How climate change has affected wheat yields in Western Australia
Historic wheat yield increases
In WA, average wheat yields doubled between 1980 and 2010, although the rate of yield increase plateaued and yield variability increased from the end of the 1990s.
The increase in wheat yields since 1980 is attributed to the adoption of technological and management improvements, such as minimum tillage, soil amelioration, stubble retention, early sowing and integrated weed control. Increases in wheat yield were not uniform across the grainbelt; the greatest increases were in southern and northern areas, and least in central and eastern areas. Wheat yield variability has been greatest in eastern and northern areas.
These changes effectively increased the water use efficiency of wheat at a greater rate than rainfall declined over the period (YouTube South West Western Australia annual rainfall animation).
Increasing influence of climate change
Modelling indicates that increased atmospheric carbon dioxide concentration of 50 parts per million over the last 50 years increased yield potential by 2–8%. The 0.8°C increase in average temperature over the period benefited wheat crops during the winter months and accelerated maturity. These changes have worked to offset some of the harmful effects of more hot days during grain filling.
Reduced rainfall is considered to have had relatively little impact on potential yield in the western and southern grainbelt because much of the reduction occurred during winter, when rainfall exceeds plant demand and the excess is effectively lost to crops and does not contribute to yield. However, shorter growing seasons and variability in annual rainfall, and rainfall distribution and intensity has profoundly affected crop management. In parts of the eastern grainbelt, declining rainfall, increasing temperatures and increasing evaporative demand are sufficient to reduce potential yield.
The reduction in the rate of increase of wheat yield and water use efficiency since 2000 has been attributed to growers reducing their inputs (risk) in response to climate conditions. Consequently, water use efficiency is lowest in more marginal and variable areas where farmers are most conservative in their farming practices and their use of inputs.
Factors affecting wheat yields in southern Australia
Hochman et al. (2017) analysed and explained how changes in climate between 1990 and 2015 affected wheat yields in southern Australia:
- Adoption of new technology and management systems held actual yields fairly steady: without these advances, water-limited yield would have dropped by 27%; this implies a much higher rate of adoption of technology gains in this period than the previous 26 years.
- Rainfall decreased by 28%: crop growth models estimated that reduced rainfall alone accounted for about three-quarters of the 27% fall in water-limited yield potential.
- Rising temperatures accounted for about a quarter of the 27% fall in water-limited yield potential.
- Increasing atmospheric carbon dioxide prevented a further 4% decline in water-limited yield potential.
- Land degradation evidence is limited, but observation for the period was that land degradation had little effect on reducing wheat yield.
- Expansion of cropping onto pasture land may have changed the average productivity of cropping soils, but there is insufficient evidence for a change.
Adoption of new technology and systems means that the actual yield as a proportion of water-limited yield potential increased from 39% in 1990 to 55% in 2015.
Projected climate change will affect yield
The greatest reductions in rainfall in WA are projected for winter and spring and the greatest increase in temperature is projected for spring when crops are flowering and filling grain.
Modelling forecasts based on representative climate futures are a concern for south-west WA:
- 'Median wheat yields modelled for the South West Australia projected declines between 26% and 38%, under a ‘most-likely’ case for RCP 4.5 by 2090, and between 41% and 49%, under a ‘most-likely’ case for RCP 8.5' (Taylor et al. 2018)
- 'Variability also changed from the baseline under all projected RCFs and across all regions…' (Taylor et al. 2018).
Modelling shows that crop yield in WA is more likely to be affected by changes in rainfall than changes in temperature. Additionally, rainfall distribution is likely to have a greater effect on crop yield than absolute reductions, with crop yields being more sensitive to reduced rainfall during May or August (germination and flowering, respectively) than June and July (months of highest rainfall).
Despite these uncertainties, some common themes emerge from studies that have modelled wheat yields under projected future climates:
- yields are projected to decline in drier eastern and northern areas and remain largely unchanged or increase in wetter western and southern areas
- increased atmospheric carbon dioxide concentration offsets a small percentage of the negative effects of decreased rainfall and increased temperatures
- higher temperatures, and to a lesser extent declining rainfall, will hasten development times and reduce the flowering period
- plant available water capacity of soils becomes increasingly important to yield potential: yield declines are greater on clay soils than sands in eastern areas
- production risks associated with climate variability in drier, marginal areas are projected to increase.
For more information
Download Bulletin 4870 Climate change: impacts and adaptation for agriculture in Western Australia (PDF 4.9MB).
Hochman, Z, Gobbett, DL & Horan, H 2017, 'Climate trends account for stalled wheat yields in Australia since 1990', Global Change Biology, vol. 23, pp. 2071–81, doi: 10.1111/gcb.13604.
Sudmeyer, R, Edward, A, Fazakerley, V, Simpkin, L & Foster, I 2016, ‘Climate change: impacts and adaptation for agriculture in Western Australia’, Bulletin 4870, Department of Agriculture and Food, Western Australia, Perth.
Taylor, C, Cullen, B, D'Occhio, M, Rickards, L & Eckard, R 2018, 'Trends in wheat yields under representative climate futures: Implications for climate adaptation', Agricultural Systems, vol. 164, doi: 10.1016/j.agsy.2017.12.007.