Net radiation frequency distribution in a corn crop

Net radiation measurements were made with a traversing system in a 250 cm tall (LAI = 4.0) corn crop with east-west oriented rows. Frequency distributions of flux density of net radiation were plotted using 0.075 cal cm^-2 min^-1 class intervals. These distributions showed a shift from predominantly sunlit to predominantly shaded conditions with depth into the plant canopy. The frequency distributions showed that net radiation in sunflecks in a canopy can exceed that above the canopy, and that negative values of net radiation can exist in shadows. A Soil-Plant-Atmosphere Model (SPAM) also predicted that net radiation in sunflecks in the canopy would exceed net radiation above the canopy. The effects of spots with high radiation load within the plant canopy on leaf temperature and convective heat exchange were investigated theoretically.Contributed by the Northeast Branch, Soil and Water Conservation Research Division, Agricultural Research Service, U.S. Department of Agriculture, in cooperation with the Cornell University Agricultural Experiment Station, Ithaca, New York. Supported in part by the Atmospheric Sciences Laboratory, U.S. Army Electronics Command, Fort Huachuca, Arizona. Deparment of Agronomy Series, Paper No. 907.