Does Felling Of Trees Increase Floods

By Professor Nils Nykvist

The amount of water in a forest soil is usually smaller than that on open ground in the same area, due to the evaporation of water that is trapped in the canopy and the forest’s high transpiration rate compared with other plant communities. Forest plantations on such land therefore mean that less water runs off the catchment.

In South Africa it was already found in the 1930s that plantations of acacia trees caused wells in the watersheds to run dry [22]. Since then, many more detailed investigations have been done with similar results. In several countries forest plantations have even been banned in areas where important water sources could be threatened.
When forests are cut down, the moisture content of soil thereby increases, if soil conditions otherwise are equal.

In M. G. Stålfelt’s classic textbook Plant Ecology from 1960, several studies are cited showing an increase of groundwater level and runoff after harvesting forest areas. In a river in eastern Colorado, USA, the runoff increased when firs and pines died due to insect damages in an approximately 1800-square-kilometre forest area [23].

In many parts of the world, excessively hard land use has transformed the original forest to a more low-productivity plant community with much lower water consumption and greater surface run-offs.

Over the past decades, many studies in small catchments have shown that logging of forests leads to increased runoff in the streams. When large forest areas are clear-felled simultaneously, local floods can occur after heavy rainfall.

Almost every year, particularly in Asia, there are major floods in the lowlands that cause many deaths and deteriorating living conditions for millions of people. One reason for this is the population growth, which forces people to live in areas where the risk of flooding is particularly high due to dam failures.

However, the growing population has also led to a serious deforestation of large areas, and many people therefore believe that the major floods are caused by deforestation in the highlands.

Although there is a strong link at the local level between deforestation and runoff from small areas, there is still a long step to find a link between deforestation and catastrophic floods in large river basins.

A wide basin consists of many small ones, where rainfall, soil, vegetation, and land use can vary considerably. In a study by the FAO/CIFOR in 2005 it was found that local flooding that may occur after rain in small river basins following clear-felling is levelled out when the catchments are greater than 50,000 hectares [24].

The results of this study are not consistent with the vivid description of the relationship between deforestation and flooding that Plato gave more than two thousand years ago [12].

However, Plato compared forest areas with eroded areas where the water storage capacity of the soil was reduced. The cutting down of forests need not lead to catastrophic flooding in the future, if the area is re-vegetated and allowed to grow back to forest.

But when the cutting down of the forests is followed by overexploitation of the soil, causing soil erosion, then the life-giving water becomes a destructive force that destroys the lives of both humans and animals.

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