Abstract

This study aims at investigating the spatio-temporal distribution of rainfall erosivity in Rwanda between the 1940s and the 1990s. The study gives new insights into rising trends in rainfall erosivity and, therefore, prepares agriculturalists to take precautionary measures for better land conservation. Modified Fournier Index (MFI) for consecutive decades from 1941 to 1990 and mean differences in Fourier Index (FI) and (MFI) values between the decades 1981-1990 and 1941-1950, were calculated. A geographical information system (GIS) was used to interpolate the values and to present spatially the variations of rainfall erosivity over Rwanda. MFI showed a clear upward pattern in rainfall erosivity from the 1960s, becoming more pronounced in the 1980s. The highest MFI values were in the northern highlands, the Congo-Nile crest and southwest region around Kivu Lake. On the other hand, the eastern lowlands accounted for the lowest MFI values. A comparative analysis of the above-mentioned decades reveals a massive FI decrease in May and January, and a significant increase in April and November, which are the wettest months of the year. The high values of MFI and FI observed in highlands with steep slopes are most likely to lead to more soil loss; the lowlands with the highest values will likewise be more exposed to flooding mainly during the wet seasons.

Key words: Rainfall erosivity, fournier index, modified fournier index, Rwanda.