date tool, soil water tool and potential yield tool.

For more information contact:

Dr Meredith Guthrie, Research Officer (climate), South Perth on +61 (0)8 9368 3058.

Dr David Ferris, Research Officer (agronomy/farming systems), Northam on +61 (0)8 9690 2117.