Contribution rates of climate variables to the change in reference evapotranspiration in northern Thailand from 1991 to 2019

Higher temperatures are expected to increase evaporation and reference evapo-transpiration (ETo). However, a decrease in ETo has been observed in numerous regions worldwide. ETo is controlled by climate variables besides temperature; wind speed, air humidity, and radiation, which vary according to regional climatic conditions, may also be drivers. Few studies have quantified the sensitivity and contributions of climate variables to ETo change in tropical savanna climates. Long-term (1991-2019) climate time series in northern Thailand was used to assess the trends in ETo and determine which climate variables exert the strongest control on ETo, during the rainy and non-rainy season, as an example for the tropical savanna climate. Furthermore, this study used the proportion of contribution and sum of positive (negative) contribution and classified the results into five groups to analyse the change in ETo. This helped to determine the climate variable with the highest contribution. From this study, ETo was the most sensitive to the maximum temperature, followed by sunshine duration in both rainy and non-rainy seasons. Furthermore, the variables that contributed the most to the long-term change in ETo were sunshine duration and wind speed. The increase (decrease) in sunshine duration and wind speed contributed to the increase (decrease) in ETo in this area. Understanding ETo changes and determining whether ETo has a strong tendency to change are the basis for water resource planning and management in the region.