Forest cover affects the absorption, reflection and transmission of light and heat from the surface of the earth, and of water from the forest canopy due to processes of evaporation and transpiration. On a global scale, only very large areas of forest appear to have a noticeable effect on climate and rainfall, although smaller areas of forest do have some effects on local microclimates.
Historically, it was believed that the presence of forests attracted rain or were instrumental in increasing rainfall. Over the last forty-five years, it has become possible to trace the movements of water vapor and atmospheric gases to develop a clearer idea of the role played by forests in moderating or regulating rainfall. In temperate regions and tropical regions such as Southeast Asia, the main source of water vapor in the atmosphere is from evaporation at the surface of the oceans. In the Amazon Basin, however, nearly 50% of water vapor in the atmosphere in the region of Manaus and Belém appears to be ‘recycled’ from the forest.
A good summary (not limited to tropical forests) of the effects of forests on rainfall is in: L. Oyebande "Effects of tropical forest on water yield" Chapter 3 in: Reynolds, E.R.C. and F.B. Thompson. 1988. "Forests, Climate, and Hydrology: Regional Impacts". Tokyo, The United Nations University.
"Lockwood (1976, 91) cited the work by Bergemann and Libby, who, in 1957, used isotopes of water to obtain the ratio of maritime water to land water in the Upper Mississippi Valley, and concluded that one-third of the average precipitation is formed of re-evaporated (i.e. continental) water and two-thirds of ocean water. One may be tempted to conclude from this study that it is unlikely that changes in the nature of land surface, such as the removal of the forest, will have any significant influence on local rainfall. In the subtropics, where continuously clear skies and large amounts of solar radiation are available to evaporate water, the main sources of water vapor are the oceans. The same assumption has been extended to South-East Asia, which receives much of its rainfall in the form of water that has evaporated from the subtropical Indian Ocean.
The reported results from the Amazon Basin, where the mean annual precipitation is 2,000-2,400 mm, are, however, different. Here, the mean recycling time for water vapour in 1979 was found to be 5.5 days (Salati et al. 1979 and Salati and Matsui 1981). Measurements of oxygen isotopes in the rain and river waters confirmed the importance of recycled water in the hydrological balance throughout the basin. The studies concluded that 52% of the precipitation in the Amazon region between Belém and Manaus was accounted for by inflowing moisture from the Atlantic Ocean; the remainder by recycled vapour within the area."
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