Evaluation Of Temperature-based Methods For Reference Crop Evapotranspiration Against Measurement Of Bowen-Ratio Energy Balance System

Accurate estimation of reference crop evapotranscpiration?ET₀?is critical for computation of crop water requirement.The currently existed numerous ET₀ methods,while bring convenience for computation of crop water requirement,also cause confusion in method choice.Knowledge on validity of these methods in different regions is thus the basis for reasonable choice.However,most evaluation studies are based on estimates of the FAO56 Penman-Monteith,and studies made with measured ET₀ is valuable.Using two years'data measured with Bowen-ratio energy balance method at Xiaotangshan,changping,Beijing in 2015 and 2016,six temperature-based methods?Hargreaves,Blaney-Criddle?B-C??,FAO24 B-C,McCloud,Hamon and Thornthwaite)were assessed.The main results were as follows:1.The Hargreaves generally performed the best,but depended on the time scale to some degree.2.On daily basis the Hamon performed the best.Based on root mean square error?RMSE?,we found the following performing order:Hamon>Hargreaves>McCloud>Blaney-Criddle>FAO24B-C.In addition,all the evaluated methods showed overestimation,ranging from 0.1 to 1.5 mm·d^-1,with the McCloud showing the minimum overestimation,followed by the Hargreaves.3.On monthly basis the Hargreaves was the best,and it overestimated by 49.4 mm?or 38.3%?in summer months the measured values while underestimated by 11.8mm?or-23.8%?in winter months.The order of these formulae in summer was:Hargreaves>Thornthwaite>Hamon>Blaney-Criddle>McCloud>FAO24 B-C,and in winter it was:Hargreaves>FAO24 B-C>Hamon>Thornthwaite>McCloud>Blaney-Criddle.4.All the methods had the common feature that they overestimated ET₀ in summer,ranging from 49.4mm to 118.1mm,and underestimated ET₀ in winter with the exception of the Blaney-Criddle,ranging from 11.8mm to 80.7mm.5.On annual basis the Hargreaves was still the best,and the performing order was:Hargreaves>FAO24 B-C>Thornthwaite>McCloud>Blaney-Criddle>Hamon.The relative deviation varied from-35.4%to 44.4%.The Hargreaves,Blaney-Criddle,FAO24 B-C overeatimated ET₀,the others undereatimated ET₀.6.The comparison of the monthly dynamic revealed that the Hargreaves was in good agreement with the measured values,and it peaked in June,while the trends of the other methods were poor,peaking mostly in July.7.For daily scale applications we recommended the Hamon and Hargreaves,but for monthly and annual scale the Hargreaves should be preferred.