Trend analysis of reference evapotranspiration in the western half of Iran

▶ A significant trend was observed in the annual ET o series in most of the stations. ▶ The main cause of the increasing trend in ET o was an increase in air temperature. ▶ The greatest significant trends in the seasonal ET o series were observed in winter. ▶ The greatest significant trends in the monthly ET o series were found in February. Reference evapotranspiration (ET o) is an important element of the hydrological cycle, and changes in ET o are of great significance for agricultural water use planning, irrigation system design and management. In this study, annual, seasonal and monthly trends in the Penman–Monteith ET o at 20 meteorological stations during 1966–2005 in the western half of Iran were examined using the Mann–Kendall test, the Sen's slope estimator and the linear regression. Annual analysis of the ET o series indicated a positive trend in 70% of the stations according to the Mann–Kendall test and the Sen's slope estimator and in 75% of the stations according to the linear regression. The magnitude of significant positive trends in annual ET o varied from (+)11.28 to (+)2.30 mm/year. On the seasonal scale, stronger increasing trends were identified in ET o data in winter and summer compared with those in autumn and spring. Meanwhile, the highest numbers of stations with significant trends were found in the monthly ET o series in February, while the lowest numbers of stations with significant trends were observed in November. Analysis of the impact of climatic variables on the significant increasing trend in ET o showed that the increasing trend was mainly caused by a significant increase in air temperature during the study period.