Deforestation in river catchment areas can increase the likelihood of a significant flood by up to 700%, new research suggests.

The study analysed the frequency of major floods in three regions across southeast Australia following years in which bushfires occurred.

The researchers then compared the flood frequency to 50 years of streamflow data from the same areas when no fires happened.  

Lead researcher and University of NSW civil and environmental engineering professor Ashish Sharma says his team found the “probability changes from a one-in-64-year flood event to a one-in-eight-year flood event if there has been deforestation.

“In any single year, the most extreme climate would cause the highest flood in one of these 64 catchments. But according to our analysis of the data, if there was major deforestation, then in the same year, the same flood would be exceeded in eight of the 64 catchments.”  

Seoul National University professor Young-Oh Kim, who co-authored the research paper, says the correlation between deforestation and flood severity “has been a question raised since the 1600s, but it’s been almost impossible to resolve because flooding in a specific location over time can be caused by many different factors, not least a change in the climate.

“What we were able to do was utilise the data when we knew there was complete tree loss from major forest fires and compare that with historical records from the same catchment areas when we could see the climate was very similar and the forest canopy was in place.”  

The research team – which also includes University of Sydney professor Lucy Marshall – considered the impact fire has on soil’s ability to absorb water, but found this is not a significant factor compared with canopy loss.  

They say the findings emphasise the role branches and leaves play in blocking rain and dispersing water across a wider area during heavy falls.  

Professor Sharma said: “If deforestation is happening, then there should be strong consideration for planning rules or regulations regarding habitats downstream, to reduce the enhanced danger they will face from flooding.

“We would also advise that if deforestation has to occur, it should be restricted to areas that do not have vulnerable communities downstream.”