Marginal methods of intensity-duration-frequency estimation in scaling and nonscaling rainfall

Practical methods for the estimation of the intensity-duration-frequency (IDF) curves are usually based on the observed annual maxima of the rainfall intensity I(d) in intervals of different duration d. Using these historical annual maxima, one estimates the IDF curves under the condition that the rainfall intensity in an interval of duration d with return period T is the product of a function a(T) of T and a function b(d) of d (separability condition). Various parametric or semiparametric assumptions on a(T) and b(d) produce different specific methods. As alternatives we develop IDF estimation procedures based on the marginal distribution of I(d). If the marginal distribution scales in a multifractal way with d, this condition can be incorporated. We also consider hybrid methods that estimate the IDF curves using both marginal and annual maximum rainfall information. We find that the separability condition does not hold and that the marginal and hybrid methods perform better than the annual maximum estimators in terms of accuracy and robustness relative to outlier rainfall events. This is especially true for long return periods and when the length of the available record is short. Marginal and hybrid methods produce accurate IDF estimates also when only a few years of continuous rainfall data are available.