Land degradation/Data uncertainties limitations

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Data, uncertainty and limitations

Both modules use the harmonised world soil database (HWSD), (FAO et al., 2009), and module B also uses the WISE soil profile database (Batjes, 2009), and the derived S-World database (Stoorvogel, 2014; Stoorvogel et al., 2017). Although the HWSD and WISE databases are the most up-to-date sources of global soil data, they are still uncertain, especially for managed soils, and their base data have a rather course resolution.

Both modules are greatly simplified representations of land degradation and its potential impacts. For module A on Water Erosion, the largest uncertainty relates to the rather coarse resolution of the terrain erodibility, as small-scale terrain characteristics determine erodibility, but are not captured in global data sets. In addition, the intensities of rainfall events are very uncertain, and here are assumed to be only a function of monthly rainfall and number of wet days per month.

Methodologically, module B could be regarded as a better representation of the ‘state of the art’, because it makes full use of available data on a global scale. However, the module is completely based on the current state of the soil properties, and no account is taken of land-use history.

A major strength of module B on Human Induced Soil Changes is quantification of changes in soil properties. In combination with other IMAGE modules, these enable assessment of all types of direct and indirect impacts, a step forward from the common qualitative assessments of land degradation.

Soil degradation is strongly influenced by many aspects of land management and specific soil conservation management measures, which are not yet accounted for in the model. The module probably underestimates human-induced degradation, for several reasons: i) soil attributes remain compatible with the original soil type (soils are not allowed to change to such an extent that they fall into another class); ii) land-use change has an immediate effect and, in its present form, land management types in broad land-use categories are not accounted for; iii) some drivers of land degradation, such as salinization and nutrient depletion, are not taken into account.