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Drought effects linger on catchments

Hydrological droughts can persist indefinitely after meteorological droughts, according to Australian-first research from Monash University, with a Victorian study showing one-third of water catchments had not recovered from a severe drought after nearly eight years.

Led by Dr Tim Peterson from Monash University’s Department of Civil Engineering, the study published in Science challenges the widely held view that rivers and underground water supplies eventually replenish following periods of severe drought or flood.

Researchers used statistical models of rainfall and streamflow at 161 water catchments across Victoria, each with more than 30 years of data and no upstream dams or water extractions.

Along with colleagues from the University of Melbourne, Peterson discovered that some rivers continue to behave like they’re still in drought for years following the event, creating a significant delay in recovery.

The study found that runoff had not recovered in 37% of water catchments in Victoria after Australia’s Millennium Drought.

Peterson said regeneration of water catchments after severe drought has major implications for global long-term water resource planning and aquatic environments, especially with climate change adding further pressure.

“Our findings suggest hydrological droughts can persist indefinitely after meteorological droughts and that the mechanism for recovery remains an open question,” Peterson said.

“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.”

Historical models show that 100mm of precipitation before the drought in 1990 created more river flow than the same 100mm in 2017, delivering a 30% reduced streamflow as a result of drought.

The number of water catchments with a low or very low runoff state increased rapidly from 1996 to the end of the meteorological drought in summer 2010. By 2011, only 15% of water catchments had recovered.

Peterson said evidence also suggested vegetation responded to the drought by increasing the fraction of precipitation going to transpiration — the process of water movement through a plant and its evaporation from leaves.

“Practically, this implies that in response to the Millennium Drought, vegetation in selected water catchments responded by maintaining similar rates of transpiration,” he said.

Long-term effects

Each water catchment analysed for this study had at least 15, seven and five years of streamflow observations before, during and after the Millennium Drought respectively.

Across all 161 water catchments, researchers found eight years into the drought, 51% of the catchments switched into a low or very low runoff state.

Importantly, by mid-2017, nearly eight years after the drought, more than one-third of water catchments still remained within a low runoff state, and have not recovered back to the pre-drought behaviour.

Peterson and his research partners have been working with Victorian and national water agencies, most recently through the Victorian Water and Climate Initiative.

“It’s exciting that the findings have already begun to be used in how water is managed,” Peterson said.

“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.”