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Analysis of spatial and temporal variations of reference evapotranspiration (ETo) is important in arid and semi-arid regions where water resources are limited. The main aim of this study was to analyse the spatial distribution and the annual, seasonal and monthly trends of the PenmanMonteith ETo for 21 stations in the arid and semi-arid regions of Iran. Three statistical tests the Mann-Kendall, Sen’s slope estimator and linear regression were used for the analysis. The analysis revealed that ETo increased from January to July and deceased from July to December at almost all stations. Additionally, higher annual ETo values were found in the southeast of the study region and lower values in the northwest of the region. Although the results showed both positive and negative trends in annual ETo series, ETo generally increased, significantly so in six (similar to 30%) of the stations. Analysis of the impacts of meteorological variables on the temporal trends of ETo indicated that the increasing trend of ETo was most likely due to a significant increase in minimum air temperature, while decreasing trend of ETo was mainly caused by a significant decrease in wind speed. At the sites where increasing ETo trends were statistically significant, the rate of increase varied from (+)8.36 mm/year at Mashhad station to (+)31.68 mm/year at Iranshahr station. On average, an increasing trend of (+)4.42 mm/year was obtained for the whole study area during the last four decades. Seasonal and monthly ETo have also tended to increase at the majority of the stations. The greatest numbers of significant trends were observed in winter on the seasonal time-scale and in September on the monthly time-scale. Copyright (c) 2011 John Wiley & Sons, Ltd.