18.104.22.168.3. Soil erosion
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.
22.214.171.124.4. Water quality and water use
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
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).