Soil erosion on Alfisols in Western Nigeria: III. Effects of rainfall characteristics

The applicability of various erosivity indices was tested for runoff and soil loss from plowed bare-fallow field runoff plots of 25 × 4 m established on an Alfisol with natural slopes of 1, 5, 10 and 15%. The correlation coefficients of percent runoff from individual rainstorms with various indices such as kinetic energy (E), EI₃₀, KE > 1, rainfall amount (A), maximum intensity (I_m), and AI_m, were generally low. The correlation coefficients of all these indices with soil loss per storm were high and did not differ significantly from one another. The use of an empirical relation (kinetic energy = 916 + 331 log₁₀, I is in inches/h) may underestimate the kinetic energy of tropical rainstorms. The kinetic energy of tropical rainstorms may be significantly influenced by other factors such as wind velocity, drop size distribution and high rainfall intensity. The index AI_m has the advantage of simplicity of computation, and it incorporates one of the most important factors, peak intensity (I_m). Further improvements can be made in this index by incorporating a factor which accounts for the kinetic energy of a rainstorm. In the meantime, the index, AI_m, may be used to prepare an “iso-erodant” map, i.e. places with equal erosion potential. There also exists a linear correlation between rainfall amount per storm and AI_m. Such a relationship may be useful in estimating AI_m for regions where data from recording rain gauges are not available.