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The Penman-Monteith (P-M) model has been applied to estimate evapotranspiration in terrestrial ecosystem throughout the world. As shown in many studies, bulk canopy resistance is an especially important factor in the application of the P-M model. In this study, the authors used the Noilhan and Planton (N-P) approach and the Jacobs and De Bruin (J-D) approach to express the bulk canopy resistance. The P-M model was applied to a maize field using the two approaches in an arid area of northwest China and evaluated on the basis of measured half-hourly values from the eddy covariance system. The results indicate that the N-P approach slightly underestimates the bulk canopy resistance, while the J-D approach overestimates it. Over the entire maize growing season, the N-P approach yielded a more consistent estimate of bulk canopy resistance than did the J-D approach. Correspondingly, the P-M model with J-D bulk canopy resistance slightly underestimated the latent heat flux throughout the maize growing season, but overestimated the latent heat flux during the dry season as compared to the N-P approach results. The good fit between the simulated latent heat flux estimated by the P-M model using the N-P approach and the data measured at half-hour time steps demonstrates that the application of this approach is reasonable in relatively homogenous maize fields that are not drought-stressed. Further research to improve the performance of P-M model to simulate evapotranspiration in the cropped fields is discussed.