Rainfall Intensity–Duration–Frequency (IDF) Curves: Effects of Uncertainty on Flood Protection and Runoff Quantification in Southwestern Saudi Arabia

Southwestern Saudi Arabia experiences occasional flash floods, possibly due to an inadequate understanding of rainfall and runoff and a lack of infrastructure. Several studies have investigated rainfall intensity, duration, and runoff, while the infrastructure is not adequate to avoid floods. One possibility for the lack of adequate infrastructure might be the limitations in handling rainfall data. In this study, rainfall intensity–duration–frequency (IDF) curves were developed using the Gumbel distribution for five areas (Abha, Al-Baha, Bisha, Gizan, and Khamis Mushait) in southwestern Saudi Arabia. Four methods of calculating depth–duration relationships were applied. The 25-year daily maximum rainfall data were converted into hourly and sub-hourly data using these methods. The methods showed considerable variability in the IDF relationships, which may influence the essential protective measures against floods and runoff collection. The log-Pearson Type III (LPT III) distribution and RainyDay were also used to develop the 24-h IDF curves. The results show that Gumbel and LPT III can be used in regions with a lack of sub-daily rainfall data, while RainyDay can be used with caution in regions with no rainfall data. This study observed significant variability in the storage capacity requirements in different areas. The effects of methodological variability can be minimized by long-term monitoring of data, calibrating the methods using these data, and constructing watersheds to store the wide ranges of runoff. The areas showed significant differences in IDF curves, emphasizing the need for studying smaller areas rather than the entire region. A better understanding of the variability in IDF relationships may assist in controlling flash floods and maximizing runoff storage.