Changes in frequency and intensity of daily precipitation over the Iberian Peninsula

1 There is a general consensus within the climate community that any change in the frequency or severity of extreme climate events would profoundly impact nature and society. Such changes can be studied in terms of rainfall indices derived from daily data. The great variability of the rainfall over the Iberian Peninsula and the irregularity of its water regime make any study of the rainfall in this geographical region very interesting. In this work, we contribute some information about the characteristics and distribution of the daily precipitation series over the Iberian Peninsula, in particular the study of trends in series of several selected indices all related to the frequency or intensity of the daily rainfall. Data used are taken from a set of 35 series of daily rainfall in the aforementioned region during the period 1958-1997. The selected observatories constitute a representative sample of the orography and geographical diversity present in the peninsula. From the daily rainfall data, series of frequency and intensity rainfall indices were constructed on the basis of a prior definition of four rainfall categories reflecting certain percentiles of the precipitation distribution in the peninsula: total rainfall ( >= 0.2 mm), light (>= 0.2 mm and < 2.5 mm), moderate (>= 2.5 mm and < 7.5 mm), intense (>= 7.5 mm), and very intense ( >= 15 mm). The indices studied were number of rainy days in each rainfall category, medians and maxima of length of dry spells ( both related to the frequency of the precipitation), accumulated precipitation in each rainfall category, proportion of rainfall in each category relative to the total accumulated rainfall, and mean precipitation per wet day in each category ( related to the amounts). All the indices were evaluated seasonally. Significant trends in many of the selected series were found. The results for all the indices are coherent and point to an increase of the light rainfall events at the cost of a decrease of more intense events.