Climate

The major dairy areas in the state are currently located between 32 and 35 degrees south. This is of similar latitude to North Cape (New Zealand), the Cape Provinces of South Africa, and regions in northern Victoria and southern New South Wales (Australia).

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Rainfall

Average annual rainfall in existing dairy areas ranges from 800 to 1,100 mm. Unlike most other dairy regions of Australia, 65% or more of total rainfall is received during winter/spring (May to September).

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Temperature

Humidity is low in summer reducing the impact of warm to hot summers on cow and human comfort. Winters are mild with mean minimum monthly temperatures of 6 C or more occurring in July and/or August.

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Temperature Humidity Index

Temperature Humidity Index (THI) is used to assess the effects of heat stress on milk production. THI is determined by the relative humidity and air temperature. As humidity and temperature increases, cow comfort decreases as they find it steadily more difficult to cool themselves. For example Milk production begins to be affected above a THI of 78, which will occur at 27 C and 80% relative humidity or at 31 C and 40% relative humidity as shown in the table below.

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Climatic trends in Western Australia

Reliability of rainfall and potential impacts of climate change are important factors to be considered as part of any investment decision in agriculture.

In the last 30 years the southwest of Western Australia has been affected less by climate change than most dairy regions in Australia. Rainfall has fallen 20-30 mm/10 yrs in the period from 1970 to 2006 compared with falls of 30-50 mm in most other dairy regions of Australia (http://www.bom.gov.au/cgi-bin/silo/reg/cli_chg/trendmaps.cgi

A similar trend has occurred with changes in maximum temperatures; smaller changes have been recorded for the southwest of the state compared with many dairy regions of Australia (http://www.bom.gov.au/cgi-bin/silo/reg/cli_chg/trendmaps.cgi)

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