SPATIAL VARIATION OF RAINFALL RUNOFF EROSIVITY (R) FACTOR FOR RIVER NZOIA BASIN, WESTERN KENYA

Rainfall is one of the key drivers of soil erosion with its force expressed in form of rainfall erosivity (R-Factor). The R-Factor is a function of rainfall amount and the intensity. Given the inadequacy of quality data, the factor is generally estimated based on rainfall data with low time-based resolution. The specific objective of this study was to model rainfall runoff erosivity (R-Factor) for river Nzoia basin. Local rainfall data were collected from 14 rainfall stations purposively selected from the basin based on sufficiency and quality. GIS techniques were used to model rainfall runoff erosivity (R) factor for river Nzoia basin. Thematic map for R-factor was generated. Results revealed that the average R-Factor value ranged between 339.39 and 855.00 MJ mmha−1year−1 with the highest values experienced in the middle extending to the lower basin of the study area. Its mean and standard deviation was 574.38 and 130.62 MJmmha−1 year−1, respectively. This could be attributed to relief kind of rainfall associated with Mt Elgon, Cherenganyi hills and the Nandi escarpment.GIS techniques were successfully used to generate rainfall runoff erosivity (R-Factor) for river Nzoia basin. There is significant spatial variation in the rainfall erosivity (R-Factor) in River Nzoia basin. If this factor is considered in isolation, then the middle extending to the lower basin of the study area will be affected more in terms of the soil erosion due to high rainfall erosivity values. Therefore, further quantitative studies need to be undertaken to evaluate the principal factors influencing spatio-temporal variations in the R-factor for river Nzoia basin.