PROBABILITY WEIGHTED MOMENTS AND THEIR EFFECTS ON THE RAINFALL DEPTH-DURATION-FREQUENCY CURVES

The generalized depth duration frequency (DDF) relationships that define rainfall depth as the function of rainfall duration (d) and frequency or recurrence interval (T) are extensively used for design purposes in hydrological applications. To generalize the DDF relationships at a site, the parameters of the selected parent distribution must provide several divergence criteria (i.e., quantile functions of different durations must not intersect each other). The divergence criteria established on the parameters of a parent frequency distribution, such as the generalized extreme value (GEV), must be satisfied by the parameters of the exponential functions that are fitted to the observed probability weighted moments. This study presents examples of the GEV-based generalized DDF curves for the annual maximum rainfalls with durations from 1 h to 24 h and recorded at the Izmir, Marmaris, and Zonguldak gauges. In addition, the current research discusses a few problems related to generalizing and using the DDF curves, as well as several reasonable solutions for these problems.