陆地生态系统CO₂和水热通量变化特征及其环境控制机制是理解生态系统水碳耦合循环过程和模型开发的基础。农田生态系统水碳通量特征的研究对把握作物生产力水平和水分利用效率的高低具有重要意义。本研究基于涡度相关技术对华北平原冬小麦农田生态系统齐穗期水碳通量的日变化特征进行了分析。结果表明：以C₃作物冬小麦构成的农田生态系统晴天CO₂通量、水汽通量和冠层导度均存在明显的“午休”现象，“午休”的原因是晴天强光造成的“光抑制”；CO₂通量对光合有效辐射呈米氏响应，其表观量子效率为0．002997 mg/μmol，潜在最大碳通量为3．3494 mg/(m².s)；水汽通量和冠层导度与光合有效辐射之间均存在极显著线性关系，但在相同光合有效辐射下，下午的冠层导度明显小于上午，其原因在于同化产物的反馈抑制；下午的水汽通量明显大于上午，则主要受控于温度和VPD的变化。由于下午的水汽通量大于上午，而CO₂通量在上午和下午无明显差异，使得农田生态系统上午的水分利用效率明显高于下午。

Characteristics of CO₂,water and heat fluxes of terrestrial ecosystems and related mechanisms involved provide insights into the coupling cycle of water and carbon and the model development.It′s necessary to investigate the characteristics of water and carbon fluxes for understanding the crop productivity and water use efficiency.In this study,we analyzed the diurnal changes of water and carbon fluxes at full heading time of winter2wheat in North China based on the eddy covariance technique.The results showed that there existed evident midday depression of CO₂,water vapor and canopy conductance under clear sky condition,as a result of photo-inhibition caused by strong radiation.The responses of CO₂ flux to photosynthetically active radiation could be expressed by Michaelis-Menten function,with apparent quantum yield 0.002997 mg/μmol photons and maximum potential of carbon flux 3.3494 mg/(m²·s).The water vapor flux and canopy conductance were significantly correlated with photosynthetically active radiation.However,at the same light condition,canopy conductance in afternoon was apparently smaller than that in the morning due to the feedback restrain of photosynthate,whereas water vapor flux in afternoon was much larger than that in the morning,resulting from the changes of air temperature and vapor pressure deficit.Note that water use efficiency in the morning was higher than that in the afternoon,because water vapor flux in afternoon was much larger than that in the morning,while no obvious difference exited for CO₂ flux between the morning and afternoon.