Changes in summer rainfall and implications for agriculture

Summary

Annual and seasonal rainfall are important drivers of agricultural productivity and profitability in Australia. Various climatological and synoptic drivers influence rainfall patterns in Australia’s diverse climate.

This study detected trends in past and future annual, seasonal and extreme rainfall across three important agricultural production regions in the Australian midlatitudes using station and gridded data from 1907 to 2018.

Apart from region-wide changes, there was a positive trend in summer rainfall for two of the seventeen studied locations and a negative trend in winter rainfall for five of the seventeen locations.

Objectives

The objectives of this project were to:

• provide detailed new knowledge about any changes in summer rainfall over the study area (in northern New South Wales, southern Queensland extended to the coast, the northern Murray Darling Basin and at similar latitudes in Western Australia – the central/southern Wheatbelt)
• assess the potential causes of any such changes and attribution to climate change, along with a discussion about the potential impact on agriculture in the area.

Key findings

• Robust trends with significant and long-term changes in seasonal rainfall and summer rainfall were noted for two out of the seventeen stations studied and for the Western Australia Wheatbelt.
• Spatial heterogeneity in rainfall is very high, which makes it challenging to establish any regional trends from local observations.
• Although some locations experienced a long period of summers with below average rainfall, this does not constitute part of a long-term trend for the time period 1907-2018/19 and current results suggest that they are part of natural variability in seasonal rainfall.
• Among the large-scale climate influences on seasonal rainfall in the Western Australia Wheatbelt, the El Nino-Southern Oscillation, the Southern Annular Mode and atmospheric blocking are the main influences on warm season rainfall.
• The decline in winter rainfall in Western Australia is expected to continue as there is high agreement in climate models for further declines in winter rainfall, while changes in rainfall in eastern Australia are less certain.

Benefits to industry

Agricultural impact studies could use the climate models and the rainfall indicators identified in this project to determine if the positive or negative trends in the past continue into the future.

For the Western Australia Wheatbelt where the direction of change in seasonal rainfall is more certain, this knowledge can assist in developing coping strategies or inform transformation processes.

MLA action

This project was commissioned and funded by MLA, GRDC, CRDC, Sugar Research Australia and AgriFutures Australia through the Managing Climate Variability Program Phase 5 project and the results will be shared with industry via the Climate Kelpie website and MLA website.

Future research

Future research should focus on extending the analysis provided to include locations in the New England and northwest region in NSW and far west and Orana region in NSW to confirm whether the high anomaly in summer rainfall in the recent decade found for Narrabri and Curlewis can be observed elsewhere.

It would also be beneficial to extend the list of rainfall variables used here to consider not only changes in the growing season rainfall and extreme rainfall across the year, but also changes in rainfall related to specific management decisions such as planting or to specific adverse events such as droughts increasing in frequency or intensity.