A Generalized Relationship between Photosynthetically Active Radiation and Solar Radiation1

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Abstract
Photosynthetically active radiation (PAR) is a necessary input in several crop growth models. Previous research from several locations with different instruments has suggested that PAR could be estimated as a constant fraction of shortwave radiation. This study was conducted to determine if a simple relationship was valid for a large geographic area in the western half of the USA. In the semiarid climate near Fresno, CA, the daily photosynthetic photon flux density (PPFD) in units of μmol m−2 was 2.04 ± 0.06 times the solar irradiance (SI). The daily irradiance within the PAR waveband (photosynthetic irradiance (PI), 0.4 to 0.7 μm) was estimated to be 45% of the daily solar irradiance. The diurnal pattern of these relationships was consistent with measurements at Phoenix, AZ and exhibited only a slight diurnal variation. Independent measurements of the solar irradiance between 0.285 and 0.63 μm with “filtered” pyranometers at the Fresno site indicated that irradiance between 0.285 and 0.63 μm was 41% of solar irradiance, a result which was about 7% lower than the estimate from the PAR/SI value converted with a published sun and sky PAR/PI factor. This difference was largely attributed to the differences in measured wavebands. Measurements of solar irradiance less than the waveband of 0.63 μm and total solar irradiance obtained with the same equipment at Brawley, CA; Weslaco, TX; Temple, TX; Manhattan, KS; Lincoln, NE; St. Paul, MN; Fargo, ND; Sidney, MT; Beartooth Pass, WY; Kimberly, ID; Davis, CA were similar to those measured at Fresno, CA. The results indicate that PAR can be estimated from solar irradiance measurements to within 10% (which is probably acceptable for most plant growth models) throughout most of the Western USA.