Selection of suitable PDF model and build of IDF curves for rainfall in Najaf city, Iraq

Abstract

Abstract The models of the Intensity-Duration-Frequency (IDF) curves relationship is considered a significant represents of the rainfall intensities used in the water resources structures, in the planning, designing and projects operation. This paper aims to estimate the Intensity-Duration-Frequency (IDF) curves and find the equations of these curves for AL-Najaf city, Iraq. Also, to identify the distribution that gives the highest rainfall intensities from the three commonly distributions used in this field. The (IDF) curves explain the relationship between the rainfall intensity and different return intervals. The study investigates the maximum daily rainfall data of 30 years using the Indian Meteorological Department (IMD) empirical formula to calculate the short durations rainfall intensity for 5, 10, 20, 30, 60 and 120 minutes, and with return periods of 2, 5, 10 and 25 years. The frequency analysis, which involves various statistical distributions, have been used to developing the relationship between rainfall intensities, durations, and return periods. The frequency analysis build based on the maximum daily rainfall data of the choosing interval (1989-2018), which had collected from the Meteorology and Seismic Monitoring Authority of Iraq. The used distributions are Gumbel, Lognormal and Log Pearson Type III distribution. The technique of the non-linear analysis has used to get the (IDF) curves equations. Also, Easy-fit software 5.5 involve the Kolmogorov-Smirnov test, which used to test the goodness of fit. The results showed that Gumbel distribution gave the best rainfall intensities for different return periods and different durations, where calculated rainfall intensities by Gumbel distribution had the highest values between the three applied distributions in this paper. The high values are more reliable to avoid the higher risks caused by sewer systems floods due to higher rainfall intensities due to climate change.