IDF curves and daily rainfall generation for Al-Madinah city, western Saudi Arabia

Due to the increase in global warming and climate change impact, especially in urban arid regions, the intensity-duration-frequency (IDF) curves and rainfall generation procedures become essential for storm design and hydraulic structures dimensioning for future planning and management. Al-Madinah, the second holiest city for all Muslims, is located in the western part of Saudi Arabia. This city needs special attention for hydrologic and hydraulic structure projects due to highly developed and rapid expansion in land use. The purpose of this study is to develop an empirical formula from IDF curves, which have been generated from 43-year records for Al-Madinah rainfall station. The absence of some daily rainfall records led to the application of the reduction method to disaggregate daily rainfall to hourly time series for developing suitable IDF curves. The formula is derived using the analysis of the best fit of Gumbel and log-Pearson Type III frequency methods. An average empirical formula can be used for predicting any return period with a given storm duration for Al-Madinah area. Generating wet, dry, and rainfall intensities are also investigated using autorun analysis on a daily time step. The results show that the dry spell continues for 71 days while the wet spell is about 1.58 days on the average based on the rainfall intensity threshold of 5.9 mm/day. Based on the goodness of fit for the dry and wet spells and the rainfall intensities, the daily rainfall sequences can be generated for any duration, which is adopted here as for the next 43 years.