Above ground biomass accumulation

Crop growth occurs during active growth until maturity, (but not after maturity is achieved) and only when potential transpiration has been determined by the evapotranspiration model.

Above ground crop growth is represented in terms of above ground biomass accumulation. Above ground biomass production is dependent on: intercepted radiation (radiation dependent), transpiration (water-dependent), and plant nitrogen uptake (nitrogen-dependent). Each of these factors is capable of limiting growth.

Radiation dependent growth

Temperature limited radiation dependent biomass production (GRTlim) ((kg/m)/day) is calculated as:

GRTlim = GR · Tlim
where

Tlim
is a temperature limitation factor.

Temperature limitation is applied if the current thermal time accumulation is less than a user selected threshold (thermal time at which temperature limitation ceases crop input parameter . (If Tlim < 0.01 it is assumed to be the most significant factor (given a value of essentially 0.0)).

Tlim =

1.0 if Tavg > Topt
0.0 if Tavg < Tbase
(Tavg - Tbase)
(Topt - Tbase)
otherwise

where

GDday (growing degree days)
is thermal time determined for the current day.

Tavg (C)
is the daily average air temperature.

Tbase (C)
is the crop input parameter: thermal time base temperature .

Topt (C)
is the optimal temperature for growth crop input parameter.

It is assumed that temperature effects, after cumulative degree-days is larger than the threshold, are accounted for in the radiation conversion factor LtBC, which is empirically derived, usually using observations during the linear phase of biomass accumulation when temperatures are warmer.

GR ((kg/m)/day)
is radiation dependent growth. CropSyst (as of version 4.11) provides two options for determining the intercepted radiation growth in the temperature corrected intercepted radiation dependent term of biomass production (GR):

Photosynthetically Active Radiation (PAR) method

GR = LtBC · intercepted_radiation
intercepted_radiation = PAR · FCCgreen
where

LtBC (kg/MJ) (converted from g/MJ)
is a coefficient representing the conversion of photosynthetically active radiation (PAR) to above ground biomass input parameter. (as of version 4.11, the user has the option to provide two values for this coefficient depending on the growth stage)

PAR ((MJ/m)/day)
Photosynthetically active radiation (Rad / 2) where It is assumed that PAR corresponds to approximately half of the total solar irradiance.

Rad ((MJ/m)/day)
is the total daily solar irradiance above the crop canopy. Rad is preferably a measured value provided in the weather, otherwise it is estimated.
FCCgreen (0-1)
is the fraction of incident PAR intercepted by the green canopy.

FCCgreen = (1.0 - emin(-kc · 1.4 , 0.9) · GAI · clumping)
clumping = (clump_factor + (1.0 - clump_factor) · ( 1 - e(-area_factor · GAI) )))

clump_factor = 0.75
area_factor = 0.25 (for photosynthetically active radiation)

Total Radiation (TR) method

GR = RUEmax · intercepted_radiation
intercepted_radiation = Rad · FCCgreen
where

RUEmax(kg/MJ) (converted from g/MJ)
Is the maximum radiation use efficiency crop parameter.
Rad ((MJ/m)/day)
is the total daily solar irradiance above the crop (see above)
FCCgreen (0-1)
is the fraction of incident intercepted by the green canopy. This is calculated as shown above but this is not PAR based so
area_factor = 0.35 (for total radiation)

Water dependent growth

As of version 4.11, the CropSyst crop submodel now has the option of using one of three equations (methods) to calculate Water Use Efficiency for crop growth:
  • Tanner & Sinclair (the original method used in all previous versions )
  • Water Productivity (as in AquaCrop)
  • Transpiration Use Efficiency (regression based on empirical data (has additional scaling coefficient parameter)).

The following text only describes the Tanner & Sinclair method.

Crop transpiration dependent biomass production (GTr) (kg/m)/day is calculated using ( Tanner and Sinclair, 1983 ):

GTr = Tract · BTR/VPD

where

BTR ((kg/m · kPa)/m)
is the above ground biomass-transpiration coefficient crop parameter.

Tract (m)
is the actual transpiration .

VPD (kPa)
is the daily mean vapor pressure deficit

Nitrogen dependent growth

Nitrogen dependent growth will be applied if nitrogen simulation is enabled.