Replacement of the natural vegetation cover by crops or grassland affects the fluxes of water and energy, as the leaf area, rooting depth, surface roughness, and albedo are altered. The vegetation changes associated with deforestation are generally accompanied by decreases in évapotranspiration and reduced soil infiltration. The infiltration capacity of a watershed may decrease substantially following deforestation, due to soil compaction by heavy logging or farm machinery, overgrazing and trampling by cattle, and increased soil erosion. The net effect of these changes is to increase runoff in newly cleared areas, at least in the short term, to create higher peak discharges in the wet season, leading to greater chances of flooding, and also to diminish streamflow during the dry season (Walling, 1987, pp. 55-57, 66-70; Jones, 1997, pp. 212-215). The long-term impacts on runoff are less clear, because of secondary regrowth and possible negative feedbacks on local rainfall, due to the reduced évapotranspiration and changes in the heat balance.
The extent of tropical forest has declined from around 1.80 billion hectares (ha) in 1990 to 1.73 billion ha in 1995 (WRI, 1998). This loss of tropical forest has been partially offset by forest regrowth in nontropical regions going from 1.71 billion ha in 1990 to 1.72 billion ha in 1995. Thus, the net forest cover reduction (primarily in the tropics) has been 56.4 million ha or 11.3 million ha/yr.
In the tropics, widespread forest clearing has been shown to increase total streamflow by as much as 125-820 mm/yr during the first three years (Bruijnzeel, 1996,1993; Fritsch, 1993). The average increase in runoff is around
4 This figure includes oxidation of above- and below-ground tree biomass, soil organic matter to 1 m depth, woody debris, and long-term decay of wood products.
300 mm/yr, when newly cleared humid tropical forest (D) is converted to cropland, tea, rubber, and cocoa plantations, or grassland (Bruijnzeel, 1996, 1993). Thus when this figure is multiplied by the net global average deforestation rate of 11.3 X 1010 m2/yr between 1990 and 1995 (WRI, 1998), the runoff comes to approximately 33.8 km3/yr, or 0.09 mm/yr SLR (Table 5.4). This estimate does not take into account vegetation regrowth or potential negative climate feedbacks that may decrease runoff, as suggested by several modeling studies (e.g., Henderson-Sellers et al., 1993; Zhang el al, 1996).
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