Land degradation

Model structure 

The different components of the model implemented in IMAGE 2.2 are discussed in more detail below.

• Rainfall erosivity index (R). The erosivity of rainfall is largely determined by the intensity of rainfall events. Soil loss occurs only during rainfall with high intensity. A proxy for R is inferred from the month with the greatest rainfall intensity (mm per day), calculated as monthly precipitation divided by number of wet days in each month. Maximum mean monthly rainfall intensities of 0-2 mm per day are assigned an index value R of 0, while those exceeding 20 mm per day are assigned a value of 1.0. A linear relation is assumed between these two extremes. Climate data is used for the historical period, the New et al. (1999). For future years, R is derived from the changes in precipitation based on different scenarios generated by the climate model. The number of wet days is assumed to be constant in time, as the IMAGE model does not generate projections for this aspect.
• Terrain erodibility index (T). The terrain erodibility expresses the combined susceptibility of soil and relief. T is defined as the average of the soil erodibility and relief indices, so that the weight assigned to the combined soil indices I_t, I_b and I_d equals that of the relief index (I_a). The terrain erodibility index is assumed unchanged in the future.
• Susceptibility to water erosion (E_p). The product of rainfall erosivity (R) and terrain erodibility (T) is the index for the potential susceptibility to water erosion (Ep) of the land based on conditions of climate and terrain.
• Soil erodibility index (SE). The soil erodibility (SE) is derived from indices for soil texture, bulk density and soil depth by taking the average of the two highest values. Although this is a rather arbitrarily chosen approach, the result is an expression of the relative differences between soils. Soil characteristics were deduced from the 0.5 by 0.5 degree resolution WISE database (Batjes, 1996a; Batjes and Bridge'; Batjes et al., 1997).
• Relief index (I_a). The relief index is a landform characterisation (slope) derived from a digital elevation model. Landforms were calculated from the difference between minimum and maximum altitude from the 10-minute grid FNOC elevation dataset (FNOC, 1985). I_a = 1 for a difference of 300 m or more and I_a = 0 for differences of 0 m; for the values in between altitude a linear relationship is used.
• Land-use pressure index (V). Given conditions of climate, soil and relief, the protection provided by land use and land cover determines the actual sensitivity to water erosion. The geographic distribution of different land-cover types generated by the land-cover model forms the basis for the land-cover index. Land-cover indices were modified from Wood and Dent (1983a,b) as described in Hootsmans et al. (2001). Natural vegetation such as forests provide a high degree of protection against water erosion, while agriculture generally leads to the higher vulnerability of the soil surface. A composite value for V is used for grid cells with agriculture. This value is based on the distribution of agricultural crops within the specific world region. The contribution of soil and crop management, and soil conservation practices, is not included in the land-cover index.
• Water-erosion sensitivity index (E_a). The sensitivity to water erosion is derived by combining the land's susceptibility to water erosion (E_p) with the land-use pressure index (V). By multiplying the indices V and E_p with opposite scales, a compensation of effects is achieved. For example, for low values of V (high degree of protection provided by the land's cover) and high land susceptibility (high value of E_p for hilly landscapes), the water-erosion sensitivity will still be low. Contrarily, when the protection provided by the land cover is low (for example, under agriculture), the sensitivity will be low if the land's susceptibility is low (for example, on non-sloping land).       

• to  Classification of water-erosion sensitivity
• to  external references
• to  MNP references