Trends in frequency indices of daily precipitation over the Iberian Peninsula during the last century

This study provides the first long-term assessment of changes in precipitation associated with different rainfall categories over the Iberian Peninsula (IP). Using recently digitized data from 27 stations in Portugal and Spain, we have examined trends of precipitation indices for the complete period 1903-2003 and the two subperiods 1903-1953 and 1954-2003. These indices were evaluated seasonally according to five rainfall categories: total rainfall (>= 0.2 mm), light rainfall (>= 0.2 and < 2.5 mm), moderate rainfall (>= 2.5 and <7.5 mm), intense rainfall (>= 7.5 mm), and very intense rainfall (>= 15 mm). For the complete period 1903-2003, we have found that the total number of rainy days and that of light rainfall are increasing at many observatories over the IP for all seasons (except in the western zone of Portugal and the Cadiz gulf, where they are decreasing). Both subperiods show opposite behaviors in fall and spring. During the first (second) subperiod spanning 1903-1953 (1954-2003), we can find a generalized decrease (increase) in the number of rainy days in fall in all rainfall categories. In spring, an increase in the number of rainy days is found mainly for total, moderate, and intense categories for the first subperiod, and a slight decrease during the second. In winter, we have verified a decrease in the number of rainy days mainly for total, moderate, and intense rainfall categories in the second subperiod. Finally in relation to the maximum length of dry spells for both subperiods, most observatories show significant trends opposite those of the total number of rainy days. It is worth noticing that the contrasting evolution of trends found between the first and second halves of the twentieth century could only be characterized in this work following a recent comprehensive digitization of historical Iberian precipitation data from the first decades of the twentieth century.