Future yields in the WA wheatbelt

Monthly Rainfall


Climate change projections for the mid-21st century for southern WA indicate an increase in temperatures, a decrease in winter rainfall and higher atmospheric CO2 concentrations. These changes could have adverse impacts on some agricultural systems, but they may also offer new opportunities (such as in areas where the risk of waterlogging may be reduced).

The aim of the GRDC-funded project is to quantify the impact of climate change on production in the wheatbelt of WA.

Climate change impact studies rely on the outputs of global climate models to define possible future climates. At present, most climate models are too spatially coarse to be used in assessing local impacts (such as on crops). Outputs from global climate models have been used that have been statistically downscaled to generate daily climate files at point locations. This allows crop simulation models, such as the Australian Production Systems Simulator Model for wheat (APSIM-Wheat), to be used to assess any differences in yields between current and possible future climate.

Initially three global climate models were compared for consistency and ability to simulate the climate at three locations in the WA wheatbelt. This indicated consistently less future winter rainfall. Summer rain tended to increase in future, but still remained a small proportion of annual rainfall. Crop simulations showed future yields were generally lower in low-rainfall locations, but tended to increase at high-rainfall sites.

Our latest results concentrate on one climate model (the CSIRO Cubic Conformal Atmospheric Model, CCAM, which reproduced WA climate quite well), extended to eight locations and three soil types of the Western Australian wheatbelt.

The APSIM-Wheat model was run with two sets of climate data for 30-year periods:

  1. current simulated climate for the period 1976-2005 with current level of CO2 (350 parts per million); and
  2. future simulated climate for the period 2035-2064, with expected CO2 level in the mid-21st century (440ppm).

Figure 1 shows the changes in monthly average rainfall simulated by CCAM. Although summer rain increased, it was still mostly ineffective. Most rain occurs from April to October, and this season was drier in the future climate simulations. Wheat yields under a future climate showed a decline in most locations (Figure 2), with clay soils being most sensitive. In Corrigin, Esperance and Wandering, yields increased in the future on duplex soil, as a consequence of reduced detrimental impact of waterlogging.

The yield decrease was due to lower rainfall and higher temperatures, which caused shorter growth duration and more water deficit in most locations.

Generally lower rainfall in autumn caused delayed sowing, shortening the growing season and increasing the chance of more severe water Impact of Future Climatedeficit during grain filling. In most locations, the positive effect of increased CO2 levels was more than offset by negative effects of lower rainfall, delayed sowing and increased temperatures.

The yield increase in some high and medium rainfall locations was due to the positive effect of increased CO2 levels and reduction of waterlogging.

Adaptation will be needed to overcome some of the projected adverse impacts of climate change. Adjusting farm management - for example fertiliser use and variety choice - may counteract some of the negative impacts of climate change. Improvements to climate models are also expected to add confidence to regional climate change projections.

GRDC Research Code DAW00088

More information: Dr Ian Foster, 08 9368 3642, ifoster@agric.wa.gov.au