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Photosynthetically active radiation

Sun, 11/09/2003 - 2:17pm — jrbloom
Generally, solar radiation is converted to "photosynthetically active radiation (PAR)" for use in algal growth calculations by multiplying the solar radiation by a factor in the range 0.4 to 0.5. Solar radiation is used in the model to calculate F(I) = I/Is exp(-I/Is + 1) where I = solar radiation Is = light saturation intensity (solar rad which provides maximum algal growth) Is is represented by the variable ASAT in W2 If Is is as PAR, then Is is generally in the range 100-400 Ly/day. In terms of units W/m^2 this would equal about 50 -200 W/m^2 (1 W/m^2 = 2.065 Ly/day) However, I can't find anyplace in the code where a conversion is performed from solar radiation to PAR. If this conversion is not done by the code, then the Is to input to the model as ASAT should be in the range of 100 - 400 W/m^2 for a PAR factor of 0.5 or 125 - 500 W/m^2 for a PAR factor of 0.4. Note that the "default" value for ASAT is 75 W/m^2. This would appear to be outside of the range of appropriate values if no conversion from solar radiation to PAR is performed by the model. Can anyone tell me whether PAR is explicitly calculated in the code and what the appropriate range for ASAT should be. Thanks Jim Bloom, Oregon DEQ bloom.james@deq.state.or.us [addsig]
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Water Quality
Wed, 11/12/2003 - 7:54am — Tom

Jim, Good question. The formulations are not based on PAR, as you determined. The default value of 75 W/m^2 is indeed somewhat low, but was arrived at through a large number of simulations that resulted in relatively accurate algal biomass predictions. The primary reason why I chose this number is that it allowed amazingly accurate predictions of subsurface chl a maximums, where the value for ASAT is very critical. This is not to say that the default value could be not be larger, but if I were to recommend a starting value, this would be the one based on past experience. Looking at Table 6-8 in the EPA manual, the range would appear to be from 44 to about 350 (the 600 value of Canale's for cyanobacteria appears to be an anomaly), using formulations not based on PAR. Keep in mind that these are values used in modeling studies rather than reported literature values. Also, the recommended value in the W2 manual was based on using only a single algal group to represent phytoplankon dynamics during the entire spring/summer/fall period. With any number of algal groups now capable of being simulated, I need to rethink the recommended value(s), but I won't do that until I get more multiple algal group simulations under my belt and get a feel for good default values. Diatoms would probably have a much lower value than cyanobacteria, with greens somewhere intermediate. In the future, I need to reformulate the algal kinetics to get more "up to date with current thinking", which would include variable stoichiometry, variable C/chl a ratios, PAR based light formulation, light history, etc., but the model has done such a good job with the current formulation that I have been reluctant to monkey around with the formulations, which would require extensive testing using existing applications to determine if the added complexity actually provided better simulation capabilities. The manual's comments regarding complexity using variable stoichiometry are still valid nearly 20 years later (see page 338). Although I am a firm believer that we should model the real world as it really behaves, additional complexity does not always allow for more accurate model results.[addsig]
Thu, 11/13/2003 - 10:41am — jrbloom

Thanks Tom! Appreciate the help Jim[addsig]