Estimation of reference evapotranspiration and crop coefficient in wheat under semi-arid environment in India

Abstract

The comparison of six standard reference evapotranspiration (ETo) estimate models was carried out with measured lysimeter evapotranspiration (ETc) in wheat crop in a semi-arid environment at Rahuri, India. The globally accepted reference evapotranspiration model of FAO 56 Penman Monteith underestimated the references ETo by 19.2% (420.5 mm) over lysimeter ET (520.7 mm). Out of the remaining five models, the Hargreaves model ranked first and was overestimated by 6.5% followed by the Blaney Criddle model (underestimated by −5.6%). The references ETo by pan evaporation model underestimated the reference ETo to the extent of −28.8%. The influence of statistical indicators like RMSE, MBE which was computed by considering Lysimeter ETc as standard, was quite low in the Hargreaves model compared to the rest of the models and hence, the Hargreaves method is quite acceptable for ETo estimates as this method requires much less climatic parameters (temperature and extraterrestrial radiation) than Penman Monteith (FAO56) and Modified Penman of FAO 24 as these models require aerodynamic and radiation terms, besides additional physical terms in former models of ETo estimates. The seasonal crop coefficients were 1.24, 1.13, 0.94, 0.85, 1.06 and 1.40 in Penman Monteith, Modified Penman, Hargreaves, Radiation balance, Blaney Criddle and Pan Evaporation models, respectively.