Bivariate Drought Recurrence Analysis Using Paleoclimatic Records

Droughts are natural phenomenon during which the moisture availability is below the normal conditions. Usually droughts are mainly characterized by their durations and severities. However most of the drought recurrence analyses study those characteristics separately due to the shortness of historic records. Valuable sources of information, that complement the historical records, are the tree ring reconstructions of droughts. An approach to adapt and include dendrochronology reconstructions combined with historical records to characterize droughts is presented. The proposed approach uses the stochastic structure of the residuals of paleo reconstructions to generate equally-likely representations of past drought events. This information allows further drought recurrence analyses. For those cases, a general methodology to evaluate the frequency and risk of the occurrence of droughts is presented using a bivariate drought characterization. The theory of runs is applied to model drought recurrence as an alternating renewal process, describing droughts simultaneously in terms of their durations and severities. The procedure was applied in an analysis of droughts in Texas climatic division 5. Bivariate and univariate analyses were compared. The application shows the advantages of bivariate analyses using tree ring reconstructions in drought characterization.