Rainfall update 7 August 2018

Rainfall for the month to date

Rainfall has continued the first week of August, with good falls along the west coast and northern grainbelt, as well as the south-west and coastal parts around Esperance (Figure 1).

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Figure 1 Western Australian rainfall totals (in millimetres) for 1–7 August 2018 (source: Bureau of Meteorology, www.bom.gov.au/climate/ [2])

Monthly rainfall to date [3] is available from the DPIRD weather stations and radar page (select Month to Date from the drop-down menu).

Seasonal rainfall

Good rain in July and the first part of August has advanced much of the northern and central grainbelt to be close to or above median seasonal rain to date (Figure 2). South-eastern parts of the grainbelt remain well below normal seasonal rain to date, with much of the south coast at decile 2. Note Figure 2 uses the years 1975-2017 as reference period for derivation of deciles. This period is drier than the baseline used by the Bureau of Meteorology (1961-1990).

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Figure 2 Rainfall deciles from 1 April to 6 August 2018 (data source: Patched Point Database)

For graphs of rainfall at individual locations, see DPIRD’s Rainfall to date tool [5].

Soil moisture

Figure 3 shows relative root zone soil water storage to 6 August 2018 from the Bureau of Meteorology’s Landscape Water Balance model [6]. Rain in early August has improved soil water storage for much of the grainbelt and parts of the south coast. A region along the south coast east of Albany remains 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, see DPIRD’s Soil water tool [7].

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Figure 3 Estimated relative root zone soil water at 6 August 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.

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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 [10].

Rainfall forecast for the next two weeks

Rainfall forecast for the next week shows continuing rain for the west coast and south west of WA (Figure 5). Smaller totals are forecast for the south coast, as has been the persistent pattern this year.

Beyond next week, the US NCEP model indicates lighter falls for the middle of the month (Figure 6).

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Figure 5 Bureau of Meteorology forecast rain in millimetres for 7-14 August 2018 (source: www.bom.gov.au/jsp/watl/rainfall/pme.jsp [12])

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Figure 6 Rainfall predicted for the next two weeks from the US NCEP model, to 23 August 2018 (source: http://wxmaps.org/pix/prec7 [14])