Rainfall intensity-duration-frequency analysis for Southeastern Nigeria
Inadequate hydrologic data and the need for proper planning of water resources development have forced engineers to analyze available data more critically. This is particularly so in developing countries. The Intensity-Duration-Frequency (IDF) relationship is one of the most commonly used basis for water resources planning and development. This study analyzed rainfall data and characteristics for locations in seven states of Southeastern Nigeria. IDF curves were developed for these locations using two methods (Graphical and Statistical) and the results were compared. The locations are Onitsha in Anambra State, Enugu in Enugu State, Abakiliki in Ebonyi State, Umuahia in Abia State, Owerri in Imo State, Port Harcourt in Rivers State and Uyo in Akwa Ibom State. Break-point, short duration, rainfall data are not generally available in the historical records at the locations. Generalized accumulated rainfall patterns developed by USDA Soil Conservation Service were matched with rainfall data for the locations of study, and the advanced pattern had the best fit with the observed characteristics and was used to break down recorded daily totals into shorter duration rainfall data. The method of annual maxima series was used to select data sets for the rainfall analysis. In the statistical method, the Type I extreme-value distribution (Gumbel) was applied to the annual maximum series for each of the seven stations to estimate the relevant parameters of the IDF model. The non-parametric Kolmogorov-Smirnov test and the χ2 test were used to confirm the appropriateness of the fitted distributions for the locations. IDF data developed from the graphical and statistical methods applied were very close for the lower return periods of two to ten years, but differed for higher return periods of 50 to 100 years. However, the difference is not significant at 5% level. The data developed by either of the methods will facilitate planning and design for water resources development in Southeastern Nigeria.
Keywords: rainfall, intensity, water resources, hydrologic data
Citation: Okonkwo G. I., and C. C. Mbajiorgu. Rainfall intensity-duration-frequency analysis for Southeastern Nigeria. Agric Eng Int: CIGR Journal, 2010, 12(1): 22-30.