Separating the diffuse and direct component of global radiation and its implications for modeling canopy photosynthesis Part II. Calculation of canopy photosynthesis

In a preceding paper, a method was presented to estimate the diurnal courses of total, direct and diffuse radiation from total daily radiation only. In the present paper, these relations are introduced into a simulation model for daily canopy assimilation. With the assimilation—light response of single leaves, assimilation rates of sunlit and shaded leaf area are calculated in relation to vertical position within the canopy and time of day. Numerical integration over canopy layers and over the day yields the daily canopy assimilation. Neglecting variation in illumination intensity of the leaves overestimates assimilation due to the convex, asymptotic assimilation—light response. For an atmospheric radiation transmission of 0.5 and a canopy with leaf area index of 5, daily canopy assimilation was overestimated by 2% when only the mean leaf angle was considered, 7% when all incoming light was treated as diffuse, 14% when light intensity was averaged over the day, and 23% when light absorption was averaged over canopy layers. These percentages increased sharply with an increase in atmospheric transmission. Neglecting the random variation in irradiance around the diurnal sine wave, overestimated assimilation by only 2–3%. Daily canopy assimilation was approximated very closely by using a weighted average of the assimilation rates at only three selected canopy depths at three times of day. A more concise model appeared to be satisfactory: a rectangular hyperbola for leaf assimilation—light response, integrated analytically over canopy leaf area index and using the irradiance averaged over the day