Is Drought Occurrence And Severity Increasing Due to Climate Change? Analysing Drought Class Transitions With Loglinear Models

SPI time series in a 12 month time scale from 6 sites in Alentejo and Algarve, in the south of Portugal, within the period from October 1896 to September 2005, were divided into three or four periods depending on the data set length and a Loglinear modeling approach has been used to investigate differences relative to drought class transitions among these periods. Four drought severity classes were considered. The drought class transitions were computed for the different periods to form a 3-dimensional contingency table. The application of Loglinear modeling to these data allowed the comparison of the periods in terms of probabilities of transition between drought classes in order to detect a possible trend in time evolution of droughts which could be related to climate change