Regional rainfall depth-duration-frequency equations for Canada

The geographical variation of short-duration rainfall extremes in Canada is first evaluated in terms of depth-duration and depth-frequency ratios. Depth-duration ratios, defined as the ratios of the t-min to the 60-min rainfall depths of the same return period, are found to be independent of return period and geographical location for any storm duration of less than 60 min. However, for storms of longer durations, depth-duration ratios are found to depend on both the return period and geographical location indexed by the at-site mean annual precipitation. Depth-frequency ratios, defined as the ratios of the T-year to the 10-year rainfall depths of the same storm duration, are also found to depend on the return period and geographical location. Generalized expressions of depth-duration and depth-frequency ratios are then combined to develop regional depth-duration-frequency equations. A split sampling experiment has verified that the proposed equations reproduce satisfactorily the design storms at long-term record stations in different hydrologic zones. The proposed equations represent a viable alternative to current interpolation procedures as they eliminate the need for isoline maps of both mean and standard deviation of annual rainfall maxima for various durations.