Simulation of diurnal variations of CO2, water and heat fluxes over winter wheat with a model coupled photosynthesis and transpiration

A model was developed that couples canopy photosynthesis and transpiration of winter wheat. The model combined a two-layer evapotranspiration model with a coupled photosynthesis–stomatal conductance model to study the diurnal variations of CO2, water and heat fluxes of winter wheat. Field experiments were conducted in Yucheng Comprehensive Experimental Station in the North China Plain to evaluate the model. Half-hourly data of weather variables and CO2, water and heat fluxes were measured by the eddy covariance method in 2002–2003. An analysis of measured flux data showed that there was an evident midday depression of photosynthesis, caused by stomatal closure due to high vapor water deficit and canopy temperature though the soil was well irrigated. There was a close agreement between simulated and measured net radiation, CO2 flux, sensible and latent heat fluxes, which proved the predictive power of the coupled photosynthesis and transpiration model. The response of CO2 flux, canopy conductance and latent heat flux to changes in climatic factors was discussed, which indicated the model could be used to predict CO2, water and heat fluxes of wheat not only in the North China Plain, but also in other climatic regions in China.