The Measurement and Distribution of Irradiance in Clear and Overcast Conditions in Four Temperate Forage Grass Canopies

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Abstract
An instrument is described for measuring the distribution of irradiance in grass canopies. The irradiance distributions for overcast and clear conditions were quite distinct; light from an overcast sky was more uniformly spread through the canopies than light from a clear sky. Each of the 4 cultivars [Lolium multiflorum cv. ''RVP'', L. perenne cv. ''S24'', Dactylis glomerata cv. ''S345'', Phleum bertolonii cv. ''S50''] had its own characteristic irradiance distribution pattern which was maintained in clear and overcast conditions. The extinction coefficients ranged 0.11-0.16 for ''S345'' and 0.80-1.20 for ''S24'' in overcast and clear conditions. The relative ranking of cultivars for the extinction coefficients, measured in clear and overcast conditions, and mean leaf angle were identical, suggesting that the angular structure of the canopy plays an important part in determining light distribution in grass canopies. In a grass canopy, the distribution of light coming directly from the sun may be satisfactorily described by Monteith''s S function.