Difference in reference crop evapotranspiration calculated by PM equation and the corresponding single crop coefficient of rice between hourly scale and daily scale
Based on the hourly meteorological data collected by an automatic meteorological station in Kunshan Experimental Station during 2012-2013, hourly ET0 were calculated by ASCE PM and FAO 56 PM formulas equations separately to show the difference in hourly ET0 calculated by two different equations. Then the hourly ET0 were summed up to obtain the daily ET0 (termed as ET0-dhA and ET0-dhF）and compared to the daily ET0 calculated by PM equation at daily scale（ET0-d）. Differences among ET0-d，ET0-dhA and ET0-dhF were discussed, as well as the difference among the single crop coefficient of rice based on daily ET0 determined by different procedures. The results showed that hourly ET0 varied in parabolic shape, with the maximum observed at noon and minimum at midnight. Hourly ET0 were slightly different between two methods, with the difference fell in range of -0.02~0.5 mm/h. Hourly ET0 by ASCE PM were found slightly higher than by FAO56 PM in daytime. Daily ET0 were highly correlated to each other, and ranked in sequence of ET0-dhA >ET0-d>ET0-dhF. Local calibrated single crop coefficient Kc of rice were higher than the value determined following the procedure by FAO, and ranked in sequence of Kc-dhAc-dc-dhF, with the difference among them less than 5%.