Changing rainfall patterns are reshaping Western Australia's wheatbelt and pushing it toward the coast.

Key points: Since 2000, rainfall zones have moved in a south-westerly direction toward the coast

Since 2000, rainfall zones have moved in a south-westerly direction toward the coast The new cropping area is estimated to be as much as a quarter-of-a-million hectares

The new cropping area is estimated to be as much as a quarter-of-a-million hectares The Southern Annular Mode (SAM) has a big impact on WA's winter rainfall

In the south-west corner of the state, home of Australia's most productive grain growing region, winter rainfall has dropped by up to 20 per cent since the 1970s.

Since the turn of the century, grain production has become commonplace in high rainfall zones previously too wet to grow viable broadacre crops.

Grain Industry Association of Western Australia's (GIWA) Michael Lamond estimates the new cropping area to be a quarter of a million hectares, roughly equivalent to all the arable land in Jordan.

"It has been occurring for some time — the increase in the percentage area cropped in the western and south coastal regions — and I suppose it's a function of the profitability," Mr Lamond said.

"At the same time, the seasons have been getting drier so a lot of those areas that weren't able to be cropped 10 to 20 years ago can now be cropped."

Agricultural evolution

Things have changed quite a bit on Peter Reid's family farm at Boyup Brook, about 250 kilometres south of Perth, in just a few decades.

In the past two decades, rainfall zones or isohyets have moved in a south-westerly direction towards the coast. ( Sourced: Department of Primary Industries and Regional Development )

It was not so long ago when the fourth-generation farmer, and many others in the area, ran livestock with just a "kelpie dog, a shovel, and a firefighter".

Now the high rainfall farming region's undulating hills are dotted with combine harvesters and tractors pulling bins filled with grain.

Mr Reid said his decision to shift towards commercial cropping was an economic one.

"In the '90s, when you couldn't make any money out of livestock, we started looking for something else to do," he said.

Mr Reid said weather patterns had also changed in that time.

"The early winter rainfall is definitely not there anymore, so establishing your pastures for livestock is difficult, but there's still enough there to actually put a crop in," he said.

Agronomist Tim Trezise moved to Kojonup, about 85 kilometres east of Boyup Brook, in 2002 at a time when the region's farmers were still quite new to grain production.

Agronomist Tim Trezise has seen the evolution of crop production in high rainfall areas over the past two decades. ( ABC Rural: Jon Daly )

He said waterlogging was the biggest challenge, followed by fungal diseases and moving machinery around small, craggy paddocks.

"It would be at least 50 per cent cropping now, whereas back then I reckon it was 20 per cent," he said.

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Mr Trezise said the drier conditions had benefited crop production in high rainfall areas like his.

"I've had guys ring up from further east, concerned about the way the way the climate is going and asking questions about this area and thinking this is a safer place to be," he said.

Anthropogenic influence

This weather pattern shift is being driven by changes to air circulation in the atmosphere, according to the CSIRO.

Winter rainfall in Western Australia's south-west has dropped by up to 20 per cent since the 1970s. ( Sourced: Department of Primary Industries and Regional Development )

WA's winter rainfall is heavily influenced by the Southern Annular Mode (SAM), a band of strong westerly wind that usually sits below Australia's south coast in winter and creates rain-bearing cold fronts that cross the coast.

However, in the last 20 to 30 years, SAM has been trending towards a 'positive' phase, meaning it contracts towards Antarctica, taking the rain-bearing weather systems with it.

Positive SAM has tended to cause widespread high rainfall in summer but dry conditions in winter. ( Supplied: Bureau of Meteorology )

Ian Foster has been a climatologist with WA's Department of Primary Industries and Regional Development (DPIRD) since 1989 and has studied climate variability and its effects on agriculture since that time.

He said the southward contraction of SAM had weakened the cold fronts WA's grain growers relied on in winter.

"Rather than us getting the stronger part of the front, we've been tending to get the weaker part of it, which means when it does cross you don't get the heavy rain and you often don't get the follow up rain," he said

Dr Foster said the southward contraction of SAM looked to be a signal of anthropogenic global warming.

"You tend to get a bit more rain in the tropics, a bit less in the mid latitudes, where we sit, and a bit more in the high latitudes again," he said.

"This shifting of the weather patterns has meant that, because our continent hasn't shifted to match this, essentially the rain has gone away from us during the winter."

Other research has also attributed the southward contraction of SAM to both rising greenhouse gases and Antarctic ozone depletion caused by human activity.

"Incredibly resilient" grain growers in low rainfall areas of WA are learning to adapt. ( ABC Rural: Jon Daly )

Climate variability

In the past two decades, rainfall zones or isohyets — basically lines drawn to denote equal average rainfall — have moved westward by up to 100 kilometres in some areas of WA, according to DPIRD.

Mr Lamond said rainfall event intensity and frequency were also changing.

GIWA crop report author Michael Lamond says WA grain growers are adapting to the changing weather patterns. ( ABC Rural: Jon Daly )

"The cold fronts tend to go through a lot quicker [and] the fronts don't drop as much rain, they aren't as frequent and in more recent times have become more erratic," he said.

"If you look at any time series of rainfall and temperature since the mid-1970s, I suppose the south-west of WA is one of the clearest examples in the world where there has been a substantial shift."

While the shifting weather pattern has benefited crops in high rainfall areas, it is having an opposite effect on farmers in low rainfall areas on the eastern edge of WA's Wheatbelt.

"If you have a whole year of less than 10-millimetre rainfall events, you're not going to grow much crop and that is what occurred this year," Mr Lamond said.

He said grain growers in low rainfall areas were already learning to adapt.

"The growers in those rainfall areas are incredibly resilient and are looking at alternatives to combat the climate they're facing at the moment," he said.

"There has been a shift back to leaving country to fallow, and having country that's not cropped for one season and cropped the next means you preserve any moisture that falls in the winter or summer."

Dr Foster said the unknown was whether the weather trend would continue on its current path and, if so, would negatively-effected farmers be able to continually chase efficiencies and stay viable.