by Dr David Stephens, Managing Director, Agrometeorology Australia, www.agromet.com.au,
What do long term rainfall trends tell us that might help farmers better manage their unfolding climate risk?
Analysis of long term rainfall and wheat yields across Australia reveals four main findings:
- Over many decades prior to 2000 there has been a steady increase in summer rainfall (April-November) in many regions, except in far southern areas of Australia.
- Since 2000 there has been a “step-down” event in winter season (May-October) rainfall across much of southern Australia.
- Consequently, there has been a 100-400 km southward shift in rainfall zones as summer dominant zones have expanded and winter dominant zones have contracted (see Figure 1).
- Wheat yield variability has increased over two-fold in the period 2000 to 2016 compared to 1980s and 1990s in southern States; yet yield variance has halved in Queensland.
The Story in More Detail
To understand longer term climatic change, rainfall data at each of scores of weather stations were weighted by the area of grain planted around each station. The weighted rainfall was also split into summer and winter seasons. The resulting weighted rainfall showed a steady increase in summer rain for southern States and a slight downward trend in Queensland over 126 years of records. However, for all States there was a step-down in winter rain around the turn of the 21st century. This equated to a 48mm reduction in growing season rainfall for Western Australia, 43mm for Victoria, 39mm for New South Wales, 33mm for Queensland and 25mm for South Australia.
In terms of the driest ten winter seasons there was a shift to more frequent droughts in Western Australia with six of the ten driest winter growing seasons since 1890 occurring this century (2000, 2002, 2006, 2007, 2010, 2012). In other States, there was only two of the driest winters in New South Wales (2002, 2006), South Australia (2006, 2007) and Victoria (2002, 2006), while Queensland only had one winter season in the driest ten years (2006).
Another feature that changed around 2000 was the shift in wheat yield variability with all southern States observing a 2-3 fold increase in wheat yield variability. Higher variability in wheat yields is partly due to better farming technology (management, varieties) producing better yields in good years, however vulnerability to very low rainfall can still produce low yields, hence increasing overall yield variability. In contrast, Queensland wheat yield variability halved mainly because of the few very high yielding years since 2000.
Given rainfall zones are defined by the ratio of summer to winter season rainfall, a new seasonal rainfall zones map (see Figure 1) was compiled for this century. This map shows the shifts in types of climate:
- The summer dominant rainfall region has shifted south from near Longreach to Charleville,
- The summer rainfall zone has shifted south from Gunnedah towards Dubbo,
- The uniform rainfall zone has shifted southwest from southern New South Wales into central Victoria, while in Western Australia the uniform rainfall zone has shifted west from Kalgoorlie to Southern Cross.
- The winter rainfall zone in the southwest of Western Australia has shifted southwest leaving a smaller winter dominant zone along the west coast.
This “new climate” has major implications for plant breeding, farm management and grain supply chains. One of the biggest adaptations to wetter summers and drier winters has been the four to six weeks’ shift to earlier planting of crops, made possible by a range of technology and practice changes over the last 30 years. Changes in crop volumes also are following the shift in climate zones, thereby affecting investments in on and off-farm storage and logistics. More accurate longer-lead seasonal and crop forecasts have become important in business planning and crop management.
Note: an update of Figure 1 for 2000-2017 will be available on the Agrometeorology Australia website: www.agromet.com.au in early 2018.