Parametric modelling of rainfall return periods in south-western Nigeria: Survival analysis approach

Abstract

Background: Rainfall is the main source of water on the earth’s surface. It infiltrates and percolates deep into the soil for groundwater recharge. Rainfall patterns, amounts, durations, and intensities can vary daily, monthly, annually, and spatially. It is therefore important to accurately estimate rainfall return periods, which can be employed in hydraulic design and flood control measures. Methods: This research considered the survival analysis approach for the prediction of rainfall return periods including intensity, and months during which these would occur in south-western Nigeria. Twenty years’ of annual rainfall data were obtained from three metrological stations and these were subjected to nine different probability plotting position methods.  Results from the plotting positions was further subjected to four  survival models using five years of censor time. The Akaike Information Criterion (AIC) was used to determine the best-fitting model for the dataset. Results: The Laplace probability plotting position in conjunction with the log-logistic distribution best describes the datasets, since it gave the lowest AIC value of 22.53. The log-logistic distribution is also suitable for the prediction of return period from the Weibull probability plotting position since the AIC values were 6.934 and -4.332 respectively.  The Hirsh plotting position in conjunction with the Weibull distribution is also suitable for the description of the dataset. Conclusion: The established parametric models are suitable for the accurate prediction of return periods of peak rainfall events during any month of the year.