Patterns of monthly rainfall shortage and excess in terms of the standardized precipitation index for Catalonia (NE Spain)

Spatial and time behaviours of rainfall shortage and excess are analysed fo r Catalonia (NE Spain) using a database obtained from 99 rain gauges with m onthly totals collected from 1961 to 1990, The distribution of monthly amou nts for each rain gauge is modelled by means of the gamma or Poisson-gamma distributions. Then, using an equiprobable transformation, monthly amounts described with these distributions are substituted by values given by the S tandardized Precipitation Index (SPI), which follows a standardized normal distribution and provides a unique pluviometric scale. After that, principa l component analysis (PCA) is applied to the set of monthly SPIs. A double regionalization of the 99 rain gauges, distinguishing between episodes of r ainfall shortage and excess, is achieved by taking into account the rotated factor loadings (RFL) correlating rain gauges and principal components (PC ). A time classification of rainfall shortage and excess episodes is also e stablished, considering in this case the factor scores (FS) obtained after a PCA of variables based on monthly SPIs. The spatial regionalization achie ved becomes a rough picture of the different topographic domains (Pyrenees, Pre-Pyrenees, Central Basin, Littoral and Pre-Littoral chains and Mediterr anean coast), the climatic diversity of Catalonia being enhanced by these r esults. The time clustering suggests a quite complex behaviour of the rainf all shortage and excess episodes. Moreover, the spatial distribution of the se time clusters is very disperse, in such a way that monthly shortage and excess sometimes affect the whole of Catalonia and sometimes just a small a rea. Besides results obtained from PCA and clustering algorithms, it is wor th noticing that the severity of the episodes increases remarkably only for rainfall shortage. In addition, an analysis of the number of rain gauges a ffected by monthly shortage and excess shows an interesting fact: whereas t he number of rain gauges associated with a shortage has an increasing tende ncy, a significant decreasing tendency for excess is detected in the period 1961-1990. Copyright (C) 2001 Royal Meteorological Society.