Precipitation on land versus distance from the ocean: Evidence for a forest pump of atmospheric moisture

Flux of oceanic moisture brought inland by winds has been conventionally considered as a geophysical parameter practically unaffected by vegetation; accordingly, models predict only slight post-deforestation precipitation reductions. Here we show that the dependence of annual precipitation on distance x from the ocean differs markedly between the world's forested and non-forested continent-scale regions. In the non-forested regions precipitation declines exponentially with distance from the ocean with an established global mean e-folding length of l similar to 600 km. In contrast, in the forest-covered regions precipitation does not decrease or even grow along several thousand kilometers inland. Using a novel physical mechanism involving the non-equilibrium distribution of atmospheric water vapor it is explained how the high transpiration fluxes developed by forests enable them to pump atmospheric moisture from the ocean to any distance inland to compensate for the gravitational runoff of water. Our results indicate that forest cover plays a major role in the atmospheric circulation and water cycling on land. This suggests a good potential for forest-mediated solutions of the global desertification and water security problems. (C) 2008 Elsevier B.V. All rights reserved.