Flooding in Dhaka, Bangladesh

Flickr/Sumaiya Ahmed

Deforestation increases the frequency and severity of flooding in developing countries, according to a recent study.

While a link between deforestation and flooding has been suspected for many years, the study, published online in Global Change Biology last week (25 September), is the first to support the hypothesis with global-scale evidence.

"There's been a lack of consensus about whether forests can have a positive effect [on flooding]," says William Laurance, a researcher from the Smithsonian Tropical Research Institute in Panama. "But I think this research will have a big impact."

The research team, led by Corey Bradshaw of Australia's Charles Darwin University, studied associations between native forest cover and flooding during the 1990s in 56 developing countries across Africa, Asia and Latin America.

Models showed that removing a tenth of existing forest cover would increase the frequency of floods by 4–28 per cent and lengthen their duration by up to eight per cent.

Forests may reduce the severity of floods in a number of ways. Firstly, rainwater is taken up by tree roots and released through their leaves. Roots also make the soil more porous, increasing the amount of water it can absorb.

The process of deforestation itself reduces soil's absorbency, Bradshaw told SciDev.Net. Removing forests compacts the soil, so there are fewer gaps underground and less capacity for absorbing water, affecting the run off, he explained.

Over the ten-year study period 100,000 people in the developing world were killed, and another 320 million displaced by floods, with economic damage exceeding US$1 trillion.

Bradshaw hopes that by demonstrating the benefits of ecosystem conservation, his team's research will encourage developing countries to protect their forests.

"It's a very economic way of looking at the value of forests, and I think governments will hopefully take notice of this," says Bradshaw.

Whilst discouraging people from living in floodplains would perhaps be the best way of preventing damage, this is not always a viable option, says Laurance. "The reality is that in developing countries people are building on floodplains, and anything that they can do to reduce flood intensity is a good thing."

Nations such as Bangladesh, China, India and Nepal, which have already been subjected to severe flooding, have already invested in forest protection or reforestation projects.

Link to abstract in Global Change Biology