Soil erosion occurs primarily when land is exposed to the action of rain. Unprotected by a cover of vegetation and the binding action of roots, each raindrop hits the naked soil with the impact of a bullet. On a slope, more than half of the soil contained in those splashes is carried downhill and either ends up in the valley below or are washed out to sea by streams and rivers.
Soil erosion by rain and snow and by streams and rivulets occurs throughout the whole world, though its seriousness varies with environmental conditions. Natural causes result in the washing away of 9.3 billion tonnes of soil a year, but human intervention increases that figure to approximately 24 billion tons of soil a year, which ends up in rivers and finally in the oceans. The FAO estimates that 11.6% of Africa north of the Equator, and 17.1% of the Near East, are subject to water erosion, as are 90 million hectares (of a total of 297 million) in India. In Nepal, the removal of topsoil by the monsoon rains does double harm, first by denuding the hillsides, and second by filling the Himalayan rivers with silt. The beds of some rivers now rise a foot every year. The swollen rivers then overflow their banks to progressively wider levels, sometimes even changing course because of the floods, thus drowning the best farmland. The Malagasy Republic is being virtually washed away; in some areas as much as 250 tonnes of soil per hectare is lost every year. The causes are both man-made and natural. Madagascar's soils tend to be erosion-prone, a condition aggravated by tropical cyclones which can deluge the island with up to 15 mm of rain in 15 minutes.