Aridity in the Iberian Peninsula (1960-2017): distribution, tendencies, and changes

Aridity is a climatic characteristic that has a significant impact on many aspects of life, mainly in agriculture but also in other economic sectors, and temperature and precipitation are the variables by which the effects of aridity can be measured. It is necessary to know its distribution and trend in different climate change scenarios. The Iberian Peninsula presents a strong contrasting climate, comprising both humid and very dry regions. Forty-five climatic time series were used to perform a spatial and temporal analysis and to identify possible points of trend change in annual aridity for the period from 1960 to 2017. Two indices were considered: the De Martonne Index (I-DM) and the Food and Agriculture Organisation (FAO) Index (I-F). By means of ordinary kriging, maps of spatial, temporal, and trend distributions were made. The trends of the aridity indices, annual temperature, and annual precipitation were analysed by applying the techniques of the modified Mann-Kendall test and Sen's estimator; furthermore, to identify gaps of years in the series, the Buishand range test was used. The results showed a significant average annual temperature increase at all stations. Although annual precipitation decreased across the board, this decrease was not significant. Semi-arid was the dominant class, although the study area showed great variability, i.e. from extremely humid areas to arid zones. Forty-one out of the 45 series presented increasing aridity during the study period, although the trend was significant in only 16 of the series. From the Buishand range test for both aridity indices, the results revealed a noteworthy changing point for I-DM and for I-F. Although the changing point was variable, it was concentrated around the late 1970s and early 1980s, and from that period, the aridity tendency was dependent on the analysed series.