Challenging the assumption that watershed streamflow always recovers from drought, a new study done seven years after the "Millennium Drought," the worst drought ever recorded in southeastern Australia, reports that more than a third of the region's affected watersheds had not yet recovered. Of these watersheds that were still dry seven years later, most showed no evidence of recovering soon, despite the rains' return. The new study's findings suggest that hydrological droughts can persist indefinitely after meteorological droughts, highlighting an amplification of climate change impacts that could present additional challenges to the sustainable use of already-threatened water resources. Watersheds are widely assumed to fully recover from droughts when precipitation resumes. However, the understanding of how watersheds recover from severe droughts is limited and often overlooked. Using the Australian Millennium Drought (2001-2009) as a natural experiment to better understand post-drought watershed recovery, Tim Peterson and colleagues evaluated annual and seasonal precipitation and runoff records for more than 160 river basins across the affected region before, during and after the drought. Peterson et al. found that, even seven years after the drought's end, runoff for 37% of affected watersheds had not yet returned to predrought levels. Furthermore, 80% of those that had not yet recovered bore no evidence of doing so within the foreseeable future. "The conclusion of Peterson et al. - that river basins may irreversibly turn into a persistent water scarcity state after severe meteorological droughts - challenges the comforting assumption that water systems naturally tend to absorb disturbances," writes Flavia Tauro in a related Perspective. "This emphasizes the necessity to change the way global water processes are conceptualized."
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