Arch. Met. Geoph. Biokl., Set. B, 17, 403--412 (1969) 551.521 : 551.586 : 63. i Faculty of Agricultural Sciences, Laboratory of Plant Ecology, University of Ghent, Ghent, Belgium Extinction of Net Radiation in Different Crop Canopies I. IMPENS and R. LEMEUR With 4 Figures Received June 23, 1969 Summary By means of a network of 14 miniature net radiometers, net radiation was measured above and at two heights within field plots of oats, beans, sunflower and corn. From the measurements, net radiation expenditure within the top and middle layer of the foliage canopies could be evaluated for clear and overcast days. The resulting data indicated that, in accordance with recent theoretical approaches, extinction coefficients within the uppermost canopy layer were not constant with time of day on clear days for the species with an erectophile leaf distribution (corn, oats). This diurnal trend did not occur on days with mainly diffuse radiation or at a greater depth within the canopy where net radiation is also more omnidirectional. Net radiation extinction with depth into the canopy was found to deviate from Beer's law. The vertical distribution of net radiation within a corn crop was approximated very closely by an equation of the follow- ing form: where (Rn)z/(Rn)l~ is the fraction of net radiation at the top (h) of the canopy transmitted at a given level z, K1 and K 2 are extinction coefficient constants and L is the downward cumulative leaf area index. Zusammenfassung Extinktion der Strahlungsbilanz in verschiedenen Feldfruchtbest~inden Mit Hilfe eines Netzes von 14 Miniatur-Strahlungsbilanzmessern wurde die Strahlungsbilanz oberhalb und in zwei verschiedenen H6hen zwischen den Pflan- zen auf Versuchsfl~ichen yon Hafer, Bohnen, Sonnenblumen und Mais gemessen. Auf diese Weise konnte f6r heitere und ffir bedeckte Tage die Verteilung der Strahlungsbilanz in den oberen und mittleren Schichten des Laubwerks bestimmt