Abstract: Daily reference crop evapotranspiration ET0 was calculated from two different time steps of daily and monthly mean meteorological data by using the Penman-Monteith equation and the monthly ET0 was accumulated,based on meteorological data in the stations of Yanzhou(1981-1986),Heze(1981-1986) and Dingtao(1997-2002),which all located in Sihu basin.The differences of monthly ET0 value were evaluated,by which analyzed whether ET0 can be computed from mean monthly weather data instead of daily mean weather data in case of unavailable daily mean weather data.The results showed that two monthly ET0 value was very similar,and there was a significant positive correlation between them.The annual ET0 value calculated with mean monthly data was higher 2.0% than that calculated with daily mean data.The former was lower 3.7% than latter in warm season,and was higher 8.1% in cold season,which showed that the key deviation was occurred in cold season.The mean monthly data was similar to that calculated with daily mean data,especially in warm season.No matter in warm or cold season,the difference of monthly ET0 values calculated with two time steps data was mainly due to differences calculation of energy item in the whole year.The lower ET0 value occurred in warm season just because of the energy items decreasing,which was about accounting for 131.8% of the difference of two monthly ET0 values,as the higher ET0 value occurred in warm season just because of the energy items increasing,which was about accounting for 92.1% of the difference of two monthly ET0 values.

Key words: Reference crop evapotranspiration, Reference crop evapotranspiration, FAO Penman-Monteith method, Daily time step, Monthly time step