Spatial analysis of the annual and seasonal aridity trends in Extremadura, southwestern Spain

The knowledge of drought (or wetness) conditions is necessary not only for a rational use of water resources but also for explaining landscape and ecology characteristics. An increase in aridity in many areas of the world is expected because of climate change (global warming). With the aim of analysing annual and seasonal aridity trends in Extremadura, southwestern Spain, climate data from 81 locations within the 1951–2010 period were used. After computing the De Martonne aridity index at each location, a geographic information system (GIS) and multivariate geostatistics (regression kriging) were utilised to map this index throughout the region. Later, temporal trends were analysed using the Mann-Kendall test, and the Sen’s estimator was utilised to estimate the magnitude of trends. Maps of aridity trends were generated by ordinary kriging algorithm, providing a visualisation of detected annual and seasonal tendencies. An increase in aridity, as the De Martonne aridity index decreased, was apparent during the study period, mainly in the more humid locations of the north of the region. An increase of the seasonal De Martonne aridity index was also found, but it was only statistically significant in some locations in spring and summer, with the highest decreasing rate in the north of Extremadura. Change year detection was achieved using cumulative sum graphs, obtaining that firstly the change point occurred in spring, in the mid-1970s, later in the annual period in the late 1970s and finally in summer at the end of the 1980s.