Differences in Sum-of-Hourly and Daily Reference Evapotranspiration for a Rice-Wheat Cropping System in Ganga Basin, India

Abstract

Estimating reference evapotranspiration (ET ) 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 ET computation. Hourly timesteps can improve the accuracy of ET 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 ET computations performed on 24 h (ET ) and hourly (ET ) timesteps for rice-wheat cropping systems in the Ganga Basin, India. The meteorological data for computing reference evapotranspiration were collected from an automatic weather station located in an experimental plot at IIT Kanpur, India. Daily and hourly ET computations were performed according to the FAO-PM (Allen et al, 1998) equation for rice and wheat cropping seasons. Diurnal variations of meteorological parameters resulted in underestimation of ET when the daily time step is considered. No significant difference was observed during wet periods. It is also observed that the hourly estimates of ET were able to capture the abrupt changes in climate variables, while the daily ET fails to get it as it considers the average values only. As a result, the sums of hourly values are more reliable for ET estimates in the Ganga Plains.