1. Modelling Crop Development and Growth in CropSyst
Marcello Donatelli
CRA-ISCI, Italy
Claudio Stockle
BSE, Washington State University, USA

2. A Generic Crop Simulator
CropSyst simulates crop development and growth using a generic crop simulator
Species and cultivars are characterized by a set of parameters which determine crop response to the environment
Crop parameters can also switch on / off specific modules of the crop simulator (e.g. vernalization, photoperiod)

3. Crop Development
Crop development is the progression of a crop through phenological stages.
The proper simulation of crop development is a key factor as it determines the length of time when the crop accesses resources such us light, heat, water etc.
Also, the proper simulation of crop phenological stages is important as it allows matching specific physiological conditions of a crop to specific environmental conditions.

4. Crop Development in CropSyst
growing degree
days accumulation
phenological stages,
leaf area duration
photoperiod
vernalization
optimum T
requirements
water stress

5. Light Interception by the Crop
The estimate of the amount of incident radiation intercepted by the crop is a key process because:
It determines the amount of PAR which is converted in biomass
it allows partitioning the potential evapotranspiration into potential crop transpiration and potential soil evaporation

6. Modelling Light Interception
For daily time step models, integration using three depths in the canopy and three times throughout the day provides useful approximation of canopy photosynthesis
Models with daily time step, and which calculate photosynthesis from transpiration, need only a simple model of radiation interception

7. Light Interception by the crop
LAI
RADabsorb
RADabovec
The shape of
the curve is
given by the
extinction
coefficient of
the crop K 1
RADabsorb
RADabovec
= 1- e (-K LAI)

8. Leaves not Randomly Distributed
When leaves are clumped (not randomly distributed), canopy transmission is still approximated by an exponential function, but LAI is multiplied by a “clumping factor”
For row crops with spaces between the rows, the effective LAI is lower than actual LAI (more light transmitted to the soil)

9. Crop Growth vs. Development
Crop growth in simulation modeling usually refers to the accumulation of biomass with time and its partitioning among different organs
This is different to crop development, which corresponds to the time sequence of stages or events in the life cycle of a crop (e.g., the progress of a crop from emergence to flowering or from the n to the n+1 leaf stage)

10. Modelling Crop Growth
One approach to crop growth modeling is to determine canopy photosynthesis, as previously discussed, and distribute the carbon fixed among the different crop organs according to some partitioning scheme
Maintenance and growth respiration must be subtracted, unless photosynthesis is estimated as net photosynthesis

11. Simple Modelling of Crop Growth
Simple approaches to model daily biomass production are available, which do not deal explicitly with photosynthesis and respiration.

12. Biomass production as a function of transpiration
KBT T
GTR =
VPD
GTR = Water-limited growth (kg m-2 d-1)
KBT = Biomass-Transpiration Coefficient (kPa)
T = Transpiration (kg m-2 d-1)
VPD = Vapor Pressure Deficit (kPa)

13. Biomass production as a function of transpiration: Assumptions
Given crop species and cultivars have a characteristic chemical composition and growth respiration requirements.
Maintenance and growth respiration losses are accounted for in the experimental determination of the constant KBT .
The difference between leaf and atmospheric vapour density can be approximated by the atmospheric deficit. This is usually expressed as the atmospheric vapor pressure deficit (VPD).

14. Limit of the Transpiration to Biomass Conversion
Water-limited growth calculations based on the previous equation are not suitable at low VPD values.
A second estimate of growth must be obtained based on the direct conversion of intercepted radiation to biomass.

15. Tanner & Sinclair
FAO AquaCrop
Transpiration-Use Efficiency Curve

18. Biomass production as a function of intercepted PAR
GIPAR = e fint PAR
GIPAR = Radiation-limited growth (kg m-2 d-1)
e = Radiation conversion efficiency (kg MJ-1)
fint = fraction of radiation intercepted
PAR = photosinthetically active radiation (MJ m-2 d-1)

19. Biomass production as a function of intercepted PAR: Assumptions
The radiation conversion efficiency must be determined for a crop well supplied with water and nutrients.
For comparison with water-limited growth, the radiation conversion efficiency must correspond to conditions with low vapor pressure deficit

20. Water and Radiation Limited Growth
GTR = KBT TR
VPD
GTR
TR/VPD
KBT
potential
growth limited
by water and
radiation
B = Min ( GTR ,GIPAR )
IPAR e
GIPAR
GIPAR = e IPAR

21. Growth Limited by Nitrogen
B = growth limited by radiation and water
Npcrit = critical plant N concentration
Np = plant N concentration
Npmin = minimum plant N concentration

22. Modelling of Crop Growth
Biomass
LAI B p
SLA
SLA = specific
leaf area
p = partitioning
coefficient
LAI
LAI = SLA
Bcum
1 + pBcum
root depth
RD = Rdmaxf(LAI, LAImax)

23. The Specific Leaf Area
It is the amount of leaf area produced per unit of carbon (biomass) partitioned to leaves.
The parameter value must be the average value for the species grown in optimum conditions.
The implementation of the effect of water stress on the reduction of the specific leaf area is on going, as a physiological mechanism that the crop uses to limit water loss in arid conditions.

24. Leaf Area Duration
Leaf area duration is set by the relevant parameter in GDD: the daily new growth starts accumulating GDD the day of formation.
Water stress accelerates GDD accumulation, thus causing premature leaves senescence.

25. Root Density
The generic crop simulator attributes a linearly to exponentially decreasing relative root density from the bottom of the soil evaporative layer down to the maximum root depth.
The relative root density sets the demand for water uptake in each soil layer, given that the total crop demand (potential transpiration) is partitioned into amounts for each layer on the basis of root relative density.