Estimating reference evapotranspiration (ETo) at 24 h timesteps has been considered sufficiently accurate for a long time. However, recent advances in weather data acquisition have made it feasible to apply hourly procedures in ETo computation. Hourly timesteps can improve the accuracy of ETo estimates, as data averaged daily may misrepresent evaporative power during parts of the day. The objective of the present study is to assess the differences between daily ETo computations performed on 24 h (ETo,d) and hourly (ETo,h) timesteps for rice–wheat cropping systems in the Ganga Basin, India. The meteorological data were collected from an automatic weather station located in an experimental plot at IIT Kanpur, India. Daily and hourly ETo computations were performed according to the FAO-PM equation for rice and wheat cropping seasons. Diurnal variations of meteorological variables resulted in an underestimation of ETo when the daily time step is considered. No significant difference was observed during wet periods. The hourly estimates of ETo were able to capture the abrupt changes in climate variables, while the daily ETo fails to represent it as it considers the average values only. As a result, the sums of hourly values are more reliable for ETo estimates in the Ganga Plains.

  • Diurnal variations of meteorological parameters result in an underestimation of daily ETo in Ganga Basin.

  • The hourly estimates of ETo are able to capture the abrupt changes in climate variables.

  • Hourly sum ETo is smaller than daily estimates when daily evaporation is low in high atmospheric demand periods.

  • The difference between hourly sum and daily ETo methods is low in monsoon months.

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