Water management programmes across the developing world are based on the mistaken belief that trees increase the available water in an area, says a report published today (29 July).
From the Mountain to the Tap summarises four years of research led by the Centre for Land Use and Water Resources Research at the University of Newcastle, United Kingdom, and the Free University of Amsterdam, the Netherlands, into water management programmes.
Although the report does not advocate an end to tree planting — which can help limit soil erosion and preserve biodiversity — it does challenge the widely-held view that forested land always conserves and supplies more water than grassland or other treeless areas.
"Contrary to popular opinion, we found that trees usually reduce the amount of available water," says Ian Calder, director of the Centre for Land Use and Water Resources Research.
Forests tend to diminish water supplies because they lose more water through evaporation than other vegetation, say the researchers.
Calder told SciDev.Net that evaporation from forests can be twice that from grasslands.
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, adds Calder.
Calder and colleagues predict that efforts to convert agricultural land to forest in the Indian states of Himachal Pradesh and Madhya Pradesh could reduce their water supplies by up to a quarter.
Previous research in South Africa, for instance, found that timber plantations reduced local water levels.
Drinking water stored on the
hillside near Rambuda village,
South Africa (Photo credit:
South Africa now requires all farmers whose crops use more water than the area's natural vegetation to pay a tax. In future, some of this money could go to farmers whose crops use less water than native plants, says Calder.
Frank Rijsberman, director-general of the International Water Management Institute, based in Sri Lanka, believes the report's findings are of most value to areas, such as South Africa, where water is scarce.
"The report makes a very important point: that forests are users, not producers, of water," he explains. "But in the Upper Yangtze region of China, for example, they are planting trees to reduce flooding, not to conserve water."
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.
Sampurno Bruijnzeel, of the Free University of Amsterdam and one of the world's leading experts on how forests affect water supply, says the report's findings must be implemented with care because they do not take into account the role trees play in ensuring soil quality.
Bruijnzeel, who contributed to the study, adds that forests also play a role in climate change because they absorb carbon dioxide from the atmosphere. They are often havens of biodiversity, and can be an important contributor to rural livelihoods, for example by providing timber.
A rain gauge for measuring
fog and mist, Costa Rica
(Photo Credit: Arnoud Frumau)
There are fears that the report could lead forestry programmes to be discarded. "I am very, very concerned it could lead to a ban on tree planting," Bruijnzeel told SciDev.Net
But Calder stresses the report's conclusions do not advocate this.
"There is no doubt that trees provide a multitude of benefits," says Calder, "but we should promote them on the basis of real benefits, not on the basis of myths."
Forestry decisions need to take into account the different effects of alternatives like crops, settlements and grasslands, on soil quality and water availability.
Further reseach needs to investigate whether forests can increase rainfall and, if so, to what extent this is compensated for by extra evaporation, says Bruijnzeel.
The report was funded by the UK Department for International Development's Forestry Research Programme.
The World Commission on Water forecasts that demand for water will increase by about 50 per cent in the next 30 years, and that around four billion people — half of the world's population — will live in conditions of "severe water stress" by 2025.