Freshwater is an essential resource which is under increasing pressure. Dams and other diversionary activities, particularly agriculture, have diverted a huge amount of the world’s fresh water for human use. Humans now use more than 50% of the available fresh water of the earth, and this proportion is en route to increase to 70% in the next half-century. Therefore it behooves us to attend to all factors which affect the water cycle. Although the role of rainforests in the global water cycle is relatively small compared to that of the oceans, it is nevertheless extremely important. Rainforests influence the hydrologic cycle in the following ways:
Rainforests release water vapor by transpiration through leaves and evaporation (evapotranspiration, or water lost through the pores in leaves and evaporated by heat). The loss of water to the air by leaves is a critical part of the water cycle of the earth, as, by this means, the water vapor content of the air is continually replenished. More than 50% (even as much as 75% in dense rainforest) of the precipitation striking a rainforest is returned to the atmosphere by evapotranspiration and, consequently, relatively little will end up in rivers and other waters. Most of the water released by evapotranspiration to the atmosphere as water vapor will be returned to the forest as rain, so rainforests provide their own rainfall. Although forests account for only about 15%-20% of global water evaporation, approximately 65% of the rainfall over land is due to them. Lowered levels of atmospheric water vapor reduce cloud cover and rainfall, so if forest is removed, rainfall in that region will be substantially reduced. This will have dire consequences for even large rainforest reserves. If they are surrounded by deforested land, they will not be able to generate sufficient rain to support themselves, and they are doomed to perish. Evapotranspiration also has a cooling effect, as it takes energy to vaporize water from leaves, and keeps the temperature in the forest relatively constant.
2) Water regulation
The movement of water into rivers and other waterways is modulated by forest vegetation. Vegetation increases the ability of soils to retain water, preventing floods and erosion. Since a forest can intercept as much as 50% of the rainfall, it will prevent much soil loss which might otherwise occur from the impact of rain on the land surface. Water passes from rainforests into rivers and streams with much less force, reducing erosion and the threat of floods.