A study of tree rings and other data suggests NT water allocations may be too generous.

Researchers have found evidence that the Northern Territory had a much drier past. They say this should come as a warning that if the NT continues to hand out unprecedented water allocations, there could be “major environmental and cultural damage” in the future.

Researchers from UNSW Sydney have reconstructed the paleoclimatic history of the Northern Territory’s Daly River region by using methods that combined streamflow records, statistics and tree-ring data from not only local trees, but many others throughout Australasia.

They were able to build an image of a rainfall period that went back more than 500 years to find that the monsoon system affecting Northern Australia swung between very wet periods of high rainfall and greater streamflow, and drier periods of substantially lower flows in the Daly and connected rivers.

Philippa Higgins, UNSW PhD candidate and lead author of the study, says the window of data the NT government might use to base its allocations on does 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 says.

In other words, if streamflow goes back to historical levels when monsoon rainfall was much less than in recent decades, we could see “major environmental and cultural damage,” Ms Higgins says.

“Overallocation of water resources combined with consecutive years of low rainfall that results in reduced streamflow in the river could reduce water quality, negatively impact aquatic species and river vegetation, and potentially damage sites of significance for Indigenous custodians.”

If calculations of water resource potential are based on gauge data that only stretches back a few decades at a time when streamflow has increased at unprecedented levels, such assumptions could be overly optimistic.

“We only have records for streamflow in the Daly that go back 50 or 60 years,” she said.

“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.”

The researchers are calling on the NT government and water managers to look at the bigger picture, as revealed by their research. 

They say that 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.

“So we caution on using just a very small amount of data to calculate water allocations for agricultural use when we have methods available to look at much longer time periods that allow us to better understand the risks of different decisions,” Ms Higgins says. 

The study is accessible here.