Estimation of reference evapotranspiration (ET 0 ) is important in hydrological studies. As climate change is predicted to cause changes in water resources and agriculture sectors, the possible implications of changes in different climate variables on ET 0 need to be understood. Sensitivity analysis was employed to illustrate the effects of perturbation in meteorological parameters (maximum and minimum air temperature (T max and T min ), sunshine hours (n), maximum and minimum relative humidity (RH max and RH min )) and wind speed (u z ) on ET 0 . ET 0 was calculated by The Food and Agriculture Organization of the United Nations (FAO) 56 Penman-Monteith approach using climate data from 1979-2017 for the Enbara and Futatsumori watersheds in forested mountain areas in Japan. We quantified the contributions of climatic factors to ET 0 at daily, monthly, seasonal and annual time scales. Daily results showed large fluctuations. According to the monthly and seasonal analysis, during warm seasons, T max and n had more influence on ET 0 , especially in May, while for the cold seasons, RH effect was dominant, especially in December. Based on the annual results, the factors most influencing ET 0 were T max followed by n, u z , RH and T min . We also found that the response of ET 0 to changes in climatic parameters differs for sites with different topographic and geographic characteristics.