Garden with Insight v1.0 Help: Plant params photosynthesis group
Photosynthate per unit of radiation: A crucial parameter that translates the amount of intercepted solar radiation into the
amount of photosynthate (sugars) produced by photosynthesis (minus
respiration). Determines daily potential biomass increase before growth constraints are considered.
WA - Biomass-Energy Ratio
Photosynthate per unit of radiation at elevated CO2 levels: The amount of
photosynthate (sugars) produced by photosynthesis with a unit amount of radiation when CO2 levels are
elevated to 660 ppm (normal levels are 330 ppm). Used to create an S
curve that alters photosynthetic rates based on atmospheric CO2.
This is the potential (unstressed) growth rate (including roots) per unit
of intercepted photosynthetically active radiation. This parameter should be one of the last to be adjusted.
Adjustments should be based on research results. This parameter can greatly change the rate of growth,
incidence of stress during the season and the resultant yield. Care should be taken to make adjustments in
the parameter only based on data with no drought, nutrient or temperature stress.
WAC2 - CO2 Concentration in future atmosphere/Resulting WA value (Split Var)
Parameter relating biomass-energy ratio to
VPD: Reduces the photosynthetic efficiency at high levels of vapor pressure (close to the saturation vapor pressure). Normal values are
from six to eight.
In EPIC, radiation use efficiency is sensitive to atmospheric CO2 concentration. WAC2 is an
"S" curve parameter used to describe the effect of CO2 concentration on the crop parameter
WA. The value on the left of the decimal is a value of CO2 concentration higher that ambient (i.e., 450 or
660 ul/l). The value on the right of the decimal is the corresponding value WA. This elevated value of
WA can be estimated from experimental data on short-term crop growth at elevated CO2 levels. Calculate
the ration of crop growth rate at elevated CO2 to crop growth at approximately 330 ul l-1 CO2. Multiply
that ration by the value of WA at 330 ul l-1 to obtain the value on the right of the decimal. Typical values
of the ratio are 1.1 to 1.2 for crops with the C4 photosynthetic pathway and 1.3 to 1.4 for C3 crops.
(Kimball, B.A. 1983 Carbon dioxide and agricultural yield: an assemblage and analysis of 770 prior
observations. Water Conservation Laboratory Report 14. USDA/ARS. Phoenix, Arizona). [Note that we
have fixed the upper level of CO2 concentration at 660 ppm.]
WAVP - Parm relating vapor pressure defficit to WA
curve - effect of CO2 concentration on biomass-to-energy ratio / 100: Determined by
two parameters: photosynthate per unit of radiation at normal and elevated CO2 levels. For plants with C4
photosynthesis, photosynthate at elevated CO2 is about 110-120% of that at normal levels. For C3 plants
the ratio is about 130-140%.
In EPIC, radiation use efficiency (RUE) is sensitive to vapor pressure deficit (VPD). As VPD increases,
RUE decreases. The crop parameter WAVP is the rate of the decline in RUE per unit increase in VPD.
The value of WAVP varies among species, but a value of 6 to 8 is suggested as an appoximation for most