Climate change in the Merredin area, Western Australia

Page last updated: Wednesday, 11 September 2019 - 4:10pm

The Department of Primary Industries and Regional Development provides this agri-climate profile of historical and projected climate information to support farm business managers in their response to a changing climate in the Merredin area of Western Australia.

Why this information is important

Climate change and climate variability have already affected Western Australian (WA) broadacre crop and animal production over recent decades, with significant reductions in rainfall and increased frost risk. Producers have been able to meet these challenges by adopting innovative farming systems to maintain farm profitability and sustainability. Future climate change will present further opportunities and challenges for producers.

Records shows that rainfall decreased and temperatures increased over the last century. Climate projections for the south-west of WA are for declining rainfall and higher temperatures.

The grainbelt of WA contributes more than $4.5 billion to WA’s economy each year. Merredin is 260 kilometres east of Perth, on the eastern edge of the central grainbelt. This agri-climate profile provides a historical analysis and future projections for a range of climate variables relevant to farm businesses in the Merredin area. 

Changes at a glance

Around the mid-1970s, there was a shift to drier winter conditions in south-west WA.

The observed trends in Merredin’s climate include:

  • little change in total annual rainfall
  • a decline in growing season rainfall (April–October) 
  • an increase in summer rainfall
  • fewer very wet years
  • lighter and less frequent winter rainfalls
  • a more variable and later start to the growing season
  • an increase in average minimum and maximum temperatures
  • a decline in the number of frosts in spring
  • an increase in the number of very hot days in summer.

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What the records show

There were shifts in climate for Merredin in the mid-1970s, then again around 2000. Therefore, the analysis for 1931–1974 (43 years), 1975–2018 (43 years) and 2000-2018 (18 years).

Rainfall

There has been little change in total annual rainfall. However, the growing season rainfall has declined by about 10% since the mid-1970s (Figure 1).

April to October rainfall for Merredin for the years 1931-2018, showing a decline in the average rainfall
April to October rainfall for Merredin for the years 1931-2018

Around the mid-1970s, there was a shift to consistently drier winter conditions. Figure 2 shows the decline in early winter (May to July) and the increase in summer (December to February) rainfall from 1931–1974 to 1975–2018.

Average mean monthly rainfall for Merredin for years 1931-2018.
Average mean monthly rainfall for Merredin for years 1931-2018.

Figures 3 and 4 show that the reduction in winter rainfall is caused by a combination of less days with heavy rainfall events and fewer rain days . The increase in summer rainfall has mainly resulted from heavier rainfall events in December to February.

Average monthly number of rainfall events greater than 5 mm for Merredin, 1931-2018.
Average monthly number of rainfall events greater than 5 mm for Merredin, 1931-2018.
Average number of rain days per month for Merredin 1931-2018.
Average number of rain days per month for Merredin 1931-2018.

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Temperature

Since the mid-1970s, mean monthly maximum temperatures significantly increased in April to July, and remained unchanged for the rest of the year (Figure 5). Mean monthly minimum temperatures significantly increased in all months except June and July (Figure 6).

Mean maximum monthly temperature for Merredin for the years 1931 to 2018.
Mean maximum monthly temperature for Merredin for the years 1931 to 2018.
Mean minimum monthly average temperatures for Merredin 1931-2018.
Mean minimum monthly average temperatures for Merredin 1931-2018.

The number of days with maximum temperature above 35 degrees Celsius (˚C) increased slightly in December (Figure 7). The number of frost day (days with minimum temperature below 2˚C) increased in August since 2000 (Figure 8).

Average number of days above 35C in Merredin.
Average number of days above 35C in Merredin.
Average number of days below 2C for Merredin for the years 1931-2018.
Average number of days below 2C for Merredin for the years 1931-2018.

 

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Projected changes

Projections for the period 2035–2064 were obtained using an intermediate emissions scenario (A2) and downscaled data from the CSIRO Global Climate Model CCAM (CMIP3). 

Rainfall

Projections are for less rainfall in autumn–winter and increased rainfall in summer (Figure 9).

Bar chart showing historic and projected falling mean monthly rainfall autumn and winter and rise in summer
Figure 10 Historical monthly rainfall for 1945–1974 and 1975–2004 and monthly projected rainfall for 2035–2064

Temperature

Projections are for increased mean maximum temperatures (Figure 10).

Bar chart showing that projected mean monthly maximum temperatures continue to rise
Figure 11 Historical mean monthly maximum temperature for 1945–1974 and 1975–2004 and projections for 2035–2064

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What are the agronomic implications?

  • Declining autumn-winter rainfall has led to a more variable and generally later start to the growing season (Figure 12). The average start to the growing season, derived from a sowing rule that uses a sowing window starting on 25 April, has shifted from 23 May for 1931–1974 to 26 May for 1975–2018 and later again 28 May from 2000.
Break of the season for Merredin for the years 1931-2018.
Break of the season for Merredin for the years 1931-2018.
  • A later and more variable start to the growing season will increase production risk for crops and pastures. Declining autumn rainfall means crops need to be established at the earliest opportunity, possibly by dry sowing. Storage and conservation of out-of-season rain is gaining importance. Effective control of summer weeds and stubble is becoming more important. Deferred grazing of pastures at the break of season is likely to be more important.
  • The reduction in heavy rainfall events in winter has led to a large reduction in reliability of run-off into farm dams. Roaded and natural catchments will need to be 15–20% larger to fill existing dams.
  • Although there has been little change in total annual rainfall, the decline in average size of rainfalls has led to evaporation losses being more important and less water stored deep in the soil. This increases the risk of moisture stress during establishment, flowering and grain fill. 
  • Even though frost risk has decreased, it still remains a significant risk in winter and spring.

What are the options for adapting to climate change?

We provide information and technical support for making changes at the incremental, transitional and transformative levels. A general guide is available for each major enterprise and for soil and water resources:

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