An Approach for Frequency Analysis of Multiyear Drought Durations

A practical approach is developed for frequency analysis of multiyear drought durations of annual streamflow series. The proposed approach utilizes a technique which smooths the frequency‐curve irregularity of drought durations and, therefore, reduces the statistical uncertainties associated with sample‐size limitations. Several hazard‐function models are examined with regard to their ability to represent the duration‐dependent termination rate of a drought data set. The logistic model with an exponential correction for short durations is considered to have sufficient flexibility for describing most portions of the termination‐rate function. The parameters of the logistic model are identified using a piecewise linear regression for the linearized termination probability curve. Once the form of the hazard model is specified arid its parameters are estimated, the probability distribution and exceedance probability of drought durations can be determined from their corresponding relationships with the hazard model. The historic droughts of Feather River, California, are used to demonstrate the methodology. Results indicate that the observed nine‐year drought of Feather River has a return period of 360 years.