Deforestation in hilly areas often leads to decreased infiltration of water and consequently higher runoff and increased peak water discharges following rainfall events and reduced runoff during dry seasons. Most importantly, it results in increased soil erosion, gully and ravine formation, flooding risk, and siltation of reservoirs and irrigation schemes. For instance, deforestation in the Himalayas has been associated with a doubling in torrent width since 1990-and a downstream cost of more than US$1 billion (Government of India, 1983).
Forestation of denuded hilly land will reduce peak runoff; lessen the risk of flooding; conserve soils; and prevent severe siltation, gully formation, and landslides. In experimental work, up to 500-fold differences in erosion rates between forest and cultivated cropland have been recorded (Maass et al., 1988). The presence of forest litter is critically important in facilitating infiltration rates and preventing soil/sediment movement by overland flow. Mono-specific plantations without an understory, however, may not provide such conditions. It is also important to conduct forest establishment activities sensitively to minimize negative impacts before a tree cover is established.
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. In the Philippines, for instance, close to 20 million people live in upland watershed areas (Cruz and Zosa-Feranil, 1988). The importance of forests as watersheds may substantially increase in the next few decades because freshwater resources are projected to become increasingly scarce, particularly in developing countries (Mountain Agenda, 1997; Liniger and Weingartner, 1998).
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. Trees in the Yangtze watershed were calculated to be worth at least three times as much for their water regulation functions as for their timber value (Smil and Yuchi, 1998, as cited by Abramovitz and Mattoon, 1999).
Deforestation and forest degradation associated with high-impact methods of logging and mining significantly erodes soils in many countries, with marked economic and environmental costs through consequent siltation of rivers, hydropower reservoirs, irrigation systems, and coastal ecosystems (Myers, 1997). In the Philippines, for instance, an estimated 8.3 Mha of a total land area of 30 Mha are severely eroded (EMB, 1990).
Forests generally are expected to use more water (the sum of transpiration and evaporation of water intercepted by tree canopies) than crops, grass, or natural short vegetation. This effect may be related to increased interception loss-especially if tree canopies are wet for a large proportion of the year (Calder, 1990)-or, in drier regions, to trees' greater root system, which allows water extraction and use during prolonged dry seasons.
Interception losses are greatest from forests that have large leaf areas throughout the year. Thus, such losses tend to be greater for evergreen forests than for deciduous forests (Hibbert, 1967; Schulze, 1982) and may be expected to be larger for fast-growing forests with high rates of carbon storage than for slow-growing forests. Consequently, afforestation with fast-growing conifers on non-forest land commonly decreases the flow of water from catchments and can cause water shortages during droughts (Hibbert, 1967; Swank and Douglass, 1974). Vincent (1995), for example, found that establishing high water-demanding species of pines to restore degraded Thai watersheds markedly reduced dry-season streamflows relative to the original deciduous forests. Although forests lower average flows, they may reduce peak flows and increase flows during dry seasons because forested lands tend to have better infiltration capacity and a high capacity to retain water (Jones and Grant, 1996). Forests also play an important role in improving water quality.
In many regions of the world where forests grow above shallow saline water tables, decreased water use following deforestation can cause water tables to rise, bringing salt to the surface (Morris and Thomson, 1983). In such situations, high water use by trees can be of benefit (Schofield, 1992).
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. Newly planted forests can use more water (by transpiration and interception) than the annual rainfall, by mining stored water (Greenwood et al., 1985). 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. In the dry zone of India, water use by Eucalyptus plantations is similar to that of indigenous dry deciduous forest: Both forest types essentially utilize all of the annual rainfall (Calder, 1992).
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