Recurrence analysis of hydrologic droughts of differing severity

Droughts are stochastic in nature, therefore, statistical assessment of droughts is an attractive approach for water resources planning and management. In this study, a methodology of frequency and risk analysis of hydrologic droughts, defined as an event during which the streamflow is continuously below a certain truncation level, is formulated. The theoretical derivation of the recurrence interval of hydrologic droughts with a certain severity or greater is obtained based on the concept of stochastic processes. For risk analysis, the distribution of the number of droughts occuring within a specific period of time and the distribution of the interarrival time of hydrologic droughts with a certain severity or greater are determined. To illustrate the feasibility of the proposed methodology, a recorded reservoir inflow is used as a case study. Satisfactory agreement is found between the observed data and the results of the proposed models.