Development of Rainfall Intensity-Duration-Frequency (IDF) Relationships for Siti Zone, In Case of Ethiopia Somali Regional State

Abstract

One of the first steps in many hydrologic design projects is determination of rainfall event. Use of inappropriate design storms could lead to malfunctions of the infrastructure systems. Hence development of IDF curves for precipitation remains a powerful tool in the risk analysis of natural hazards. Thus, this study was undertaken to develop IDF curves with their mathematical expressions in three selected rainfall stations of Ethiopian Somali region. To provide IDF information, historically observed hourly rainfall data was gathered and annual maximum data series for durations of 1, 2, 3, 6, 12 and 24 hours were extracted and fitted to different probability distribution functions for selecting best fitting distribution function to perform the frequency analysis. Extreme rainfall quantiles (X_T) were then computed for the return periods of 2, 5, 10, 25, 50, and 100 years and taking these as an input the IDF parameters (A, B and C) were estimated with the help of MIDUSS IDF Curve fit tool software and their performance were evaluated using the regression coefficient (R²) and the Nash and Sutcliffe simulation efficiency (E_NS). Finally, mathematical relationships were developed and IDF curves were constructed for the respective return periods of all the selected stations. Generally, the outcomes of this study indicate that Gumbel (EVI) was found to be the best fitting probability distribution in describing the given data series for all the selected stations. The IDF curve fit parameters (A, B and C) generated with the MIDUSS software shown an increasing or decreasing trend with an increase or decrease in return period and vary from 752 to 8239.3 for ‘A’, 0.21 to 101.49 for ‘B’ and 0.86 to 1.19 for parameter ‘C’ and parameter ‘C’ was found to be the most sensitive parameter. Since the R-squared and ENS values was very high, over 0.99 that depicts a strong relation between the observed and computed intensities, the developed IDF parameters can be used as a useful tool for the estimation of the rainfall design parameters.

Keywords: Rainfall, Intensity Duration Frequency Curve, Probability Distribution, Water resource management infrastructures, Ethiopian Somali region