Measurements of photosynthetically active radiation (PAR) and global solar irradiance (Rs) were made at a LBA (The Large Scale Biosphere‐Atmosphere Experiment in Amazonia) experimental site, at Fazenda Nossa Senhora (10°45′ S; 62°21′ W), in Rondonia, in the years of 2004 and 2005, with the objective of estimating the seasonal variation of the ratio between the photosynthetically active radiation and the global solar irradiance. The relationship between PAR and Rs were made by using linear regressions equations with data from year 2004 and tested with data from the year 2005. The seasonal variation of the ratio PAR/Rs ranged from 0.43 (September) to 0.48 (January). The linear regression equations between PAR and Rs obtained were: a) On an hourly basis: PAR = 0.747+0.478*Rs, (R2 = 0.99; wet season) and PAR = −4.578+0.452*Rs (R2 = 0.99; dry season); b) On a daily basis: PAR = 4.956+0.466*Rs (R2 = 0.99; wet season) and PAR = −6.762+0.457*Rs (R2 = 0.96; dry season).

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