Widespread flooding is not caused by deforestation

By Henrylito D. Tacio
On the morning of November 5, 1991, water from a heavy rainfall brought about by tropical storm creating a flash food that swept down from the hills into Ormoc City in Leyte, Philippines, killing approximately 8,000 people and living an additional 50,000 homeless.
In 1998, the flooding of Yangtze River devastated large areas of central China, which resulted in damage in excess of US$30 billion.  Floods in 2000 affected 3.5 million people in Cambodia (one-third of the population) and 5 million in Vietnam.   In the same year, floods in Bangladesh displaced more than 5 million people and in India 30 million.
“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 to road and bridges, buildings and other economic infrastructure, in the loss of agricultural crops and livestock, loss of productivity in industry, commerce and trade – not to mention the incalculable loss of human life.
People oftentimes blame the destruction of forests when rains lead to severe flooding.  But United Nations official Patrick Durst begs to disagree with this conventional wisdom.  He says there is no scientific evidence to link major, large-scale floods to the loss of forests.
“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.  He adds that such ideas have, in the past, led some governments to force poor farmers from their lands and away from forests while doing nothing to prevent future flooding. “Such actions are totally misguided,” he points out.
Are floods caused by nature or by human activities like logging?  Indeed, a hard question to answer but Forests and Floods: Drowning in Fiction or Thriving on Facts? tries to separate fact from fiction on issues related to 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,” explains the 30-page report published by FAO.   “Rain then has nowhere to go but into rivers where it fills them to overflowing.”
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.
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 Center. 
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,” explained 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.
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.
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 FAO report noted.
“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 lead to aid for reforestation projects.  Concerning the landslides that buried hundreds of people in Central America in 2005, Greenpeace’s John Sauven told the Independent newspaper: “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.

Here’s what Durst said about the study: “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 localized 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’ and tomorrow’s upland and lowland populations.” — ###


One response to “Widespread flooding is not caused by deforestation

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