The Impact of ENSO Event on Reference Crop Evapotranspiration Variability in Some Typical Cold Climates of Iran

The main aim of this research is investigating the effect of ENSO phenomenon on reference evapotranspiration (ET0) on monthly، seasonal and annual time scales، using Southern Oscillation Index (SOI). For this purpose، 13 sites located in cold climate regions with 50 years (1957-2006) meteorological data were selected. In the first step، the reference evapotranspiration rates were determined for the selected sites by using FAO recommended approach. In the second step، different phases (El Nino، La Nina and normal) were separated in terms of SOI and the mean deviation of ET0 values at each phase were compared by Mann-Whitney test. At statistical significant levels (p< 0. 1)، good correlation were found between the ET0 values and SOI. About 72% of correlations were positive and the rest (28%) were negative. In positive SOI-ET0 correlations، the monthly averages of ET0 values during El Nino phases were 14. 8% and 10. 8% lower than ET0 of La Nina and Normal phases، respectively. On the contrary، the average ET0 rates in La Nina phases were 13. 1% higher than the corresponding values of normal pahses. The mean time lag to observe the highest impact of ENSO on ET0 was 3. 2 months. The highest effective months in the study sites was found to be November، October and December، respectively. In seasonal time scale، 68% of the statistical significant affecting cases were occurred in autumn. It was found that the cold climates were more sensitive to the ENSO signals than warm climates. The results can be useful for policy makers in water resources management and agricultural sectors.