Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 12 6 2008 10.5194/hess-12-1309-2008 http://www.hydrol-earth-syst-sci.net/12/1309/2008/ http://www.hydrol-earth-syst-sci.net/12/1309/2008/hess-12-1309-2008.html http://www.hydrol-earth-syst-sci.net/12/1309/2008/hess-12-1309-2008.pdf 1309 1321 2008-12-10 A precipitation-based regionalization for Western Iran and regional drought variability T. Raziei I. Bordi isabella.bordi@roma1.infn.it L. S. Pereira Soil Conservation and Watershed Management Research Institute (SCWMRI), Tehran, Iran Department of Physics, University of Rome "La Sapienza", Rome, Italy Agricultural Engineering Research Center, Institute of Agronomy, Technical University of Lisbon, Lisbon, Portugal The spatial distribution of the seasonal and annual precipitation was analyzed in western Iran using data from 140 stations covering the period 1965–2000. Applying the Precipitation Concentration Index (PCI), the intra-annual precipitation variability was also studied. Furthermore, nine precipitation-derived parameters were used to regionalize climate in western Iran using principal component analysis and clustering techniques. Results suggest that five spatially homogenous sub-regions can be identified characterized by different precipitation regimes. The spatial pattern of seasonal precipitation seems to be highly controlled by the wide latitudinal extent of the region and by the pronounced orographic relieves, and the time of occurrence of the maximum precipitation varies from spring in the north to winter in the south. <br><br> The time variability of dry and wet periods in the identified sub-regions was analyzed using the Precipitation Index (PI) and the existence of any long-term trend was tested. Results show that the northern and southern regions of western Iran are characterized by different climatic variability. 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