Clustering analysis of regional reference evapotranspiration and its components based on climatic variables across northeast China, 1961–2010

Evapotranspiration integrates atmospheric demand and surface conditions. The Penman-Monteith equation was used to calculate annual and seasonal reference evapotranspiration (ET₀) and thermodynamic and aerodynamic components (ET_rad and ET_aero) at 77 stations across northeast China, 1961–2010. The results were: (1) annual ET_rad and ET_aero had different regional distribution, annual ET_rad values decreased from south to north, whereas the highest ET_aero values were recorded in the eastern and western regions, the lowest in the central region; (2) seasonal ET_aero distributions were similar to seasonal ET₀, with a south–north longitudinal pattern, while seasonal ET_rad distributions had a latitudinal east-west pattern; and (3) in the group for ET₀ containing 69 sampling stations, effects of climatic variables on ET₀ followed sunshine hours > relative humidity > maximum temperature > wind speed. Changes in sunshine hours had the greatest effect on ET_rad, but wind speed and relative humidity were the most important variables to ET_aero. The decline in sunshine duration, wind speed, or both over the study period appeared to be the major cause of reduced potential evapotranspiration in most of NEC. Wind speed had opposite effects on ET_rad and ET_aero, and therefore the effect of wind speed on ET₀ was not significant.