The effect of estimated PAR uncertainties on the physiological processes of biosphere models |
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Authors: | Jaeil Cho Taikan Oki Shinjiro Kanae |
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Institution: | a Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan b Department of Mechanical and Environmental Informatics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan c National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba 305-8604, Japan |
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Abstract: | 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. |
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Keywords: | Photosynthetically active radiation (PAR) Solar radiation Biosphere model Stomatal conductance |
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