Rainfall erosivity estimation for Sierra Leone using non-parametric indices

Rainfall erosivity (R factor) has been determined for Sierra Leone using monthly precipitation data covering 1960-2013 using Fournier (FI) and modified Fournier (MFI) indices. Mann-Kendall, Pearson's correlation and Sen's slope tests were used to establish the trend in time series rainfall data and their correlation. The study showed that there is a decreasing trend in annual rainfall in all the districts and the average annual rainfall varies between 1400.7 and 3027 mm, with high variability in the southeast and western regions compared to northern region. Rainfall erosivity was in the low to extremely severe category ranging from 21.7 to 166.8 for Fournier index and 95.4 to 264.6 for modified Fournier index. The year 1998 recorded the highest erosivity with values ranging from severe to extremely severe. The rainfall aggressiveness varied from low to extremely severe. Trend analysis was significant for values of - 1.82 >= Z <= - 1.57 at the 99% and 95% confidence levels, respectively, with two-tailed tests. No significant change in the precipitation trend was observed for some districts at the 95% confidence level. However, a significant change was noted for Bo, Pujehun and Tonkolili districts at the 99% confidence level. Sen's slope (Q) test revealed a non-significant decreasing slope magnitude for all 14 districts. Pearson's correlation coefficients showed significant correlation between annual rainfall and erosivity but with stronger correlation for modified Fournier index than Fournier index. The decreasing rainfall trend and high erosivity may have challenging implications for natural resource management including vegetation, soil and water resources under current climate conditions.