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Garden with Insight v1.0 Help: Plant next day functions: calculate potential biomass increase (new growth)

Potential biomass increase by photosynthesis is calculated by considering solar radiation and the plant's conversion of it to sugars. We start with the solar radiation today, as simulated by the weather component.

The first step is a reduction of solar radiation for shading, which is not in the EPIC model but was added to simulate garden plots. The solar radiation is reduced proportionately by the shading index, which goes from zero (no shade) to one hundred (complete darkness).

Next the photoactive radiation, or radiation useful for photosynthesis, is calculated by reducing the shaded solar radiation by half to eliminate wavelengths that plants cannot use in photosynthesis. Plants use wavelengths of light in the range of 400-700 nanometers, which is about the same as our visual range. Interestingly enough, that range brackets the peak of the solar radiation spectrum (500 nm) and includes about half of the radiation coming from the sun.

Intercepted photoactive radiation, or radiation useful for photosynthesis that actually falls on plant leaves, is calculated with an exponential function based on the leaf area index. Light normally dissipates in an exponential pattern as you move down a canopy of plant leaves, and the rate of the exponential decline is modified by the light extinction coefficient of the canopy. This simulation uses a constant light extinction coefficient of 0.65 which is "representative of crops with narrow row spacings". The equation is 1.0 - exp(-0.65*LAI).

Now with a little hand waving, the amount of photosynthate (sugars) produced is calculated from the intercepted photoactive radiation and an energy-biomass conversion factor. The conversion factor represents a crude approximation of the elegant mechanism of photosynthesis, and it is crucial because it controls to a large extent how fast the plant grows. The conversion factor is modified by carbon dioxide and vapor pressure.

The amount of carbon dioxide in the atmosphere affects the energy- biomass conversion factor as determined by an S curve parameter. Generally more CO2 is better up to a point where the effect starts to level off. You may think this is strange, because if the amount of carbon dioxide in our atmosphere is going up, as everyone says, and that's supposed to be a bad thing, how can it be bad when the plants will grow faster? If increased plant growth were the only effect, it might be fine. But CO2 in the atmosphere absorbs infrared light, and this causes the earth to retain more heat, and this causes global temperatures to increase (warming), and this can cause all sorts of havoc to ice caps and rainfall and temperature patterns and so on. However, it is true that increased CO2 does improve plant growth, and it is regularly used in greenhouses to improve yield.

The second thing that influences the energy-biomass conversion factor is the vapor pressure deficit. The vapor pressure deficit is the saturation vapor pressure minus the actual vapor pressure, so it is the amount of water vapor the air could still hold before becoming saturated. You could say that the vapor pressure deficit is an index of the drying power of the air, or the power of the air to suck water out of something that has water in it. As the vapor pressure deficit increases and the air becomes drier, the plant begins to close the stomata (little pores) in its leaves to keep from losing too much water. However, closing the pores also reduces gas exchange between the inside and outside of the leaf, which reduces the photosynthetic rate because photosynthesis requires a constant supply of CO2 from the air.

It is important to understand that the calculation of photosynthesis here is for net photosynthesis, that is, after the energy used in respiration has been subtracted. We assume that the amount of energy expended in respiration is directly proportional to the amount of energy converted in photosynthesis, and so respiration can be included simply by reducing the energy-biomass conversion factor. Most calculations of photosynthesis use this simplification. That does not necessarily mean it is perfectly correct to do so, but it does mean the impact of simulating respiration separately is probably small.

Note that there is no provision here for differential shading of plants. If you plant a simulated corn plant right next to a radish plant, they will both use the same light extinction coefficient and so will act as if they were both shading each other. A more complete simulation of shading would model the falling of light rays from the sun (in relation to the orientation of the soil patch) and the shading of some plants by others, taking into account the position, height, width, and leaf area of the different plants.

calculation of solar radiation
EPIC Potential Biomass Increase
Model contents

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Updated: March 10, 1999. Questions/comments on site to
Copyright © 1998, 1999 Paul D. Fernhout & Cynthia F. Kurtz.