A method for simulating the direct solar radiation regime in sunflower, jerusalem artichoke, corn and soybean canopies using actual stand structure data

Abstract

A method has been developed to incorporate actual canopy architecture in radiation models. The mathematical procedure involved, introduces a leaf angle and azimuthal distribution function, and yields an integrated extinction coefficient which can be substituted into basic light extinction formulae.Leaf angle and azimuthal distribution functions were determined for sunflower (Helianthus annuus), Jerusalem artichoke (Helianthus tuberosus), corn (Zea mays) and soybean (Glycine max var. Amsoy). A special computer program was written to calculate the extinction coefficients for each canopy at a number of solar elevations.Applying these coefficients in the extinction formulae for direct solar radiation, the influence of the four stand structures on penetration, interception, absorption and sunlit leaf area index was evaluated. The variation of these variables with time of the day is influenced by the leaf angle distribution function, while the non-random azimuthal distribution is reflected in an upper and lower limit for each variable. The span of this variation increases with low solar elevations and with higher ratios of maximum and minimum azimuthal density.Although the four crops displayed an almost equal daily absorption of direct solar radiation, it was shown that erectophile plant stands are more efficient in their light intercepting capacity than planophile canopies.An error analysis indicates considerable deviation on the prediction of the radiation regime, when calculations are based on a uniform leaf inclination. Deviations on extinction coefficient, interception and sunlit leaf area index are expected to be 10 to 20% at low solar elevations.