Deforestation may cause rainfall in the Amazonian basin to decline disastrously, British scientists said in a study published on Wednesday by the journal Nature.

Rainfall across the vast basin could lessen by 12 percent during wet seasons and 21 percent during dry seasons, potentially inflicting astronomical costs on farmers and reducing hydro-electricity output from receding river flows.

University of Leeds researcher Dominick Spracklen and colleagues put together a computer model based on satellite data of forest cover and rainfall patterns.

Air that passes over dense tropical vegetation carries at least twice as much rain as air that passes over land with sparse vegetation, they found.

The reason for this, they said, lies in a phenomenon called evapotranspiration.

Tropical forests are highly efficient at sucking water out of the soil, much of which is then delivered to the atmosphere as vapour through leaf pores.

This not only helps to keep the local humidity of the forest at a constant level -- it also charges the winds with droplets which are deposited further afield as rain.

Deforested land, though, is far less effective at recycling water this way, which means the air above it is less moist.

Factoring in logging trends in the early part of the century, which indicate 40 percent of the Amazon will be deforested by 2050, the team say the loss of rainfall across the river basin, from east to west, will be dramatic.

Luiz Aragao, an environmental scientist at the University of Exeter, said the change in rainfall would be especially worrying for eastern and southern Amazonia.

On the assumption endorsed by many climatologists that global temperatures will rise by some three degrees Celsius (5.4 degrees Fahrenheit) by century's end compared to pre-industrialisation levels, the impacts there "could be huge," he said in a commentary.

"Changes in regional climate could exacerbate drought-related tree mortality, which in turn would reduce carbon stocks, increase fire risks and lower biodiversity.

"Such changes might also directly threaten agriculture, which generates $15 billion (12 billion euros) in Amazonia, and the hydropower industry which supplies 65 percent of Brazil's electricity."

On the plus side, Aragao said the logging trends used in Spracklen's model could be pessimistic, as Brazil has pledged to limit historical deforestation rates by 80 percent by 2020.