Each year extreme floods kill or displace hundreds of thousands of people and cause billions of dollars in damage to property. The consequences of floods are particularly catastrophic in developing countries generally lacking the infrastructure to deal adequately with above-average water levels.
For centuries it has been believed that native forest cover reduced the risk and severity of catastrophic flooding, but there has been strong scientific debate over the role of forests in flood mitigation.
Forest loss is currently estimated at 13 million hectares each year, with 6 million hectares of that being primary forest previously untouched by human activities. These primary forests are considered the most biologically diverse ecosystems on the planet, but this realisation has not halted their immense rate of loss.
Last year my colleagues and I published a paper entitled Global evidence that deforestation amplifies flood risk and severity in the developing world in Global Change Biology (highlighted in Nature and Faculty of 1000) that has finally provided tangible evidence that there is a strong link between deforestation and flood risk.
Using data from 56 developing nations in Central/South America, Africa and Asia, we correlated information on flood frequency and severity with country-specific forest data. After controlling for differences in rainfall, elevation, soil moisture and degraded areas, flood risk was strongly correlated with increasing deforestation. The models constructed predicted a 4 – 28 % increase in flood frequency with only a 10% increase in deforestation.
An important additional finding was that only the amount of native forest was correlated with reductions in flood risk – plantation forests had the opposite effect. This has huge implications for governments of developing nations trying to save lives and reduce expenditures. Promoting native forest conservation also has the added benefits of slowing climate change by storing vast quantities of carbon, reducing wildfires, and conserving species.
The study also investigated how deforestation affects the severity of flooding. We examined flood duration as an index of damage potential, as well as direct measures such as the number of people killed and displaced by floods, and the total estimated damage measured in dollars caused by powerful flood waters.
Although the correlations were not as strong, we found real evidence that deforestation also leads to more intense and devastating floods that kill more people and damage more property.
The implications of this study are measured potentially in the trillions of dollars over the next coming decades, so we hope it is used wisely as yet another good reason why we should preserve forests.