Evaluation of a Model Adaptable to Satellite Data for Estimating Daily Gross Photosynthesis

A model for instantaneous gross canopy photosynthesis, considering sunlit and shaded leaves, variation of maximum rate of leaf photosynthesis within the canopy, and a solution of the radiative transfer equation for propagation of direct and diffuse photosynthetically active radiation, is numerically integrated in space (angular variation of radiances and depth within the canopy) and time (diurnal cycle) to obtain daily total photosynthesis for varied incident irradiances and leaf and canopy characteristics. These numerical results are used to evaluate an analytic model for daily photosynthesis that requires daily total irradiance, recognizing that readily available global satellite data provide daily total irradiance rather than diurnal values of direct and diffuse irradiances. The analytic model is derived by considering the light response function for leaf photosynthesis to be a rectangular hyperbola. The evaluation was done by using field observations for 27 canopies representing four cultivated and seven uncultivated species and having a leaf area index between 0.6 and 5.9. The evaluation gives a mean maximum uncertainty of 32% for the analytic equation. The magnitude of uncertainty can be reduced by knowing the diffuse fraction of incident radiation.