(CN) --- Rain has long signaled the end of a drought, but a new analysis of Australia’s worst dry spell in a century reveals many water reservoirs remained low nearly a decade after the rain returned. Research published in the journal Science on Thursday studies 15 years of data collected before, during and after the Millennium Drought dried out Victoria, Australia, from 2001 to 2009 and reveals flaws in the long-held belief that reservoirs naturally replenish after drought ends.
"Whether or not watersheds always recover from prolonged droughts has major implications for global long-term water resource planning and aquatic environments, especially under climate change,” study authors wrote.
Around the eighth year of the Millennium Drought, half of the 161 watersheds studied were operating in a low runoff state. Streamflow then remained low several years after the drought had ended --- when recovery is generally expected to occur.
In 2011, the year following the drought, only 15% of watersheds studied recovered their pre-drought streamflow levels. Seven years after the drought ended, runoff in 37% of watersheds studied remaining in a low state. And only 20% of those showed signs of improving.
Researchers generated 64 annual and 32 seasonal models for each watershed in efforts to understand post-drought watershed recovery — a topic of vital importance as droughts becomes more common under climate change.
While they have not found the prolonged non-recovery elsewhere, the shift into a low-streamflow state has been observed among watersheds in California and China.
“Given the prerequisite for a shift during a drought has been met elsewhere, it is reasonable to assume that some of these U.S. [and] China catchments may also not recover, at least within the time frames we've examined,” the paper’s author Tim Peterson said in an email. Peterson is a senior lecturer of environmental engineering at Monash University, in Clayton, Victoria, Australia.
“We're not saying in our paper that watersheds never recover. We're staying that they appear stuck in a low runoff state,” Peterson added. “One day they may recover, but they do not simply recover once the weather returns to normal after the drought.”
Notably, the post-drought problem isn’t driven by a lack of rain. Instead research suggests more rainwater than usual soaks into the vadose zone — the space between the earth and the watershed which grows in droughts. Additionally plants which adapted to low water conditions during the drought increase transpiration, putting more water back into the air rather than downstream.
"Evidence suggests that the vegetation responded to the drought by increasing the fraction of precipitation going to transpiration,” the study authors wrote. "Our findings suggest that hydrological droughts can persist indefinitely after meteorological droughts and that the mechanism for recovery remains an open question.”
The Victoria Department of Environment, Land, Water and Planning funded this research.
As his team develops mathematical models to understand control recovery and watershed thresholds, Peterson is looking to recruit doctoral students to continue this work. Peterson wants to understand where the tipping point lies before a watershed switches to a low flow state — an elusive discovery that just might provide a better benchmark for communities to measure water use by.
“To build resiliency we first need to understand that watersheds appear more complex than was thought --- and do not have an infinite resilience to disturbances like drought,” Peterson said. “We don't yet have the tools and science to resolve these modelling limitations and so for now it would be prudent for governments and communities to understand that their model estimates may consistently overestimate the water resources after droughts.”
He added a warning: “It is very difficult for governments and communities to know if a particular watershed will or will not recover. So they should be cautious."
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