Abstract: In the main growth stages of winter wheat,the changes of every component of canopy energy balance by using Bowen-ratio system,diurnal pattern of canopy surface temperature and daily ambient air temperature in the clear days by using infrared thermometer,leaf area index every week were measured.The diurnal pattern and seasonal variation of canopy energy balance were analyzed.The evapotranspiration of the cropland was estimated by using Brown-Rosenberg method and validated by comparison with that measured using Bowen-ratio method,based on crop canopy temperature measured by remote sensing method and surface energy balance principle.The analysis of canopy energy balance indicated that the allocation of net radiation flux to soil heat flux was only about 3% when components of nocturnal canopy energy balance was considered.The latent heat flux in the nighttime was approximated to zero.The estimated evapotranspiration based on canopy temperature was slightly higher than that measured by Bowen-ratio system,and relative average deviations between the two methods were 16%(ignoring soil heat flux) and 10%(considering soil heat flux),with no significant differences.The results certified that the soil heat flux and nocturnal evapotranspiration of cropland can be ignored approximately in estimating cropland evapotranspiration during jointing stage and grain filling stage as LAI was above 3.It is feasible to estimate cropland evapotranspiration in the North China Plain by applying Brown-Rosenberg formula.The results justify the possibility for the combination of remotely sensed thermal infrared crop canopy temperature with crop growth model to estimate crop yield at the regional scale.

Key words: Winter wheat, Winter wheat, Canopy surface temperature, Brown-Rosenberg formula, Evapotranspiration