How a crop model works: Drivers of growth, development and phenology

1. How a crop model works: Drivers of
growth, development and phenology
Mukhtar Ahmed
mukhtar.ahmed@slu.se
ahmadmukhtar@uaar.edu.pk
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2. Objective
Understanding about climatic variables impacts on growth,
development and phenology of crop.
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3. 3
Modeling
CropEnvironment
Soil
o Temperature
o Solar radiation
o Rainfall
o CO2
o Phenology
o Growth
o Development
o Yield
o Soil texture
o DUL
o DLL
o Nutrients dynamics

4. 4
CO2
Light Temperature Rainfall/Irrigation
Management
Carter 2013
Soil

5. Phenology
• Timing of visual events which changes from one stage to next
• Study of the timing of recurring biological events, the causes of their
timing with regard to biotic and abiotic forces, and the interrelation
among phases of the same or different species (Leith 1974).
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Zadoks Growth stages (Zadoks et al., 1974)

6. Growth and Development
Growth Development
Growth of crops, plants or
plant parts is defined as the
irreversible increase in size
Development is the continuous
change in plant form and function
with characteristic transition phases
It is primarily associated with
capture and allocation of
resources
Development is mostly related to
non-resource environmental cues
such as temperature, photoperiod
and light quality.
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Development is rate of progress through an organism life cycle
Ontogeny: Time course of development through phases of life cycle

7. Drivers of Phenology
• Temperature
• Thermal time calculation (ΔTT)
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Growing degree
days accumulation
phenological stages
Photoperiod
(fD)
Vernalization
(fv)
optimum T
requirements
Other environmental factors
Soil water stress(fw, pheno),
N stress (fN, pheno)
P stress (fP, pheno)

8. Cardinal Temperature
• Tbase: Base temperature below which development rate =0
• Topt1:1st Optimum temperature at which development rate is most
rapid
• Topt2:2nd Optimum temperature; highest temperature at which rate is
still at its maximum
• Tmax: Maximum temperature at which development rate=0
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9. 9
Tbase & Topt from field data
Plot reciprocal of days to anthesis vs temperature
Tb= The x-axis intercept
Topt= Temperature at which rate is maximum
(99%)

10. Non Linear function
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11. Crown Temperature
in response to air temperature (T) for different snow depth (Hsnow)
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Snow depth
Hsnow default
is set to zero
Source: Zheng B, Chenu K, Doherty A, Doherty
T, Chapman L (2014) The APSIM-Wheat
Module (7.5 R3008). In. APSRU Toowoomba,
Australia

12. CardinalTemperatures
Crown temperature and thermal time
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Source: Zheng B, Chenu K, Doherty A, Doherty
T, Chapman L (2014) The APSIM-Wheat
Module (7.5 R3008). In. APSRU Toowoomba,
Australia

13. Degree day Approach
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o If T > Tbase
o Degree day=Tavg-Tbase
o If T<Tbase
Degree day=0
o If T > Topt
Degree day=Topt-Tbase
o Tbase=8
o Topt=30
Avg. Temp Degree Day
7 0
15 7
30 22
40 22

14. Four methods used to calculate
thermal time (DTT and HTT)
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cumulative daily thermal time (DTT)
Where Tmax is the maximum
temperature, Tmin is the minimum
temperature, Tavg = (Tmax + Tmin)/2, Tb is
the base temperature, and Tu is the
upper threshold temperature.
Method 1:
Method 2:
Where Tm = min (Tmax, Tu), Tn = max (Tm,
Tb), and Tavg′ = (Tm + Tn)/2.
Tb is compared with Tu before the average temperature (Tavg′) is calculated. Tm and
Tn are adjusted if they are <Tb or >Tu. In this method, DTT is given by

15. Four methods used to calculate
thermal time (DTT and HTT)
Method 3:
Method 4:
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16. 16
Comparison of the four methods used to calculate thermal time (DTT and HTT)

