Seasonal rainfall update

Rainfall update 4 September 2018

Rainfall for the previous month

Given this update is very early in the month, Figure 1 shows rainfall deciles for August, which was above average for much of the grainbelt and South West.

Map of Western Australia showing rainfall as deciles for August 2018
Figure 1 Western Australian rainfall deciles for August 2018 (source: Bureau of Meteorology)

Monthly rain to date is available from the DPIRD weather stations and radar page (select Month to Date from the drop-down menu).

Seasonal rainfall

While August was wetter than average for most of the grainbelt and South West, July was near-average or slightly wetter, and June was slightly drier than average. Rainfall for June to August was a mixed pattern of near-average, slightly wetter and drier regions. The dry start to the season means much of the South Coast remains at decile 2 or less for growing season rainfall to date (Figure 2).

Map of Western Australia showing rainfall as deciles for April to  August 2018
Figure 2 Rainfall deciles for April to August 2018 (source: Bureau of Meteorology)

For graphs of seasonal rainfall at individual locations refer to the Rainfall to date tool.

Soil moisture

Figure 3 shows relative root zone soil water storage to 3 September 2018 from the Bureau of Meteorology’s Landscape Water Balance model. Good rain in August has improved soil water storage for much of the grainbelt and the South Coast. Parts of the south-eastern grainbelt remain with relatively low levels of soil water storage for this time of year.

For soil water estimates at individual locations with and without crop water use refer to the Soil water tool.

Map of Western Australia showing relative soil water storage  at 3 September 2018
Figure 3 Estimated relative root-zone soil water at 3 September 2018 from the Bureau of Meteorology’s Landscape Water Balance model

Potential yield

Potential crop yield is estimated using the French-Schultz relation, and uses seasonal rainfall from 1 April to date. Rainfall for the rest of the growing season (to 30 September) is assumed to be decile 5. This model does not account for crop diseases or soil constraints. There is little change in yield potential from the previous report, as most of the seasonal rainfall is unchanged

Map of Western Australia showing potential crop yield in tonnes per hectare at 3 September 2018
Figure 4 Potential crop yield based on seasonal rainfall to date and assuming an average finish to the season

To estimate yields at individual locations refer to the Potential yield tool.

Rainfall forecast for the next two weeks

Rainfall forecast for the next week shows continuing rain for the lower west coast and south west of WA (Figure 5). Most of the grainbelt is expected to receive less than 10mm.

Beyond next week, the US NCEP model indicates only light rainfall for the second week. See Figure 6. Almost all climate models surveyed indicate September is more likely to be drier than normal, with most also expecting below normal rainfall in October. This appears to raise the risk of a drier than normal finish to the 2018 growing season.

Map of Australia showing forecast rainfall 4 to 11 September 2018
Figure 5 Bureau of Meteorology forecast rain in millimetres for 4 to 11 September 2018

Map of Australia showing forecast rainfall 3 to 19 September 2018
Figure 6 Rainfall predicted for the next two weeks from the US NCEP model, to 19 September 2018 (source: Center for Ocean-Land-Atmosphere)

In this Issue

Seasons