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DEFORESTATION AND FLOODS:
Not the root cause

EVERY year, massive floods cause misery to millions of people living in Bangladesh, Cambodia, China, India, Thailand, Vietnam and elsewhere in the Asian lowlands. Politicians and environmentalists alike commonly attribute such disasters to the clearing of Asia's upland mountain forests. But this blame is unfairly attributed, according to a United Nations report published this week.

Conventional wisdom says that forests prevent flooding by acting as giant sponges. Tree roots, the forest soil and leaf litter soak up water during heavy rainfall, delaying or even altogether preventing its arrival in nearby watercourses. Contrary to such popular belief, the report says, in reality the forests have only a limited effect on major floods. It is true that, on a local scale, trees can reduce the volume of water arriving in nearby streams and rivers, or at least, spread its arrival over a greater time span. However, this holds true only for relatively short periods of light, drizzly rain over small areas.

If the rain is heavy, prolonged and falls over larger areas, the presence of trees will not prevent catastrophic floods. This is because, when major floods do occur, it is most often towards the end of the rainy season, when heavy rain falls on areas that are already waterlogged. That is to say, the giant sponge that is the forest has already become saturated.

Under these circumstances, flooding is unavoidable. Indeed, not only is it impossible to prevent such flooding, but doing so would lay waste some of the benefits of becoming temporarily waterlogged. Flooding is important for maintaining biodiversity and preserving fish stocks. It also replenishes the fertility of flood plain soils. Farmers in many flood plains plant crops that grow underwater, such as deep water aman rice or jute.

The problem is not a lack of trees

The report's numerous authors also attack claims that the presence of trees prevents catastrophic landslides. It is true, they say, that tree roots can trap sliding soil but only for shallow landslides of up to a metre in depth. If the slipping soil is deeper than three metres, the presence of trees will not be sufficient to stop the slide.

The report, "Forests and floods: drowning in fiction or thriving on facts?", was compiled by researchers based at the Centre for International Forestry Research-a research institute based in Bogor, Indonesia, dedicated to forest conservation and human welfare-working with a host of other international teams. It was published in collaboration with the UN's Food and Agriculture Organisation.

So, if the removal of the forests from the mountains whose rivers feed the coastal cities is not to blame for disastrous floods, what is? The report contends that the main cause of the damage is the growth in the human population and associated economic activities. Towns and cities have long been established on flood plains, despite the risk of periodic flooding. This is because the numerous social and economic benefits of living near water tend to outweigh the risk. Historically, such settlements have clung to the higher ground on the flood plain. But as towns and cities have grown, new housing estates and commercial zones have been built on ever more flood-prone areas.

The flooding of the Yangzi river in China in 1998 caused $30 billion-worth of damage. Further floods in 2000 in Cambodia and Vietnam caused $145m and $285m, respectively. Globally, floods affect the personal and economic fortunes of more than 60m people every year.

Yet measuring the severity of floods in terms of economic losses rather than the geographical extent of the waters can give the impression that flooding has become much more severe in recent times. In reality, the huge economic losses attributed to flooding in recent years are mainly a reflection of economic growth, increased investment in infrastructure and rapidly growing flood plain populations. Floods are no more frequent today than they were 120 years ago, when lush forests were abundant. The problem is just that more people now live and work on flood plains.

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