The watershed function of forests has particular public health importance because forests often are the primary determinant of water quality and quantity for household use in developing countries. Watershed areas are important sources of water for irrigation, hydro-electric power, and industrial use. Watershed protection is important in maintaining the quantity and quality of water supply, as well as flood avoidance. China s decision to halt logging in the Yangtze watershed in 1998 following severe flooding illustrates the magnitude of these effects. Interception losses are greatest from forests that have large leaf areas throughout the year.
Forests also play an important role in improving water quality.
In the dry tropics, forest plantations often use more water than short vegetation because trees can access water at greater depth and evaporate more intercepted water. Extensive afforestation in the dry tropics therefore can have a serious impact on supplies of groundwater and river flows. It is less clear, however, whether replacing natural forests with plantations, even with exotic species, increases water use in the tropics when there is no change in rooting depth or stomatal behavior of the tree species.