Key messages

• For ten locations in the Western Australian grainbelt we found a decline in winter (June to August) rainfall and an increase in out of season rainfall (November to March) in the past two decades compared to the previous two decades, and a trend to more negative values of a drought index in some locations.
• The number of days to complete full leaf emergence after June 1 (measured as ninety growing degree days) is up to a day sooner, leading to faster crop growth and the potential to move grain filling into a cooler window.
• The number of rain days is fewer and number of days between rain events in winter is longer. All these elements mean that crop production in the grainbelt is more challenging and will require continued innovation to maintain yields.

Aims

To examine current and potential rainfall and temperature patterns in the grainbelt and some of the implications for plant growth.

Introduction

The South West Land Division (SWLD) in which the Western Australian grainbelt is located, has seen a decline in growing season rainfall since the mid-1970s (Bates et al 2008; Hope 2006), and an increase in temperature. There has been another decline in rainfall from 2000 (Bureau of Meteorology 2020), while temperature increases are parallel to the global rise. The increased frequency of dry seasons has been a consequence of these changes. These altered weather and climate patterns have a range of consequences which may be to the advantage or disadvantage of crop growth. We analysed a range of different weather variables to better understand the consequences of these changes.

Method

Ten locations in the SWLD were chosen to represent low (Mullewa, Beacon and Kondinin), medium (Ravensthorpe, Mingenew, Goomalling, Gnowangerup and Newdegate) and high rainfall zones (Williams and Katanning). Rainfall and temperature data from 1975 was obtained from SILO patched point data.

To map the difference in winter (June to August) and out of season (November to March) rainfall averages for years 1975-1999 and 2000-2020, data from 332 Bureau of Meteorology weather stations was used. A simple assumption is that this rainfall difference can be further projected for the next twenty years (2021-2040) and so projected rainfall difference from 1975-1999 using this assumption was calculated and mapped.

Monthly Standardized Precipitation Evapotranspiration Indexes (SPEI) for each location from 1975 to 2018 were sourced from the SPEI website (Vincente-Serrano et al 2010). These were then averaged for out of season (November to March) and winter (June to August) for the years 1975-1999 and 2000-2018 to determine any trends in the data. The SPEI is a drought index, which uses the concept of standardized precipitation, and includes a temperature component, allowing the index to account for the effect of temperature on drought development through a basic water balance calculation. SPEI has an intensity scale in which both positive and negative values are calculated identifying wet and dry events.

As temperature drives crop growth we calculated growing day degrees (GDD) for the months of May to September and identified the average number of days to reach a value of 90 GDD. Daily GDD’s are calculated as the cumulative total of average daily temperatures.

Results

Rainfall and SPEI

Winter rainfall throughout the SWLD has generally declined in the past twenty years with a 15 to 45mm reduction across much of the central and northern grainbelt. Some southern locations have shown little change, though small increases have occurred in two southern shires. The 2040 winter rainfall predictions, based on the past twenty-year changes, and projected forward twenty years show the greatest potential for a reduction in the western parts of the northern and central grainbelt (Figure 1).

Out of season (November to March) rainfall has increased in the past twenty years with an increase of 15 to 45mm in average rainfall across much of the grainbelt. Ravensthorpe and Esperance have increased by as much as 45-75mm. The 2040 out of season rainfall predictions show a further increase in the majority of locations in the SWLD (Figure 2).

Since 2000, the average out of season SPEI for six locations, has been trending more negative, with the lowest values in Beacon, Kondinin and Mingenew indicating warming temperatures in these locations. SPEI values became more positive in locations along the south coast (Ravensthorpe, Gnowangerup, Newdegate and Katanning). The average winter SPEI has decreased in all locations, with locations along the southern grainbelt closer to the coast having values closer to zero (Table 1). The SPEI values may be largely reflecting the better out of season and winter rainfall across these locations and the shift to cooling from south easterly winds.

Table 1 Average out of season (November to March) and winter (June to August) Standardized Precipitation Evapotranspiration Index (SPEI) for 1975-1999 and 2000-2018 for ten locations in the South West Land Division.

Number of rain days

Since 2000, the number of rain days per month (where rain is determined as > 2mm) has decreased by up to two days in May to July, one day in August and September and increased by one day in January in some locations (Table 2).

Table 2 Change in average number of rain days from 1979-1999 to 2000-2020 in ten locations in the South West Land Division for selected months of the year. Positive numbers indicate more rain days, negative numbers fewer rain days. Standard error in parenthesis.

Average number of days between rain events in winter

Since 2000, all locations have seen an increase in the average number of days between rain events in June. The trend continues for most months but is less extreme for August. The largest change was for Beacon in June, where currently there is an average of ten days between rain events compared to six days pre 2000 (Table 3). The variability in June is also much higher in the past 20 years compared to the previous twenty.

Table 3 Average number of days between rain events in winter (June to August) in 1979-1999 and 2000-2020 at ten locations in the South West Land Division. Standard error in parenthesis.

