ENSO teleconnection impacts on reference evapotranspiration variability in some warm climates of Iran

摘要

The Southern Oscillation Index (SOI) is an acceptable scientific index for determining the strength of El Ni?o Southern Oscillation (ENSO) phenomenon. Because of the importance of reference evapotranspiration (ET_0) in determining crop water demand, this study was conducted to assess the impacts of different ENSO phases on ET_0 variability in some warm climates of Iran. For the estimation of ET_0, 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 ET_0 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 ET_0 values and their corresponding variations in each phase were verified. The results of seasonal ET_0 showed that in 54% of the study sites, significant (P < 0.05) correlations between ENSO events and the ET_0 variations exist. In the monthly timescale, 88% of the significant SOI-ET_0 correlations experienced positive signs. In most of the cases, the spring and winter ENSO events influenced the ET_0 values one or two seasons after the occurrence of the ENSO. On average, the mean monthly ET_0 values during El Ni?o phases were 10.1 and 9.3% lower than the corresponding ET_0 values during La Ni?a and normal phases, respectively. On the contrary, the mean monthly ET_0 values during La Ni?a were 8.4% higher than that in normal phase. It was found that the degree of impact of ENSO on ET_0 variability is sensitive to the timescale of analyses. Furthermore, the ET_0 variations in warm arid sites were more sensitive to teleconnection impact of ENSO than the humid sites.