Effects of ENSO and Climate Change on Reference Evapotranspiration in Southern Vietnam

This study aims to evaluate the effects of climate change and El Niño–Southern Oscillation (ENSO) on reference evapotranspiration (ET_o) in the climate sub-regions of South Vietnam (SVN) and the role of the related meteorologi-cal variables. The trend analysis shows that ET_o has been increasing quite clearly, especially during the rainy season. By examining the contribution of meteorological variables to the increase of ET_o, it is indicated that temperature is the main contributor. Among the meteorological variables that are related to ET_o, temperature is also the factor that has gained the most significant increase. Analysis of variance reveals that there is no difference in the increase of ET_o among the climate sub-regions of SVN. The effect of ENSO on ET_o is assessed based on the difference in the mean value of ET_o between the El Niño and La Niña phases. The results show that this difference is most obvious from October to May, and the main factor contributing to the increase is not temperature but sunshine hours. The difference in sunshine hours between the warm and cold phases in these months is around one hour per day, contributing about 58%–86% to the ET_o difference. Further analysis of variance shows that ENSO has different levels of influences on ET_o in the climate sub-regions. Compared to the increase in ET_o due to climate change over the past 40 years, the ET_o difference between El Niño and La Niña phases is many times higher. In addition, since the effect of ENSO on ET_o is most obvious in the study area during the dry season, it is much stronger. In order to mitigate the effect of ENSO on drought in this area, monitoring and forecasting meteorological variables that have the main contribution to the variation of ET_o, including the number of sunshine hours, should be promoted.