Agricultural Meteorology, vol. 9 (1971) pp. 21-37
Measurements of direct and diffuse solar radiation were made at fifteen randomly distributed locations beneath a pine canopy and a hardwood canopy. A model that identifies four components of the solar radiation appearing beneath tree canopies is used to analyse the data. Direct beam radiation appearing below the canopy was found to follow an exponential-extinction law in the pine forest and a constant-ratio law in the hardwood forest. The large number of randomly distributed radiometers permitted precise estimates of the number of instruments needed to sample the radiation for different averaging periods. Under the pine canopy, 412 radiometers would be needed to estimate the instantaneous radiation with a standard error of the mean of 10 mly/min. For a full-day average, the number is 10. Beneath the hardwood canopy, the number needed for an instantaneous average is 18, while only one is needed for a full-day average.
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