CANOPY PHOTOSYNTHESIS AND TRANSPIRATION ESTIMATES USING RADIATION INTERCEPTION MODELS WITH DIFFERENT LEVELS OF DETAIL

To simulate radiation interception, the physical structure of plant canopies is defined by the inclination, orientation, and location of their foliage elements. Detailed models must consider all these factors, which has a significant impact on the required computation time. Simplifications of these models are possible, sacrificing accuracy for computational speed. An existing canopy radiation model was modified to incorporate an ellipsoidal distribution of foliage elements, which was continuous over the range of leaf angles and allowed for accommodation of horizontal or vertical tendencies of the canopy. The canopy was divided into thin layers, and the foliage elements within each layer were classified into nine leaf inclination and azimuth angle classes. Simplifications of the model including options for varYing the thickness of the canopy layers and the number of angle classes, for modeling the canopy as a single layer, and for partitioning the foliage elements in each layer into sunlit and shaded fractions, with disregard of leaf angle classes were also incorporated. This radiation model and its simplifications were implemented in a computer simulation module for estimating canopy photosynthesis and transpiration. Simulation runs were performed for conditions with low and high leaf area index (LAI) and irradiance. It was found that canopy photosynthesis and transpiration estimates, determined using canopy layer thickness of 0.5 LAI and three inclination and azimuth angle classes, compared well (negligible error) with simulations using 0.1 LAI and nine leaf angle classes, an standard recommendation for detailed models. If only photosynthesis estimates were of interest, even thicker layers could be used. Furthermore, errors were small if only average irradiance over sunlit and shaded leaf fractions by layer were considered, fluctuating from 0% to 4.3% for photosynthesis estimates, and from 0% to 2.7% for transpiration estimates. In many instances, a single layer divided into sunlit and shaded elements yielded reasonable results. Simplified canopy radiation models resulted in a dramatic decrease of computation time, up to 1/60 of that required by the standard.