Water management programmes across the developing world are based on the mistaken belief that trees increase the available water in an area.

Forests tend to diminish water supplies because they lose more water through evaporation than other vegetation, say the researchers.

In wet climates, this is because of the ‘clothesline’ effect: just as wet clothes hanging on a line will dry faster than those laid on the ground, tall trees lose more water than small shrubs.

In dry conditions, trees evaporate more than other plants because their deeper roots take up more water to evaporate.

Forestry programmes sometimes also receive far more attention than water-intensive activities such as irrigation, which if better regulated could contribute to water conservation.

Without trees, soils can become degraded and less able to absorb water. This can lead to sudden floods during the rainy season and inadequate replenishment of groundwater supplies to sustain livelihoods during the dry season.

Forestry decisions need to take into account the different effects of alternatives like crops, settlements and grasslands, on soil quality and water availability.

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