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  

 
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) 

 
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  

 
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 
crops. 

 

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