European Water 35: 23-29, 2011. © 2011 E.W. Publications Rainfall Erosivity Pattern of Ogun River Basin Area (Nigeria) using Modified Fournier Index G.C. Ufoegbune * , N.J. Bello, Z. O. Ojekunle, A.R. Orunkoye, A.O. Eruola and A.A Amori 1 University of Agriculture, Abeokuta, Nigeria * gidufoes2000@yahoo.co.uk Abstract: The study was based on 20 years rainfall data representing twenty rainfall stations under Ogun River Basin Area with the objective as; to determine rainfall erosivity pattern of Ogun River Basin Area using modified Fournier index by FAO. An FAO linear regression model which relates monthly EIзo index of Universal Soil Loss Equation (USLE) values with monthly rainfall erosivity values of the Modified Fournier Index was used. The annual rainfall erosivity table showed a range of 5756.68 MJ.mm/ha.h.yr to 19,583.82 MJ.mm/ha.h.yr. The highest rainfall erosivity values are 19,583.82 MJ.mm/ha.h.yr for Lagos, 13,296.25 MJ.mm/ha.h.yr for Ijebu – Ode and Ibadan with 11382.91MJ.mm/ha.h.yr. While the least values, 5756.68 MJ mm/ha.h.yr, 6905.18 MJ mm / ha.h.yr and 7395.25 are for MANR Ilaro, Ebute Igboore and Igbogila respectiviely. Regression analysis between the annual rainfall and annual rainfall erosivity showed a high correlation of r2 = 0.77, which suggest that the annual rainfall erosivity for Ogun River Basin Area is closely related to the annual rainfall values. The Iso – erodent map using the both annual / rainfall erosivity valves and geographical location of each rainfall stations, interpreted with FAO erosivity class range showed that Southwestern zone of Nigeria is a zone with high erosive rainfall. Keywords: Iso-erodent, precipitation values, erosivity. 1. INTRODUCTION Soil loss estimation is a capital intensive and time consuming exercise by which conservation practices may be based on the quantification of the relevant nature of soil, land topography, vegetation and climatic factors and the relation of these factors to regional and temporal characteristics. Soil loss estimation started in first decades of the 20th century and has increased in number and variety using either the Universal Soil Loss Equation (ULSE) of Wischmeier and Smith (1978) or Revised Universal Soil Loss Equation, of Renard et al (1997). Both Universal Soil Loss Equation (USLE) and Revised Universal Soil Loss Equation (RULSE) have empirical relationships, while USLE quantifies soil erosion as the product of six factors representing rainfall and run-off erosivity (R), soil erodibillity (K), Slope length (L), Slope steepness (S), cover and management practice (C) and supporting conversation practice (P). The equation is thus: A=R.K.L.S.C.P (1) where, A is the computed spatial and temporal average soil loss per unit of area. RUSLE utilizes the same basic equation but in a computerized version. Rainfall erosivity factor (R) of the USLE and RUSLE in estimating soil loss depends on the availability of suitable quantifiable rainfall erosivity parameters. These parameters must describe adequately the ability of rain detachment and down slope of soil particles according to intensity of rain and amount of energy. Since the early stage of soil loss research, Kinetic E, of falling rain and its maximum 30 minutes intensity Iзo, designated EIзo, has attained wide recognition as a causative factor in the erosion process and spatial distribution of mean annual rainfall energy for use in soil loss estimation model. However, the use of EIзo alone is not sufficient to describe the relative rainfall erosivity of any two locations with varying intensity of rainfall especially in developing countries (Cohen et al. 2005). Therefore, index based on kinetic and momentum of run-off can also