Drop size distribution and energy load of rain storms at Ibadan, western Nigeria

Abstract Rainfall erosivity factors were measured during 1980 and 1981 at Ibadan, Nigeria for two rainy seasons, Season 1 from late March to late July and Season 2 from late August to early November. Rainfall characteristics monitored included drop size distribution, kinetic energy, and momentum using a DISTROMET. Additional factors/characteristics monitored were rainfall amount and intensity using a daily recording rain gauge, and sand splash using an Ellison Cup technique. Rainstorms received in March, June and September had a relatively bigger median drop size ( D 50 ) ranging from 2.75 to 3.00 mm. Similarly rains in May and September had high kinetic energy of 2000–2500 J/m 2 , and high momentum of 300–450 kg/m/s. Kinetic energy of rainstorms was related to D 50 ( r =0.63), 5 min maximum intensity ( r =0.79), 30 min maximum intensity, I 30 ( r =0.83), and rainfall amount ( r =0.98). Therefore, rainfall erosivity may be estimated using data of rainfall amount from the daily recording rain gauge. Regression equations proposed for computing kinetic energy (J/m 2 ) from rainfall amount ( A in mm) differ among seasons, and need to be validated for different ecoregions of west Africa. The proposed equation is KE=25.80 A +54.58 for Season 1 and KE=29.86 A −43.18 for Season 2.

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