Battling Floods: Can Forests Contribute?

There has been much debate about how to prevent, mitigate and manage flood-related disasters in the Asian lowlands, and indeed much has been accomplished in terms of better land use planning, flood preparedness and forecasting, and helping affected people cope with floods. Each flood puts the link between the poor state of upland forests and floods on the political agenda. Calls are made for logging bans – imposed after major floods in China, Thailand and the Philippines – reforestation projects, or resettlement of upland dwellers who are often ethnic minorities. Without doubt, such actions carry socio-economic costs. The question that has not been properly answered is whether the costs are balanced by benefits, which mainly lowlanders living in flood-prone areas realize.

A closer look at the relationship between floods and forests shows that much of our existing “knowledge” is based more on fiction than facts. Hydrological systems are complex and it is difficult to disentangle the impacts of land use from natural phenomena. Although much is known about the relationship between floods and forests, such knowledge is often used to make generalizations that are frequently inappropriate or even misleading. Little difference is made between what we know and what we think we know, which contributes to general confusion.

Much of this confusion has a long history and relates to the “sponge theory”, according to which forests act as a sponge soaking up water during rainy spells and releasing it evenly during dry periods. The simplicity of the theory makes it appealing. It is true that deep soils have higher water storage capacities than shallow soils irrespective of vegetative cover. It is also true that forest soils generally have higher water infiltration and storage capacities than non-forest soils. However, much of the absorbed water is consumed again, quite extravagantly, by trees and does not replenish low dry-season river flows.

This is not to say that forests are not beneficial in hydrological terms. On a small scale (up to 500 km²) they can affect peak river flows and thus floods. However, even in smaller basins, the extent to which forests can absorb excess water during heavy rainfall depends on forest type and management, and more importantly, on the underlying geological conditions and antecedent rainfall conditions. In large basins, the positive effects of forests on catastrophic floods are negligible.

Floods occurred long before deforestation was a problem. For example, eight major floods were recorded in Bangladesh between 1870 and 1922. Flood intensities have most likely also not increased. Damage has been due to economic growth, investment in infrastructure and a growing floodplain population, with people and institutions that have forgotten how to live with rivers and floods. New developments have expanded into flood-prone areas that used to be avoided, and urban areas have transformed formerly vegetated land into impermeable surfaces. Today’s floodplains bear little resemblance to yesterday’s and it should not surprise that even minor floods can cause major damage.

Well-managed forests provide a multitude of products and environmental services. There are plenty of reasons to use them wisely. However, the scope of forests to reduce the severity of major floods derived from an extended period of intense rainfall is rather limited. Effective approaches to reducing flood damage need to focus on downstream areas and floodplains. Above all, whatever approach we choose, we need to be more open to facts, although they may not confirm our beliefs. The lives of millions of people are affected when we embrace fiction.

Dr Thomas Enters is presently with the Food and Agriculture Organization of the United Nations (FAO) as Forestry Sector Analysis Specialist. He can be reached at Thomas.Enters@fao.org