Trend analysis of reference evapotranspiration in the western half of Iran

Reference evapotranspiration (ETo) is an important element of the hydrological cycle, and changes in ETo 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 ETo at 20 meteorological stations during 1966–2005 in the western half of Iran were examined using the Mann–Kendall test, the Senʹs slope estimator and the linear regression. Annual analysis of the ETo series indicated a positive trend in 70% of the stations according to the Mann–Kendall test and the Senʹs slope estimator and in 75% of the stations according to the linear regression. The magnitude of significant positive trends in annual ETo varied from (+)11.28 to (+)2.30 mm/year. On the seasonal scale, stronger increasing trends were identified in ETo 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 ETo 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 ETo showed that the increasing trend was mainly caused by a significant increase in air temperature during the study period.