Growing degree days

A cumulative ninety growing degree days (90 GDD) from 1 June is equivalent to approximately one leaf emergence in cereal plants once seedling emergence occurs. Since 2000, the average number of days for the 90 GDD to be met is now sooner than before 2000. All locations showed a more rapid accumulation of 90 GDD, with the largest difference at Mullewa (Table 4) while Newdegate has remained about the same.

Table 4 Average number of days to reach 90 growing day degrees (GDD) starting from June 1 for 1975-1999 to 2000-2020 at ten locations in the South West Land Division.

Conclusion

Since 2000, the WA grainbelt of the South West Land Division has seen: an increase in out of season rainfall (although this has shown a very high annual variability), a reduction in winter rainfall, a trend for more negative SPEI values in winter, fewer rain days in winter, an increase in the number of days between rain events in winter and warmer temperatures.

The more negative SPEI values for out of season and winter indicates a trend towards more water stress periods than in the past. For southern grainbelt locations (Ravensthorpe, Gnowangerup, Katanning and Newdegate), the out of season SPEI has increased (more positive) since 2000. This is due to the position of these locations relative to changes in the high-pressure cells and highlights the climate variability in the SWLD. SPEI values below-1 during the growing season of a crop are often indicative of drought stress and have been shown to be positively correlated with yield loss.

While localised drought and heat events seem to have become more frequent in the past two decades, water use efficiencies have not necessarily declined and for some locations water use efficiencies of 20mm/kg/ha were reported in 2020 with a decile 3 growing season rainfall (GIWA November 2020). While furrow sowing has allowed for better water harvesting of smaller rainfall events post-seeding and stubble retention is helping conserve water deeper in the profile, these savings may have limits. Warmer winters accelerate crop growth, which may result in flowering and grain fill occurring under cooler temperatures and this could help offset lower soil moisture accumulation in winter.

Of concern is the longer delay between rainfall events in winter. Future climate predictions from a range of different sources include a further decrease in winter rainfall with more time under water stress, but with the possibility of an increase in intense heavy rainfall throughout Australia (Bureau 2020). The rainfall patterns of 2010 for WA should be a reminder as to how future rainfall might play out – many locations had only four rainfall events separated by a month apart but with each event about 20-30mm. An increase in the frequency of low rainfall years will impact on water scarcity (as seen from 2018 to 2020, with many shires in the southern grainbelt declaring water scarcity) and future climate predictions indicate that dry-year frequency (once in a century drought) will increase in Australia and particularly in the south-west corner once the rise in global average temperature exceeds 1.5°C (Takeshima et al 2020).

The two most important synoptic weather systems associated with rainfall in Australian grain growing regions during the growing season are cut-off low pressure systems and westerly frontal systems. In the South West, fronts account for half of the rainfall, while cut-off lows account for a third (Pook et al 2014).  Since 1975, both weather systems have shifted south, resulting in less winter rainfall (Hope et al 2006). The majority of November to March rainfall is from a series of thunderstorms and tropical cloud bands. Future climate projections have these events increasing in intensity (Bureau 2020). The possibility of this rainfall being saved as soil water is one positive, but this will depend heavily on stubble retention, good weed management and improved furrow systems that hold water in place to allow sufficient time for infiltration.

References

Bates, B.C., Hope, P., Ryan, B. et al. Key findings from the Indian Ocean Climate Initiative and their impact on policy development in Australia. Climatic Change 89, 339–354 (2008).

Bureau 2020 - State of the Climate – 2020 Bureau of Meteorology and CSIRO Commonwealth of Australia http://www.bom.gov.au/state-of-the-climate/

GIWA Crop Report November 2020 http://www.giwa.org.au/2020

Hope, P.K. Projected future changes in synoptic systems influencing southwest Western Australia. Clim Dyn 26, 765–780 (2006).

Hope, P.K., Drosdowsky, W. & Nicholls, N. Shifts in the synoptic systems influencing southwest Western Australia. Clim Dyn 26, 751–764 (2006).

Pook, M.J., Risbey, J.S., McIntosh, P.C. A comparative synoptic climatology of cool-season rainfall in major grain-growing regions of southern Australia. Theor Appl Climatol117(2014), pp.521-533

Takeshima A., Kim H., Shiogama H., Lierhammer L., Scinocca J.F., Seland O., Mitchell D. 2020 Global aridity changes due to differences in surface energy and water balance between 1.5°C and 2°C warming. Environ. Res. Lett. 15

Vicente-Serrano S.M., Begueria S, Lopez-Moreno J.I. 2010. A multi-scalar drought index sensitibe to global warming: the Standardised Precipitation Evapotranspiration Index. Journal of Climate 23:1696-1718

Contact details

Meredith Guthrie,
Department of Primary Industries and Regional Development
Ph: 9368 3058,
Email: Meredith.guthrie@dpird.wa.gov.au