Generation Rainfall Intensity Equations for Intensity Duration Frequency (IDF) Curves (Case Study: Salah Al-Din, Iraq)

Abstract

The intensity of rainfall can be considered an important influence in designing and operating hydraulic structures. The intensity-duration-frequency (IDF) curves are very important for planning, managing, operating, and designing all water resource projects. The current study aimed to derive the curves of IDF and equations for the stations (Tikrit, Samraa, Baiji, and Tuz) in Salah Al-Din/Iraq. Using the maximum daily rainfall during the period between 1990 and 2022, developed the empirical equations for estimating rainfall intensity with the various rainfall durations and different return periods (IDF equations), by using three methods of frequency distribution techniques (Gumbel, Log Pearson III, and Log-Normal). By finding all the missing rainfall data using the homogeneity curve expectation maximization (EM) algorithm adopted in SPSS, and checking the consistency of the data using the double mass curve technique. The results showed that the rainfall intensity reduced as the duration increased, while rainfall of any duration showed a higher intensity if the return period of the rainfall was large. A comparison among the three distributions was made using the methods of testing goodness (Chi-Square, Anderson-Darling, and Kolomokorov-Simornov) using Easy Fit software 5.6. The test results proved that the Log Pearson III distribution was the best method for the study area having correlation coefficients 0.92, 1, 0.96, and 0.92 for Tikrit, Samraa, Baiji, and Tuz stations, respectively. Also, Bernard's equation with an error ratio of (α< 0.05), can be adopted as a general empirical equation for all hydraulic projects in the study area.