Droughts can affect river flows permanently, Australian study suggests

Australian researchers have warned that drought-affected rivers could experience reduced flows indefinitely even after the drought has broken in a study which found more than a third of Victorian water catchments have not fully recovered from the millennium drought.

The millennium drought ran from 1996 to 2010 and was the longest uninterrupted period of low rainfall in south-east Australia since 1900.

Researchers analysed up to 40 years of stream-flow data from 161 catchments in Victoria and found that 37% had not recovered from the drought by mid-2017. Of those, 80% showed no sign of recovery.

A paper published in the journal Science on Friday said the findings “suggest that hydrological droughts can persist indefinitely after meteorological droughts”.

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Lead author Dr Tim Peterson, from Monash University, said the reason for the reduced water flows was not clear. It was not linked to land use or an increase in ground water, and did not change in response to the wetness of the catchment.

“The groundwater data says it’s not going to groundwater,” he said. “It’s got to go somewhere. The only place that we found was reasonable was increased evapotranspiration per millimetre of rainfall.”

That means that 100mm of rain in 1990 would have resulted in more water flowing into rivers than 100mm falling in the same spot in 2017. In other words, that drought could result in a permanent reduction to the water supply.

Peterson said the research undermined the assumption that water catchments would always return to normal when normal conditions were restored.

“All of our models had this assumption that no matter how big a disturbance is, once that disturbance ends eventually the rivers or aquifers will come back,” he said. “I have always been very sceptical of that.

“What this really means for Australia is that our catchments are more complex than we thought … It can mean that our hydrological droughts can continue long after the climate drought has ended.

“It also means that the models and equations that are used to understand stream flow can’t account for this that we’re showing in the paper.”

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The worst affected catchments were in central and western Victoria, but Peterson said neighbouring catchments could behave “quite differently”. The study did not find any indicators of what made a system less likely to recover from drought, or how to help recovery.

The Victorian government, which funded the research through its water and climate initiative, has begun adjusting its models to include reduced stream flows.

“We don’t yet know what will cause the catchments to recover but the water industry in Victoria has been trying to use the more recent historical record so that in a way they’re doing their best to account for this shift,” Peterson said.

The climate emergency could amplify the problem due to an increase in rainfall variability. Even without a period of sustained drought, an increase in year-on-year rainfall variability could result in a change in run-off, he said.

“We can probably expect greater variability in the stream flow than we thought we were going to have,” he said. “Knowing which catchments will have these changes is something we can’t predict yet.”