Two efforts by UN organizations to diagnose and map the distribution and trends of soil degradation in drylands are briefly described and compared.
Soil degradation by water is greatly accelerated under poor vegetation cover in the Tropics and Subtropics, probably for two major reasons: high rates of decomposition of organic matter and high intensities of rainfall. The consequences of soil losses and high water runoff can reach far downstream in the river basin, causing damage by sedimentation on floodplains and in reservoirs.
Damages by wind erosion and deposition are also much accelerated by overexploitation of the meagre vegetation cover of drylands. Overgrazing: overcultivation, firewood collection are the consequences of human and animal pressure, leading to more or less long-lasting desertification. The fine1 material, clay, silt, and organic matter, is blown by dust storms over lon~ distances and can be deposited as dry or wet fallout in oceans or on Iand under the dust trajectories. Salinization often affects highly productive soils under poor irrigatior practices. It is therefore a form of dryland degradation which causes particularly high losses of potential crops.