Forest loss not to blame for major flooding, say UN experts

Posted: 20 May 2008

Author: Henrylito Tacio

It is commonly thought that deforestation is a major cause of severe flooding. But United Nations official Patrick Durst, and some other experts, disagree. Planting trees and watershed management has many environmental benefis, but preventing large-scale floods is not one of them, they argue in a recent report. Henrylito Tacio investigates.

"Floods are among the most destructive calamities man has to cope with," says the Philippine Atmospheric, Geophysical and Astronomical Services Administration. A really big flood can result in millions even billions of dollars of damages - not to mention the incalculable loss of human life.

Denuded forest, Philippines
Denuded forest, Philippines
Bare mountains where the forest has been felled, Philippines. Photo © Henrylito Tacio
Examples abound, from the 1991 flash floods that swept down from the hills into Ormoc City in Leyte, Philippines, killing approximately 8,000 people to the 1998 flooding of the Yangtze River in China that devastated large areas of central China. Two years later floods in Cambodia affected 3.5 million people, or a third of the population, and 5 million people in Vietnam. In the same year, floods in Bangladesh displaced more than 5 million people and in India 30 million.

"Government officials, aid groups and the media are often quick to blame flooding on deforestation caused by small farmers and tree cutters," says Durst, regional forestry officer of the Food and Agriculture Organization (FAO) in Bangkok, Thailand. Such ideas have, he says, have led some governments in the past to force poor farmers from their lands and away from forests while doing nothing to prevent future flooding. "Such actions are totally misguided," he adds.

So, are floods caused by nature or by human activities such as logging? The FAO report Forests and Floods: Drowning in Fiction or Thriving on Facts? tries to separate fact from fiction, at least in terms of forests and water. It also dispels some of the commonly held misconceptions about the role of forests in flood mitigation. "Clearly, floods are caused by nature, but in some cases they are exacerbated by human activities," Durst says.

"The floods blamed on deforestation almost always occur after prolonged rains, which saturate the soil, including forest soil, so that it can no longer absorb more water," saysa the report. "Rain then has nowhere to go but into rivers where it fills them to overflowing."

Simple equation

At its root, the flood equation is pretty simple: If a river cannot handle the load of water it's required to carry, it must rise. With enough water, it must rise above its banks and flood. The faster water runs from the watershed into the river, the higher a flood will be. Thus anything that increases runoff speed - like excessive pavement or ditching of farmland - will contribute to floods.

And the economic and human losses from floods have increased over the years mainly because more people live and work in areas where floods are common. "People need to stop blaming floods on those who live and work in and around forests," says Pal Singh of the World Agroforestry Centre.

Trees and forests may partly play a role in environmental protection. "Planting trees and protecting forests can have many environmental benefits, but preventing large scale floods is not one of them," says David Kaimowitz, director-general of the Center for International Forestry Research.

"If deforestation was causing floods, you would expect a rise in major flood events paralleling the rise in deforestation, but that is not the case. The frequency of major flooding events has remained the same over the last 120 years going back to the days when lush forests were abundant."

Large-scale floods in the Chiang Mai valley in northern Thailand are well documented for events in 1918-1920 and again in 1953. "These floods all occurred when lush forests were still abundant in Thailand," the FAO report said.

Giant sponge

The conventional view for 100 years has been that forests prevent floods by acting as a giant sponge. According to this theory, developed by European foresters at the end of the 19th century, the complex of forest soil, roots and litter acts as a giant sponge, soaking up water during rainy spells and releasing it evenly during dry periods, when the water is most needed.

Despite relatively good rainfall and soil rich in minerals, erosion on Madagascar is some of the worst in the world, the result of centuries of chronic deforestation. Photo © CI/Russell A. Mittermeier
When major floods do occur, it is most often towards the end of the rainy season, when heavy rain falls and soils are already saturated and incapable of soaking up additional water, the report says.

"The reality is far more complex," says He Changchui, FAO regional representative for Asia and the Pacific. "Although forested watersheds are exceptionally stable hydrological systems, the complexity of environmental factors should cause us to refrain from overselling the virtues of forests and from relying on simple solutions."

After all, there can be a political interest in leaving the conventional wisdom about forests and floods unchallenged, the report points out. Governments can respond to floods with logging bans and give the appearance to the public they are taking decisive steps to stop flooding. The practical effect of such policies is to force poor farmers - who are routinely portrayed as major perpetrators of illegal logging - to abandon their lands. "Politicians and policymakers should stop chasing quick fixes for flood-related problems," urges Kaimowitz.

International agencies might also have an interest because the traditional beliefs can lead to aid for reforestation projects.

Counter claims

Commenting on the landslides that buried hundreds of people in Central America in 2005, Greenpeace's John Sauven told The Independent: "Unfortunately, something nearly always happens this time of year. Mudslides are becoming more and more common and deforestation certainly plays a role."

This claim has recently been supported by a study done by Dr. Cory Bradshaw of Charles Darwin University in Australia. "We found real evidence that deforestation also leads to more intense and devastating floods that kill more people and damage more property," he said.

But Durst is not convinced. "Bradshaw and his colleagues excluded from their analysis extreme flooding events caused by major storms such as cyclones and typhoons, the exact types of extreme rainfall events that cause major floods." In other words, Bradshaw's conclusion was based from localised conditions and non-major flooding events.

"While the ability of forests to prevent catastrophic floods is limited, watershed management should definitely not be abandoned," the FAO publication urged. "Forests provide a variety of environmental services, which need to be protected and nurtured for the benefit of today's and tomorrow's upland and lowland populations."

Henrylito Tacio is Planet 21 contributing editor in South East Asia.