Probabilities of Repeated Long Dry Episodes Based on the Poisson Distribution. An Example for Catalonia (NE Spain)

Summary A formulation, similar to the procedure employed in seismic risk analysis, has allowed us to quantify probabilities concerning repeated long episodes of dry days for an arbitrary number of years and their return periods. This formulation is based on both a cumulative expression, similar to the well known Gutemberg-Richter seismic law, and the Poisson distribution. We will assume that this latter distribution is applicable, provided that some constraints affecting the length of the dry episodes and their average number recorded on a year are satisfied. In comparison with other methodologies, based on either Markov chains or Gumbell and Jenkinson formulations, we have been able to quantify probabilities concerning repeated long episodes, including extreme events, contributing significantly to the generation of drought episodes. The formulations that we introduce has been applied to sets of dry episodes obtained from daily pluviometric recordings belonging to 69 gauges of the Instituto Nacional de Meteorología located in Catalonia (NE Spain). The results obtained in terms of return period maps, return period-length curves and probabilities for repeated episodes and for an arbitrary number of years are in agreement with previous pluviometric studies and the spatial diversity of the region due to its orographic complexity. The more relevant synoptic situations causing the long dry episodes are summarized and briefly described. Received June 24, 1997 Revised October 28, 1997