The transmission of global and photosynthetically active radiation within a dwarf apple orchard

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Abstract
Measurements of incoming global radiation, K↓, beneath a dwarf apple orchard canopy were made with a traversing sensor and a randomly located set of 10 stationary sensors. The location of the latter was changed once during the period of the experiment. On a daily basis, satisfactory agreement between the two sets of measurements was attained only for the second network location.Above-canopy and track measurements were used to calculate diurnal and daily values of canopy transmissivity, absorptivity, and reflectivity. Average values for the whole period were 0.53, 0.28, and 0.19. Analogous coefficients were obtained for photosynthetically active radiation (PAR). Extracanopy values of PAR were evaluated from PAR = 0.49 K↓ on 'bright' days (days with cloud cover less than ) and PAR = 0.5 K↓ for cloudy days. These relationships were obtained by regression from sample measurements of PAR and K↓. Average PAR transmissivity, absorptivity, and reflectivity were evaluated as 0.42, 0.51, and 0.07.

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