Tree rings test what we know about rain in the north

Even though the bulk of the Territory's new cotton crops are expected to be grown using wet season rain alone, there are fears future plantings will take irrigation water from Top End rivers.

Researchers from UNSW in Sydney said a study of tree rings in the Daly River area found irrigation potential could be based on overly optimistic expectations about streamflow.

In a paper published in the journal Water Resources Research researchers from UNSW Sydney said they reconstructed the area's ancient climate history by combining streamflow records, statistics and tree-ring data from not only local trees, but many others throughout Australasia.

They claimed more than 500 years ago the Top End's monsoons swung between very wet periods of high rainfall and greater streamflow, and drier periods of substantially lower flows in the Daly and connected rivers.

Environmentalists are waging a campaign in the NT in opposition to cotton growing.

The Daly River in particular is well known for its barramundi fishing.

Pastoralists are looking to broadacre crops like cotton or rice to provide an income stream other than beef.

There has been increased cotton growing in the Territory with a cotton gin being built in the Katherine region for this year's harvest so growers can avoid the high transport costs to have it processed in eastern states.

UNSW PhD candidate and lead author of the study, Philippa Higgins, said modern data did not take into account the longer history of streamflow in the river.

"If you base water allocations on a period that's a lot wetter than the historical period, you're at a big risk of over-allocating that resource," Ms Higgins said.

She said there were about 50 or 60 years of actual streamflow records for the Daly River.

"However, our new method of deriving the paleo climate based on tree-rings goes back almost 600 years. And we see in that time, even as late as the mid 20th Century, that the monsoon season was drier, leading to much lower streamflow."

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The researcher said their reconstruction of 592 years of streamflow using tree-ring data and complex statistical analysis showed that wet periods invariably give way to years of reduced rainfall and lower streamflow.

The researchers compared tree rings with known rainfall and streamflow data of the Daly River.

They said because rainfall records are much longer than the streamflow data, this allowed the team to set up a model of the climate's relationship with tree rings.

Researchers now had the ability to look at the tree rings corresponding to Daly River records of streamflow and work backwards to deduce the climatic changes going back hundreds of years.

Ms Higgins says the longest tree ring record in the Daly River study was 250 years, but the team was able to use much older tree ring data from south-east Asia that were subject to the same monsoon climatic conditions.

"One of the strengths of this study is we've used a network of tree rings," she said.

"Regionally, the Australian monsoon is pretty well related to the Indian monsoon, so we used that correlation to also incorporate tree rings from Asia into our reconstruction."

They found streamflow had increased gradually since the 1800s, but strong increases in the most recent 40-year period were unprecedented in the last 600 years.

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