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Garden with Insight v1.0 Help: Plant Growth - Potential Biomass Increase

Photosynthetically active radiation

Interception of solar radiation is estimated with a Beer's law equation (Monsi and Saeki, 1953) [Equation 255] where PAR is intercepted photosynthetic active radiation in MJ/m2, RA is solar radiation in MJ/m2, LAI is the leaf area index, and subscript i is the day of the year. The constant 0.5 is used to convert solar radiation to photosynthetically active radiation (Monteith, 1973). Experimental studies indicate that the extinction coefficient varies with foliage characteristics, sun angle, row spacing, row direction, and latitude (Thornley, 1976). The value used in EPIC (0.65) is representative of crops with narrow row spacings (Uchijima et al., 1968). A somewhat smaller value (0.4 - 0.6) might be appropriate for tropical areas in which average sun angle is higher and for wide row spacings (Begg et al., 1964. Bonhomme et al., 1982. Muchow et al., 1982).

Equation 255

PAR = 0.5 * RA * (1.0 - exp(-0.65 * LAI))
PAR = PhotoActiveRadiation_MJPm2
RA = radiationForDay_MJPm2
LAI = leafAreaIndex

Potential Increase

Using Monteith's approach (Monteith, 1977), potential increase in biomass for a day can be estimated with the equation [Equation 256] where deltaB(p) is the daily potential increase in biomass in t/ha, BE is the crop parameter for converting energy to biomass in kg/ha*MJ/m2.

Biomass energy conversion is affected by vapor pressure deficit (VPD) and by atmospheric CO2 level. The biomass conversion factor BE is adjusted using the equations of Stockle et al. (1992) [Equation 257] where CO2 is the atmospheric CO2 level in ppm and bc(1) and bc(2) are crop parameters.
(in other code)

The VPD correction is accomplished in the equation [Equation 258] where VPD is the vapor pressure deficit in kPa and bc(3) is a crop parameter. The value of VPD is e(a) (saturation vapor pressure at mean air temperature) minus e(d) (vapor pressure at mean air temperature). Finally, BE' is substituted into equation 256 to obtain the corrected biomass estimate.

Equation 258

deltaB(p) = 0.001 * BE' * PAR
BE* = 100 * CO2 / (CO2 + exp(bc(1) - bc(2) * CO2))
BE' = BE* - bc(3) * (VPD - 1.0), VPD > 0.5
deltaB(p) = PotentialIncreaseInBiomass_tPha
BE = biomassConvFactor_kgPhaTMJPm2
PAR = photoActiveRadiation_MJPm2
CO2 = carbonDioxideInAtmosphere_ppm
bc(1) = biomassToEnergyRatioCO2Params[0
bc(2) = biomassToEnergyRatioCO2Params[1
bc(3) = biomassToEnergyRatioVPDParam
VPD = vaporPressureDeficit_kPa

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Updated: May 4, 1998. Questions/comments on site to
Copyright © 1998 Paul D. Fernhout & Cynthia F. Kurtz.