Scientists believe the Northern Territory may have had a much drier past, warning that if it continues to hand out unprecedented water allocations based on current data, there could be "major environmental and cultural damage".
Tree rings from the Katherine region have provided researchers with a rare glimpse into data dating back almost 600 years that shows rivers have not always flowed at the rate they do today — in fact, they flowed at much lower levels.
"The last few decades have been the wettest that we've seen in 600 years," UNSW Associate Professor Fiona Johnson said.
"The Northern Territory has some of the wildest and most pristine rivers in the world historically, and we can see what goes wrong in other parts of Australia where water has been over-allocated.
"There are plenty of examples where water has been allocated, particularly at times of higher flows, and then we find that when things returned to dry periods, there's not enough water left over for the environment."
In a study published by UNSW, teams examined 63 tree rings — which contain a weather history that can date back centuries — alongside the Daly River by drilling a hole and extracting a small portion for analysis.
As trees grow they form a distinct new circle of wood around their trunk every year. Within that ring, information about age, temperature, and rainfall is recorded.
"We assume that in good conditions, in wet years, the trees will grow faster, and in dry years the trees will grow less and so the tree rings will be less thick," Associate Professor Johnson said.
The oldest tree ring record the team could find in the Daly study was 250 years old, but by examining trees in South-East Asia, which were subject to the same monsoon climatic conditions, the scientists were able to go back further to reconstruct streamflow patterns.
Government's allocations too generous
Lead author of the study, Philippa Higgins, has taken aim at the cotton industry in particular and its plans to extract 520 gigalitres of water from the Daly River.
She said the data the government based its decisions on was too small — only dating back 50 to 60 years.
The Douglas Daly region is about 200 kilometres south of Darwin and sits on top of the Olloo Dolostone Aquifer, which has almost been fully allocated in terms of sustainable water yields for industry.
It also incorporates the Katherine Tindall Aquifer, which is also under immense pressure from over-allocations.
"Over-allocation 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."
With two significant dry spells — 2018 and 2019 — just behind us, Ms Higgins was calling on the government to "look at the bigger picture".
"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."
A government spokeswoman said the Daly River had been subject to more research than any other system in the Northern Territory.
"To date, water allocations in the Daly River are based on dry season flows and the Erskine rules," she said.
"These scientific rules reduce the water that is available under licence to account for dry years and are based on maintaining a specific flow rate.
"This means in a dry year, the announced allocation process kicks in and water licences are reduced so that there is no impact on threshold flows."
The future of rainfall is unpredictable
Kirsty Howey, coordinator of the Environment Centre NT, said climate change would spell "very erratic and quite unpredictable" rainfall into the future in northern Australia.
"It's quite a troubling picture for rivers," she said.
"In fact, it's very likely to be repeated again and we could even see longer dry spells of more successive failed wet seasons."
She said another factor was evapotranspiration, which was essentially just evaporation due to increased temperatures that could reduce streamflow.
"We just don't have the modelling in place to be able to incorporate those kinds of predictions into our current water allocations," Ms Howey said.
She said the UNSW study, published in Water Resources Research, sounded a warning bell that if the NT government continued down the path of using limited data ranges, "we might end up with our river systems in trouble".