17. Photoperiod impact on phenology
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Source: Zheng B, Chenu K, Doherty A, Doherty
T, Chapman L (2014) The APSIM-Wheat
Module (7.5 R3008). In. APSRU Toowoomba,
Australia

18. Photo Growing Degree Days (PGDD)
PGDD= WEDD x Photoperiod
https://www.frontiersin.org/articles/10.3389/fenvs.2017.00057/full
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19. Vernalisation impact on phenology
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Source: Zheng B, Chenu K, Doherty A, Doherty
T, Chapman L (2014) The APSIM-Wheat
Module (7.5 R3008). In. APSRU Toowoomba,
Australia

20. Devernalisation (ΔV d)
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Source: Zheng B, Chenu K, Doherty A, Doherty
T, Chapman L (2014) The APSIM-Wheat
Module (7.5 R3008). In. APSRU Toowoomba,
Australia

21. Cumulative or Total vernalisation (V )
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V= σ(∆𝑉 − ∆𝑉𝑑)
Vernalization factor (fv) is calculated just from Emergence to Floral initiation
𝑓𝑣 = 1 − 0.0054545𝑅 𝑣 + 0.0003 × (50 − 𝑉)
RV = Sensitivities to vernalisation, which is cultivar-specific and is specified by vern_sens (1.5)
Source: Zheng B, Chenu K, Doherty A, Doherty
T, Chapman L (2014) The APSIM-Wheat
Module (7.5 R3008). In. APSRU Toowoomba,
Australia

22. Photosynthesis
Potential Growth= f(cover, radiation, rue)
∆DM = RUE x radn x cover
RUE = f(water, N, P,T, CO2)
cover = 1 - e-k x LAI (Beer-Lambert law)
RUE – radiation use efficiency. (grams biomass / MJ)
K – extinction coefficient; crop specific, affected by row spacing.
Water limited growth = sw supply * TEC /VPD
TEC = Transpiration efficiency coefficient.
Actual growth = minimum of potential and water limited growth.
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23. Biomass accumulation (Photosynthesis)
• Daily biomass accumulation (ΔQ) = Radiation interception (ΔQr)
∆𝑄 𝑟 = 𝐼 × 𝑅𝑈𝐸 × 𝑓𝑑 × 𝑓𝑠 × 𝑓𝑐
Radiation interception is calculated from the leaf area index (LAI, m2 m−2) and
the extinction coefficient (k)
𝐼 = 𝐼 𝑜(1 − exp(−𝑘 × 𝐿𝐴𝐼 × 𝑓ℎ)/ 𝑓ℎ)
where I0 = Total radiation at the top of the canopy (MJ)
fh = Light interception modified to give hedge-row effect with skip row (fh=1)
𝐼 = 𝐼 𝑜(1 − exp(−𝑘 × 𝐿𝐴𝐼))
Extinction coefficient (k) varies with row spacing
𝑘 = ℎ 𝑒 𝑊𝑟
Wr = Row spacing which is specified by the user
he = Function of rowing spacing which is defined for both green leaf
and dead leaves by parameters x_row_spacing, y_extinct_coef
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24. Radiation use efficiency (RUE)
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Source: Zheng B, Chenu K, Doherty A, Doherty
T, Chapman L (2014) The APSIM-Wheat
Module (7.5 R3008). In. APSRU Toowoomba,
Australia

25. Biomass production
Partitioning to leaf, stem, ear/pod and root is stage dependant.
Grain filling takes priority during reproductive growth.
Sink size is determined by one of
Harvest Index,
Grain number/size
Grain cohorts (ears).
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26. Stress factor
Temperature factor fT, photo
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Source: Zheng B, Chenu K, Doherty A, Doherty
T, Chapman L (2014) The APSIM-Wheat
Module (7.5 R3008). In. APSRU Toowoomba,
Australia

27. CO2 factor
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Source: Zheng B, Chenu K, Doherty A, Doherty
T, Chapman L (2014) The APSIM-Wheat
Module (7.5 R3008). In. APSRU Toowoomba,
Australia

28. Thanks
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