Uncertainties in Photosynthetically Active Radiation (PAR) Experimental Data Collected in Brazil and Spain
Abstract
Photosynthetically Active Radiation (PAR) plays a critical role in evaluating agricultural productivity and ecosystem dynamics. Despite its importance, ground-based radiometric measurements of PAR are limited and often exhibit large uncertainties due to sensor degradation and calibration issues. Consequently, PAR is frequently estimated as a fixed fraction (typically 42–52%) of global horizontal irradiance (GHI), both in ground-based and satellite-derived datasets. This approach may introduce bias besides omitting the spatial and temporal variability of PAR fractions. This study is part of a collaborative effort between Brazil and Spain to investigate uncertainties in PAR measurements and satellite products. The methods depart from field campaigns by comparing long-term radiometric stations to a more accurate and calibrated rotating shadow-band spectroradiometer. Three field campaigns were accomplished at Spain while Brazilian field campaigns will take place during 2025/2026 season. Preliminary results points that conventional PAR sensors induced bias in PAR/GHI fraction varying from +6% to -12% for a monthly average. Temporal variability showed minimal differences. The variability of PAR fraction on the direct and diffuse components was also assessed showing values for diffuse spectra around 0.54 while 0.43 for direct. However, variability of PAR fraction for diffuse is much larger ranging from 0.3 to 0.8 for the 5th to 95th percentiles. These results emphasize the need of a cautious verification on PAR sensor biases prior to scientific use, as well provide more details on PAR variability for direct and diffuse components.
Keywords
photosynthetically active radiation, radiometry, instruments