Rainfall temporal variability and erosivity in subhumid and humid zones of southern Nigeria

Soil erosion by water is a major cause of land degradation in the tropics. Its quantitative assessment requires evaluation of factors such as rainfall variability and erosivity. Therefore, pluviograph data from 1977 to 1999 for the subhumid (Ibadan) and humid forest (Port-Harcourt) zones of southern Nigeria were analysed to determine the temporal and spatial variability of rainfall erosivity. Analyses were carried out generally at 15-minute intervals to obtain rainfall amount, duration, intensity and kinetic energy. Descriptive statistics and t-tests were used for data analyses. Rainfall intensities between 100 and 150 nun h(-1) occurred every year. Rainfall intensities were higher in the subhumid (SH) than humid forest (HF) zone whereas the kinetic energy of rainfall was higher in the HF than the SH zone. Thus, the high erosivity in the SH was mainly due to high intensities whereas it was mainly due to high amounts in the HF zone. In both zones, 42 percent of the years exceeded long-term annual average of erosivity. The mean annual EI30 was 11107 MJ mm ha(-1)h(-1) in the SH zone and 17 988 MJ nun ha(-1) h(-1) for the HF zone. Rainfall erosivity indices (E and EI30) correlated significantly with rainfall amount, with 86-99 per cent of variations in daily erosivity explained, and 55-99 percent of variations in annual erosivity explained by rainfall amount. Spatial and temporal variations were not only influenced by rainfall characteristics but also by inherent differences in rainfall erosivity indices. The data presented are required in the prevention or control of land degradation as well as in the development of watersheds, particularly in the tropics. Copyright (c) 2006 John Wiley & Sons, Ltd.