Crop development in CropSyst is based on thermal time accumulation. The thermal time accumulation is modulated by photoperiod, vernalization requirements, and water stress as explained below. The thermal time (sum of growing degree days) required for each developmental phase must be specified. Degree days are accumulated from planting.
Depending on crop type, certain growth stages are relevant: emergence, tuber initiation, beginning of flowering, end of leaf expansion in determinate crops, end of flowering, beginning of grain filling, and physiological maturity. CropSyst represents a crop as “big leaf” (and a “big root”) and individual plants or plant cohorts are not simulated separately.
Water stress tends to increase the crop canopy temperature, which may accelerate the accumulation of degree-days in most crops. The parameter "Phenological sensitivity to water stress", which ranges from 0 to 1, permits to determine the degree of the effect of water stress.
For some crops like sorghum, however, phenology may actually be delayed by water stress
(negative adjustment factor). This is likely a quiescence mechanism reminiscent of the response of some perennial grasses.
Growing Degree-Day Accumulation
In CropSyst, growing degree days are accumulated either daily or hourly, as selected by the user.

Daily Accumulation
Daily thermal time accumulation is computed with the following equation:
GD
day
= T ave
- T base
CGD day
= CGD day-1
+ GD day
if T avg
< T
GDdaybase
T avg
=



T base
T cutoff
(T' max
+ T min
) / 2
if T avg
> T cutoff
otherwise
GD day is today's thermal time.
CGD day is today's accumulated thermal time since planting.
T base
T cutoff are crop input parameters that define the range of temperatures for viable development.
T min is the daily minimum air temperature.
T' max is the daily maximum air temperature (maybe adjusted by crop water stress as discussed below)
Hourly Accumulation

Hourly calculation of degree days requires three parameters:
T base
= Base temperature (no accumulation when T hour
<= T base
)
T opt
= Optimum (maximum accumulation when T hour
= T opt
)
T max
= Maximum temperature (no accumulation for T hour
>= T max
)
T hour
is hourly temperature. The following is an example of hourly degree-day accumulation for a given T hour
, where
T base
= 3 o
C, T opt
= 25 o
C, and T max
= 35 o
C.
15
10
5
25
20
T base
T opt
= 3 C
= 25 C
T max
= 35 C
0
0 5 10 15 20 25 30 35 40
Crop water stress adjustment
To account for possible effect of water stress on canopy temperature, a correction is implemented by adding temperature to the T max
input according to the following equation:
T' max
= T max
+ ( 1 + 1.5 · VPD max
)· StressIndex water
· PSWS where
T' max
T max
(°C)
(°C) is the adjusted daily maximum air temperature. is the input daily maximum air temperature.
StressIndex water
(0-1) ranging from 0 (no stress) to 1 (maximum stress) is the daily crop water stress index
VPD max
(kPa) is the maximum vapor pressure deficit of the day

PSWS is the phenologic sensitivity to water stress crop input parameter. For crops whose development is not expected to be affected by water stress, this correction can be deselected by the user (PSWS = 0). For some crops, water stress may actually delay development (PSWS < 0).
Photoperiod
Floral induction in many plants is sensitive to daylenght (nicto-period actually). In the so-called long-day plant the rate of development accelerates as days become longer within a photoperiod length bracketed by the threshold and critical photoperiod. Conversely, short-day plants accelerate the development rate when the daylength shortens. There is a mixture of qualitative
(i.e. induced or not induced) and quantitative responses to daylenght. Other crops are insensitive to day length. Operationally, the photoperiod affects the thermal time requirements of the preflowering phase. Therefore, the input parameters to calculate photo-period effects on thermal time accumulation have different definitions.
In the model the user selects whether there is to be consideration of photo-period for either shortday or long-day crops. The meaning of the parameters is as follows:
Day length that stops development toward flowering (hours) : For short-day crops, this is the day length above which development towards flowering stops. For long-day crops, this is the day length below which development towards flowering stops, and is referred to as day length to inhibit flowering (daylength if
) in the equations below.
Day length that does not constrain development toward flowering (hours) : For short-day crops, this is the day length below which development towards flowering is not constrained. For longday crops, this is the day length above which development towards flowering is not constrained.
Referred as “day length for insensitivity” (daylength ins
) in the equations below.
The response to photo-period is approximated with a simple linear relation, where the photoperiod factor (f
photo
) is restricted to values from 0 to 1. On a daily basis, this factor multiplies the thermal time accumulation for the day to obtain the actual accumulation.

For long-day crops: daylength - daylength if f
photo
= daylength ins
- daylength if
For short-day crops: daylength if
- daylength f photo
= daylength if
- daylength ins
The weakness of this approach is that it is plausible that the time at which the photoperiod becomes irrelevant at controlling the progress towards flowering is ambiguously defined. This model implicitly assumes that temperature and photoperiod act in a multiplicative form.
Some suggestion for improvement:
- define more accurately the period in which a crop is sensitive to the day length
- test this approach with a broad data set, this is very difficult to do without a broad database
- study the effect of the daily thermal amplitude; there are some indications that the magnitude of the temperature swings and not just the average are important at determining the developmental rate