Evapotranspiration is critical to many applications including water resource management, irrigation scheduling, and environmental studies. Many models based on meteorological data have already been developed to estimate reference evapotranspiration ( ET0 ) in various climatic and geographical conditions. The main purpose of this study was to evaluate the performances of the Makkink, Priestley-Taylor, and Hargreaves models versus the Penman-Monteith FAO-56 (PMF-56) method in arid and semiarid regions of Iran during 1993–2005 and to identify the alternative ET0 model that presents results closest to the PMF-56 method. Additionally, a regional estimation of monthly ET0 with the best-performed model is presented by using the spatially distributed physical parameters and geographical information system. The results indicated that the Hargreaves model was the best model to estimate ET0 in eastern arid and semiarid regions of Iran. The spatial distribution maps of ET0 showed that ET0 values increased from north ...

The Southern Oscillation Index (SOI) is an acceptable scientific index for determining the strength of El Niño Southern Oscillation (ENSO) phenomenon. Because of the importance of reference evapotranspiration (ET₀) in determining crop water demand, this study was conducted to assess the impacts of different ENSO phases on ET₀ variability in some warm climates of Iran. For the estimation of ET₀, the daily meteorological variables from a set of stations during a period of 50 years (1957–2006) were used in an aerodynamic energy balance approach and the correlation between SOI and the estimated ET₀ values for two scenarios (with and without time lag) was constructed. Using Spearman, Pearson and Mann‐Whitney approaches, the correlation coefficients (r) and the statistically significant relative differences between the mean ET₀ values and their corresponding variations in each phase were verified. The results of seasonal ET₀ showed that in 54% of the study sites, significant (P < 0.05) correlations between ENSO events and the ET₀ variations exist. In the monthly timescale, 88% of the significant SOI‐ET₀ correlations experienced positive signs. In most of the cases, the spring and winter ENSO events influenced the ET₀ values one or two seasons after the occurrence of the ENSO. On average, the mean monthly ET₀ values during El Niño phases were 10.1 and 9.3% lower than the corresponding ET₀ values during La Niña and normal phases, respectively. On the contrary, the mean monthly ET₀ values during La Niña were 8.4% higher than that in normal phase. It was found that the degree of impact of ENSO on ET₀ variability is sensitive to the timescale of analyses. Furthermore, the ET₀ variations in warm arid sites were more sensitive to teleconnection impact of ENSO than the humid sites. Copyright © 2010 Royal Meteorological Society