Effects of Vertical Profiles of Plant Area Density and Stomatal Resistance on the Energy Exchange Processes within a Rice Canopy

Abstract

A multilayer energy budget model within and above a rice canopy is developed to show physical and physiological effects of the vegetation on the energy exchange processes and to evaluate the characteristics of the energy budget. Micrometeorogical observations within and above a rice canopy are analyzed to optimize model parameters and to test the model. Model parameters of the inclination factors of a leaf F_l(z) and a panicle F_p for radiation, the effective drag coefficient c_d, the transfer coefficient for sensible heat c_h and stomatal resistance r_s(z) are optimized by fitting the modeled vertical profiles of downward shortwave radiation S^↓(z), mean horizontal wind speed, air temperature, plant surface temperature and water vapor pressure to the observations in the morning, near noon and in the afternoon on 4 September 1998, when the growing stage of rice was early ripening.
The optimized model parameters demonstrate that the radiation exchange processes depend on the solar altitude and the vertical distribution of the plant area density and its components, i.e. the morphology of the canopy, and that panicles are as important as leaves for the energy exchange processes in the paddy field, while culms are unimportant. The optimized vertical profiles of the stomatal resistance r_s(z) show inverse correlations with F_l(z)S^↓(z), and positive correlations with vapor pressure deficit VPD(z). Thus, the model demonstrates that the stomatal resistance is also influenced by the morphology of the canopy, and interacts with VPD adjacent to the leaves. According to the calculated vertical profiles of sensible and latent heat fluxes, the negative sensible heat flux in the afternoon originates mainly from lower canopy layers, which occurs even under the condition of higher r_s(z).