Reference evapotranspiration change and its sensitivity to climate variables in southwest China

Reference evapotranspiration (ET0) is the key factor for hydrologic water balance, irrigation scheduling, and water resources planning. Based on Food and Agriculture Organization (FAO) Penman–Monteith method and the climate variables of 57 meteorological stations from 1960 to 2010 in southwest China, the spatial and temporal distributions of ET0 were analyzed by using Mann–Kendall test and Sen’s slope estimator. Sensitivity coefficient was used to analyze the sensitivities of ET0 to four climate variables, and the key climate variables attributed to ET0 change were determined. Result showed that there was a slight downward trend of ET0 from 1960 to 2010 and spatially increasing trend from northeast to southwest in annual time scale. Results also showed that ET0 had relatively higher sensitivity to wind speed and mean air temperature, and wind speed was the dominant variable for change of ET0 in southwest China. The inverse relationship between increasing air temperature and decreasing evaporation, “evaporation paradox,” existed in southwest China, and the negative contribution of wind speed to the changes of ET0 offset the positive contribution of air temperature.