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In arid and semi-arid lands with warm climates, aridity and associated water scarcity is more severe because of greater populations and associated water use. The goal of this study was to explore the spatial and temporal variations of the de Martonne and Pinna aridity indices over Iran based on temperature and precipitation data from 41 stations for 40 years (1966-2005). The spatially interpolated maps of the aridity indices were prepared using the Ordinary Kriging technique in a GIS environment. The arid and semi-arid regions cover about 88% of Iran according to the de Martonne index, while about 96% of the country’s areas are classified as dry and semi-dry based on the Pinna index. A strong relationship was found between the values of the de Martonne and Pinna indices, confirming their similar spatial distribution. Around 63% of the two indices series had a decreasing tendency. The significant decreasing trends of the aridity indices were observed mainly in the western and northwestern regions of the country. The relative changes of the aridity indices at the stations with significant decreasing trends were in the range of 18%-54%. (C) 2014 Elsevier Ltd and INQUA. All rights reserved.