- The Washington Times - Monday, August 8, 2005

LONDON — Millions of dollars in overseas aid are wasted every year planting trees in dry countries in the hope that they will attract rainfall and act as storage for water, a British government-funded study has found.

In fact, forests usually increase evaporation and reduce the amount of water available for human consumption or growing crops, said the four-year study.

Research on water catchments on three continents found it is “a myth” that trees always increase the availability of water.

Even the cloud forests of tropical Costa Rica had less effect in stripping water out of clouds than assumed, according to the study, funded by Britain’s Department for International Development and released last week.

In one part of a large study, Sampurno Bruijnzeel, of the Free University of Amsterdam and one of the world’s leading specialists on tropical forests, measured the overall amount of water flowing from wet, forested catchments in Costa Rica and found there was no more water than was flowing from surrounding grassland.

Although cloud forests were more effective at stripping water out of clouds, the researchers found, the evaporation was higher than on short vegetation.

The study, created for Britain’s tropical Forestry Research Program, shows that converting cloud forests to pasture does not cause major reductions in water yields.

In India, South Africa and Tanzania, the study showed, plantation forests actively wasted water and were “ineffective” or “counterproductive” at retaining water.

Studies by Newcastle University in the Indian states of Himachal Pradesh and Madhya Pradesh found that planting trees had a “negative” effect on the water supply.

The study suggests that efforts to convert agricultural land to forest caused a 16 percent to 26 percent reduction in water yield. The study suggests that rainfall evaporates up to twice as fast in forests as it does in treeless areas.

“Putting in more forestry in water-stressed catchments where the water table is 500 feet down and the groundwater is being mined for uncontrolled crop irrigation will only aggravate matters,” said Ian Calder of Newcastle University, one of the researchers.

“The public perception is that where you plant trees, you will be increasing the groundwater recharge. The evidence is the opposite.”

Another “water myth” the study debunks is that trees, particularly plantation trees, are always beneficial in preventing erosion and stopping floods.

Mr. Calder said the study showed that there might be some benefit with plantations taking up water in most years, and reducing annual floods, but in major one-in-20-year floods, the benefits were “negligible.”

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