Differences in sum-of-hourly and daily reference evapotranspiration for a rice–wheat cropping system in Ganga Basin, India

Abstract

ABSTRACT 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. This study investigates the differences between daily ETo computations performed at 24 h (ETo,d) and sum of hourly (ETo,h) 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 sum-of-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 timestep was considered. No significant difference was observed during wet periods. The sum-of-hourly estimates of ETo were able to capture the abrupt changes in climate variables, while the daily ETo failed to represent it as it considered the average values only. As a result, the sums of hourly ETo estimates are more reliable in the Ganga Plains.