One-third of Victorian water catchments have not recovered eight years after severe drought, according to a new paper published in Science, and the rivers may not recover anytime soon.
The Australian Millennium Drought between 2001 and 2009 devastated communities that relied on the Murray-Darling Basin. It is commonly thought that rivers and underground water will eventually replenish following severe droughts like this.
Now, a team of researchers, led by Tim Peterson of Monash University, challenge this idea. They found that groundwater supplies had not recovered from the Millennium Drought by mid-2017, and around 80% of those catchments show no evidence they will recover in the near future.
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“Our findings suggest hydrological droughts can persist indefinitely after meteorological droughts and that the mechanism for recovery remains an open question,” says Peterson.
“This new discovery just appears to be the way catchments naturally behave. It’s not explained by factors like land use. They are just more complex than we thought.”
Many watersheds rely on runoff – the water that flows over the land after rain – to replenish, but the amount of runoff was very low over the millennium drought. The team found that runoff behaviour was not yet back to what was needed in 37% of runoff-dependent catchments in Victoria.
While runoff is a result of precipitation, this drop in catchments shows runoff water post-drought isn’t actually making rivers. In other words, 100mm of rain before the drought made more rivers than 100mm of rain did in 2017, and this results in catchments not recovering.
Peterson also says this shows that plants responded to the drought by using more water, relative to the amount of rain, in transpiration – the process of plants taking up water and later losing it though evaporation.
“Practically, this implies that in response to the Millennium Drought, vegetation in selected water catchments responded by maintaining similar rates of transpiration,” he says.
The team used statistical modelling of rainfall and stream flows to assess 161 Victorian catchments over 30 years.
“It’s exciting that the findings have already begun to be used in how water is managed. We are now developing mathematical tools to further help water management use these findings to ensure long-term water supply within a challenging and changing climate.”
The study was predominantly funded by the Victorian Government’s Victorian Water and Climate Initiative with more recent support from the Australian Research Council.