Abstract: Evapotranspiration(ET) is an important component of farmland water cycle, and its accurate estimation is of great significance for precision irrigation and water-saving agriculture. The Penman-Monteith (P-M) model is one of the most commonly used estimation methods, but reliable representation of canopy resistance (r_s) has been a difficult problem in applying the P-M. In this paper, seven commonly used r_s models were selected to assess if their simulated r_s could be used with P-M to directly estimate winter wheat ET. The P-M simulated ET was compared with measured values by Bowen ratio energy balance (BREB) system in Shunyi, Beijing for two years (2020 and 2021), and the main factors affecting wheat rs were analyzed. The results showed that the seven models generally underestimated wheat canopy resistance and overestimated evapotranspiration. Overall, the Todorovic model (TD) performed the best, and the R² for simulated r_s and ET were >0.605, mean bias error (MBE) being −82.8s·m⁻¹ and 10.4W·m⁻², respectively, with root mean square error (RMSE) of 254.4s·m⁻¹and 33.5W·m⁻²; the other six models performed poor, and the R² for simulated r_s was between 0.113−0.241, MBE and RMSE, were between −236.4 to −61.3s·m⁻¹ and 277.2 to 373.8s·m⁻¹, respectively. The R² for simulated ET was between 0.046−0.184, MBE and RMSE were between 44.5−97.4W·m⁻² and 81.4−147.9W·m⁻², respectively. On basis of RMSE, the performing order was TD>FAO56-PM>Katerji-Perrier (KP)>Garcıá-Santos (GA)>idso (IS)>Jarvis (JA)>CO. The correlation between canopy resistance and various factors suggested that net radiation (Rn) affected the most on wheat rs, while air temperature (Ta) and canopy temperature (Tc) affected the least with the following specific order: Rn>leaf area index (LAI)>relative humidity (RH)>vapor pressure deficit (VPD)>soil moisture (θ)> canopy-air temperature difference (∆T)>Tc>Ta. This results better explained the good performance of TD model, and it considers the key factors affecting canopy resistance such as Rn, VPD and ∆T. In addition, it has no parameters to be calibrated, which makes it easy to use. The results of this paper provided a scientific basis for applying one-step approach to calculate the water consumption of winter wheat.

Key words: Canopy resistance model, Winter wheat, Evapotranspiration, Penman-Monteith