Text by Henrylito D. Tacio

Photo courtesy of Dr. Teofredo Esguerra

“Floods are among the most destructive calamities man has to cope with.”

The statement comes from the Philippine Atmospheric, Geophysical, and Astronomical Services Administration (PAGASA).

A really big flood can result in millions, even billions of pesos 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.

More often than not, people blame the destruction of forests when rains lead to severe flooding. But former United Nations official Patrick Durst begged to disagree with this conventional wisdom. 

Durst was then the regional forestry officer of the UN Food and Agriculture Organization in Bangkok, Thailand. In a conversation, he said 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,” he said.

Such belief in the past, he pointed out, 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 said.

Now, this is getting interesting. Are floods caused by nature or by human activities like logging and farming?

That’s a tough question to answer if it is going to be asked in a beauty pageant question and answer segment.

The FAO came up with a book that tries to separate fact from fiction on issues related to forests and water. Forests and Floods: Drowning in Fiction or Thriving on Facts? 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 said.

“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,” explained the 30-page FAO publication. “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.

Some experts believe that 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,” said 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, who was then the director-general of the Center for International Forestry Research when he said those words.  

“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,” he said. “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 that happened 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 rainfalls and soils are already saturated and incapable of soaking up additional water, the FAO report noted.

“The reality is far more complex,” said Dr. He Changchui, former 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. 

“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.”

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