Introduction

Soil erosion is perhaps the most serious mechanism of land degradation in the tropics generally and the humid tropics in particular (El-Swaify et al., 1982). In the humid tropics, erosion by water, rather than by wind, assumes primary importance. The following discussions, therefore, will be restricted to rainfall-induced erosion.

Opinions differ as to which forms of water erosion are most serious, although gully erosion and mass wasting are the most catastrophic and visually impressive forms. However, any ‘accelerated’ erosion, i.e., erosion with a rate which exceeds the natural rate (prevailing geologically prior to human disturbance) or the rate of soil genesis from underlying parent strata is likely to produce detrimental impacts on the productivity and quality of the ecosystem. As such, even the insidious and visually least impressive sheet (inter-rill) and rill erosion can be seriously detrimental. A complete picture of erosion effects cannot be painted without a total consideration of onsite and offsite impacts. Since runoff water (including floods) and runoff-borne sediments recognize no political boundaries, erosion is always a watershedbased and often a multi-nation problem. Assessing the problem, and the planning and implementation of effective counter-measures, therefore, may well require the institution of not only national commitments and policies but also regional and international ones as well (FAO, 1982a).

The setting

Climatic classifications do not agree on a uniform definition for the humid tropics. Fig. 10.1 shows Troll's classification of tropical climates (El-Swaify and Dangler, 1982).