A comparison of three empirical methods of calculating Areal rainfall was carried out using 10 years of rainfall data in Ogun river basin, Southwestern Nigeria. The optimum network density for estimating areal rainfall and its reliability for water resources purpose was also evaluated. Data were analyzed by one – way ANOVA using the Genstat statistical package (Release 4.24 Discovery Edition) taking each yearly mean as the average of the 10 years (2001-2010) for the basin. Results show the least deviation in areal rainfall between Thiessen polygon and Isohyetal methods but the deviation was higher between Arithmetic mean and Thiessen polygon /Isohyetal methods. Hence, under topographical condition of the Ogun river basin, either the method of Thiessen polygon or Isohyetal methods can be used for computation of areal rainfall. The relative reliability of each method in terms of accuracy of both measured and extrapolated rainfall data utilized was also discussed.
 

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