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New research, based on forest fires in Australia, proves there is a significantly higher risk of large-scale flooding when major deforestation has occurred in catchment areas. The chance of large-scale flooding in a specific catchment area can increase by as much as 700% if widespread deforestation has occurred.

That is the finding of a new paper by academics who analyzed decades of flooding data from regions in Australia that frequently suffer forest fires.

The loss of forest canopy has long been claimed to increase the risk of flooding—but actually proving the direct link has proven difficult, since the impact of other factors such as changing climate are hard to exclude.

But a team of researchers, led by Professor Ashish Sharma from UNSW Sydney, have now confirmed the correlation and determined there is a very significant increase in the risk of flooding if forest canopies are removed from catchment areas.

"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 Prof. 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 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.

"Deforestation and mega forest fires due to climate change are happening all around the world, so this is a very important topic."

Decades of data

The results of the research have been in Communications Earth & Environment and were based on information related to three mega forest fires in south-east Australia, as well as streamflow data in the same region covering 50 years when no major forest fires were reported.

The team were able to identify nine historical years when the climate strongly matched those of 2003, 2007 and 2009 (when the devastating fires occurred), 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.

However, in the period immediately following the megafires, 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 Ph.D. candidate and is now a Senior Researcher at the K-water Institute in Korea, while 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 utilize 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 megafires in 2003, 2007 and 2009—not two or three years afterward.

"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 research team, which included Professor Lucy Marshall who was at UNSW but has since taken up a role at University of Sydney, also took into consideration the fact that flooding events could be impacted by the effect of the forest fires on the surface soil.

Fires can make the soil hydrophobic—which means it repels water rather than absorbs it—thus potentially causing an increase in the chance of a flood event after rainfall.

But analysis showed this was not a significant factor in the flood risk increasing when compared to the impact of the loss of the forest canopy and leaf litter which absorbs and disperses rainfall.

And to double-check their conclusions, the research team also 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 Professor Young-Oh Kim from Korea's Seoul National University and a co-author of the study.

"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 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. With more forestation, there is a two-fold impact that reduces the impact of rainfall and reduces the risk of flooding.

The first is the canopy of the trees—that is the branches and the leaves—which first 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.

Second, with more trees in a catchment area, there are also more fallen leaves and that dampens the flow of any rain that does reach the ground.

When there is significant deforestation, the canopy and leaf litter are severely diminished, and the water they were previously absorbing goes 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

Prof. Sharma hopes the research will be taken on board by anyone involved in the process of man-made deforestation, given the devastating impact flooding can have on human populations who may be living in areas at risk.

"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."