Developing rainfall intensity–duration–frequency relationship for two regions in Saudi Arabia

Intensity–duration–frequency (IDF) relationship of rainfall amounts is one of the most commonly used tools in water resources engineering for planning, design and operation of water resources projects. The objective of this research is to derive IDF relationship of rainfall at Najran and Hafr Albatin regions in the kingdom of Saudi Arabia (KSA). These relationships are useful in the design of urban drainage works, e.g. storm sewers, culverts and other hydraulic structures. Two common frequency analysis techniques were used to develop the IDF relationship from rainfall data of these regions. These techniques are: Gumbel and the Log Pearson Type III distribution (LPT III).An equation for estimating rainfall intensity for each region was derived using both techniques. The results obtained using Gumbel distribution are slightly higher than the results obtained using the LPT III distribution. Rainfall intensities obtained from these two methods showed good agreement with results from previous studies on some parts of the study area. The chi-square goodness-of-fit test was used to determine the best fit probability distribution. The parameters of the IDF equations and coefficient of correlation for different return periods (2, 5, 10, 25, 50 and 100) are calculated by using non-linear multiple regression method. The results obtained showed that in all the cases the correlation coefficient is very high indicating the goodness of fit of the formulae to estimate IDF curves in the region of interest.