Photoperiod
The Photoperiod function calculates the duration of daylight (photoperiod) in hours for a given day and location. It is essential for simulating plant development processes that depend on day length within the APSIM NG framework.
Overview
The Photoperiod function determines the number of daylight hours based on the latitude (from the weather file) and a specified twilight sun angle threshold. This function is crucial for models where photoperiod sensitivity affects growth, flowering, or other physiological processes.
Inputs
| Input | Type | Description |
|---|---|---|
Twilight |
double | The sun angle (in degrees) below the horizon to define the start/end of daylight. |
MetData |
IWeather | Weather data provider, supplies latitude and other meteorological information. |
Clock |
IClock | Provides the current day of year for the calculation. |
Methodology
The function calculates day length using the latitude and the twilight angle. The calculation is performed according to the weather data providers.
Controlled Envronment
Use the specified day length from the controlled environment.
Specified Day Length in the Weather File
Use the actual day length from the weather file.
Calculated Day Length
The day length is calculated using the following astronomical formula, which estimates the duration of daylight (in hours) for a given day of year, latitude, and twilight sun angle:
Let: - \(D\) = day of year - \(\phi\) = latitude (degrees) - \(\theta\) = twilight sun angle (degrees below the horizon; e.g., \(-6\) for civil twilight)
- Convert angles to radians:
- \(\phi_{rad} = \phi \times \frac{\pi}{180}\)
- \(\theta_{rad} = \theta \times \frac{\pi}{180}\)
- Calculate solar declination:
- \(\delta = 0.4093 \times \sin\left(0.0172 \times (D - 79.25)\right)\)
- Compute intermediate values:
- \(a = \sin(\phi_{rad}) \times \sin(\delta)\)
- \(b = \cos(\phi_{rad}) \times \cos(\delta)\)
- \(x = \frac{\sin(\theta_{rad}) - a}{b}\)
- Clamp \(x\) to the range \([-1, 1]\) to avoid domain errors.
- Calculate day length (hours):
\[ \text{DayLength} = 2 \times \arccos(x) \times \frac{24}{2\pi} \]
This formula accounts for the Earth’s tilt and the observer’s latitude, providing an accurate estimate of daylight hours for any day and location.
Events
Events Listened For
These are signals or notifications that the function listens for from other parts of the simulation:
| Event | Purpose |
|---|---|
| DoWeather | Triggers calculation of day length for the current day. |
Events Raised to
No events are raised by this function.
Properties
Configurable and Reportable Properties
| Property | Type | Description |
|---|---|---|
Twilight |
double | The sun angle (in degrees) below the horizon for defining daylight. |
Read-Only Reportable Properties
| Property | Type | Description |
|---|---|---|
DayLength |
double | The calculated duration of daylight (hours) for the current day and location. |
User Interface
PhotoperiodFunction can be added as a child of a parent function node (e.g., a model requiring day length) in the model tree. Right-click the parent node, select “Add Model…”, and search for PhotoperiodFunction in the Filter Box.
Practical Example
No practical example is available for this function.