Professor Ashish Sharma from UNSW Sydney, who led the research team said:

“What we have shown is that the probability changes from a one-in-64 year flood event, to a one-in-eight-year flood event if there has been deforestation.

“So we can say there is an eight-fold increase in the likelihood of there being a flood,” says Professor Sharma, from UNSW’s School of Civil and Environmental Engineering.

“Another way of thinking about it is to imagine there being 64 different forested catchments. In any single year, the most extreme climate would cause the highest flood in one of these 64 catchments.

“But according to our analysis, if there was major deforestation, then in the same year, the same flood would be exceeded in eight of the 64 catchments.

“Deforestation and mega forest fires under climate change are happening all around the world, so this is a very important topic.”

The research was published in the Communications Earth & Environment journal.

Decades of data

The analysis used information related to three mega forest fires in south-east Australia. As well as streamflow (how much water is flowing past a certain point) data in the same region covering 50 years when no major forest fires were reported.

The team identified nine historical years when the climate strongly matched those of 2003 (Eastern Victorian Alpine Bushfires), 2007 (Great Divides Fire) and 2009 (Black Friday Bushfires), and also ensured there were no extreme rainfall patterns in any of the datasets which could distort the comparisons.

That meant any years when El Niño (lower average rainfall) or La Niña (higher average rainfall) were declared would be ignored, as they can dramatically change the risk of flooding by themselves.

Analysis of the remaining records showed that in the years when there were no forest fires in the region, the chance of exceeding the normal annual flood levels in any given catchment area was 0.016 – or roughly 1 in 64.

In the period immediately following the mega fires, the chance of an unusually large flood in a catchment area rose to 0.127 – or roughly 1 in 8.

“Our paper tries to assess whether the loss of forest canopy increases the risk of flooding or not in large-scale watershed processes,” says Dr Tae-Ho Kang who worked on the project as a PhD candidate. He is now a Senior Researcher at the K-water Institute in Korea where he is continuing the work with the team at UNSW.

“This 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 to historical records from the same catchment areas when we could see the climate was very similar and the forest canopy was in place.

“And we only looked at flooding data in the wet season immediately following those mega fires in 2003, 2007 and 2009 – not two or three years afterwards.

“So basically we could see in the comparisons that the climate was similar, the rainfall was similar, and the locations and the geology were obviously the same.

“The only difference was whether there was forest canopy in the catchment area, or if there was widespread deforestation caused by the fires. That really allowed us to contrast the data with regards to the risk of significant flooding.”

Absorbing the rainfall

The authors considered how flooding events could be impacted by the effect of the forest fires on the surface soil.

Their analysis revealed that the loss of forest canopy and leaf litter impacted flood risk much more than the changes to surface soil.

To confirm their conclusions, the research team compared streamflow data in relevant years from areas where damage from forest fires was extensive, versus that in areas where the burning was not so severe.

“We contrasted those two sets and we could see there was a dampening effect in the forested catchments which was not happening in the more heavily deforested areas,” says co-author Professor Kim from Korea’s Seoul National University.

“And that is comparing the levels of flooding in proximal areas in the same year, so obviously the climate was the same for both and the only thing significantly different is the loss of forest canopy.”

“Recent mega wildfires in South Korea highlight the country’s increasing exposure to severe forest loss driven by climate change, a phenomenon that has become increasingly common worldwide. According to our study, after each disastrous wildfire, societies need to seriously consider the flood risk increase that will occur.”

The paper helps explain the specific process by which a large forest canopy helps lower the risk of flooding.

A bigger canopy helps reduce rainfall impacts and flood risk in two different ways.

The first is via the branches and leaves forming the canopy – which block the rain from reaching the ground. If there is heavy rain it can also disperse the water over a wider area and over a longer time which reduces the peak amount of moisture reaching the ground and therefore lowers the potential flood levels.

Secondly, catchments with more trees have more fallen leaves to dampen the flow of any rain that reaches the ground.

When there is significant deforestation the canopy and leaf litter is severely diminished, and more rain is absorbed straight into the soil, which can then more quickly get saturated. And when that happens the risk of a flood subsequently goes up.

Implications for the future

Professor Sharma hopes the research will inform policy around man-made deforestation.

“The data is collected from south-east Australia, but we see no reason why the conclusion does not apply in the same way anywhere else where forest canopy is being removed,” he says.

“Our main message is that extensive deforestation has a significant increase in flood risk. 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.”

Declaration: The authors declare no competing interests.