rainfall riddle | i8 Storm duration (min) Bell / SCS ratios TRMM ratios Modified Bell’s ratios 5 0.139 15 0.239 30 0.343 60 0.435 120 0.565 180 0.626 0.7 360 0.750 0.84 0.840 720 0.877 0.92 0.920 1440 1.000 0.156 0.267 0.384 0.486 0.632 0.700 1.000 Table 1 Ratios between 24-hour duration intensity and other storm duration intensities Undertaking frequency analysis and selecting the best fitting distribution We undertook frequency analysis of the merged record of maximum daily rainfall using several distributions. We then selected the best fitting distribution, which was the Gamma distribution, based on discriminant plots of the distribution tails, the moment ratio diagram and Bayesian criteria. The frequency analysis fitting is shown in Figure 2. We then adopted a theoretical ratio of 1.13 to transform the daily rainfall values and 24-hour values. Estimating short duration ratios when no data is available In the absence of short duration records or any similar information, sub-daily rainfall duration ratios should be assumed between rainfall intensities of the 24-hour ratio and those of the 12-, 6-, 3-, 2-, and 1-hour, and 30-, 15- and 5-minute ratios as shown in Table 1. Using the TRMM data from 3 hours to 1 day, we found that the ratios between X-hour storms and 1 day storms were 0.7, 0.84 and 0.92, for 3, 6 and 12 hours, respectively. These ratios were slightly different from the ratios of Bell with the SCS Type II extension shown in Table 1. The Bell/SCS ratios were modified to conform to the TRMM derived ratios, as shown in the same Table 1. It is well known that ratios for durations from two hours to five minutes are fairly constant in different climates because of the similarity of convective storms patterns (Awadallah & Younan, 2012; Bell, 1969). Obtaining the IDF curves Finally, multiplying the rainfall depths by 1.13 and then by the ratios of Table 1, we calculated the intensity-duration rainfall values to obtain the IDF curves (Figure 3). Figure 1 Monthly vs. daily TRMM rainfall data Figure 2 Fitting of the gamma distribution Figure 3 Intensity-Duration-Frequency curves for Namibe Bibliography Awadallah, A.G., & Younan, N.S. (2012). Conservative design rainfall distribution for application in arid regions with sparse data. Journal of Arid Environments, 79, 66-75. Bell, F.C. (1969). Generalized rainfall-duration-frequency relationship. Journal of Hydraulic Division, 95(HYI), 311-327. United States Department of Agriculture. (1986). Urban hydrology for small watersheds. Technical release 55. 39