The effect of estimated PAR uncertainties on the physiological processes of biosphere models

Jaeil Cho, Taikan Oki, Pat J.F. Yeh, Shinjiro Kanae, Wonsik Kim

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11 Citations (Scopus)


Photosynthetically active radiation (PAR) energy reaching on the vegetated surface is a key determinant of plant physiological processes. Most of biosphere or crop models use the ratio of PAR to incoming solar radiation (Rs), PAR/Rs, to convert Rs into PAR in order to reduce weather data-input requirements. Several existing models simply specify a constant ratio, PAR/Rs = 0.5. However, some field experiments have reported that the ratio PAR/Rs may not be constant. Previous empirical equations of PAR/Rs were derived based on the data of monthly or daily timescales collected from only a few measurement sites, hence they may not be appropriate to be used in current global biosphere models usually with hourly simulation time steps. Here, we represent the exponential correlation between PAR/Rs and sky clearness index (0-1) using hourly data from 54 Ameriflux measurement sites. It is found that PAR/Rs increases up to 0.6 in cloudy conditions when the clearness index (CI) is below ~0.2, whereas it is nearly constant at ~0.42 when CI is above 0.2. When the identified empirical equation is used in the model simulation, it results in -4 to 2% difference in the stomatal conductance compared to that using the constant ratio PAR/Rs = 0.5.

Original languageEnglish
Pages (from-to)1575-1579
Number of pages5
JournalEcological Modelling
Issue number12
Publication statusPublished - 24 Jun 2010
Externally publishedYes


  • Biosphere model
  • Photosynthetically active radiation (PAR)
  • Solar radiation
  • Stomatal conductance

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