Light interception​#

Light-interception models estimate how much incoming radiation a leaf or canopy absorbs. Photosynthesis uses the photosynthetically active part of the spectrum (PAR); leaf energy balance also needs near-infrared radiation (NIR).

Choose a model​#

For a leaf with known incoming light, use ConstantAbsorption to absorb a fixed fraction. For a canopy, use a Beer-Lambert model to calculate absorption from leaf area index (LAI, leaf area divided by ground area).

The Beer-Lambert coefficients describe light extinction in the canopy. For each band, the absorbed fraction is 1 - exp(-k * LAI). The shorter form BeerShortwave(k_PAR) uses k_NIR = 0.48. That number is an extinction coefficient, not a PAR-to-shortwave conversion factor.

Another model is ArchimedLight, which calculates 3D light interception in a 3D canopy. This model is in a separate package external to PlantBiophysics, but is still fully compatible with PlantBiophysics's models.

Absorb a fixed fraction at the leaf​#

ConstantAbsorption(α_PAR=0.8, α_NIR=0.2) absorbs 80% of incident PAR and 20% of incident NIR. These illustrative fractions are supplied explicitly; they must lie between 0 and 1. The equations are:

Absorbed photon flux aPPFD is Ra_PAR_f * constants.J_to_umol. All radiation uses the same represented surface area, so the model couples directly to leaf photosynthesis and energy balance. Add it to the simple CompositeModel constructor, as in the several-timestep tutorial.

For controlled lighting, incident light is often specified as photon flux. Here we simulate a leaf under 1000 µmol photons m⁻² s⁻¹, assuming no incident NIR and an absorbed PAR fraction of 0.8:

using PlantBiophysics, PlantSimEngine, PlantMeteo, Dates

constants = Constants()
incident_ppfd = 1000.0
weather = Atmosphere(
    T=25.0, Wind=1.0, P=101.3, Rh=0.6, duration=Minute(1),
    Ri_PAR_f=incident_ppfd / constants.J_to_umol,
    Ri_NIR_f=0.0,
)
scene = CompositeModel(
    ConstantAbsorption(α_PAR=0.8, α_NIR=0.2);
    environment=weather,
)
run!(scene; constants=constants)
leaf = only(model_objects(scene))
(aPPFD=leaf.status.aPPFD, Ra_SW_f=leaf.status.Ra_SW_f)

The absorbed photon flux is 800 µmol photons m⁻² s⁻¹. To reproduce a specific chamber experiment, choose the absorption fractions and J_to_umol for the leaf and lamp spectrum; see LI-COR's light settings. Imported aPPFD values from the LI-COR readers are already absorbed photon fluxes: use them directly rather than applying absorption a second time.

The model has no imposed timestep or object scale. At any timestep duration, it returns fluxes for the current incoming light, without multiplying by duration. At other spatial scales, incoming and absorbed radiation must still refer to the same surface area. It does not calculate shading or convert ground-area radiation to leaf-area radiation.

Run a canopy light simulation​#

Here we use LAI = 2.0, k_PAR = 0.6, and the default NIR coefficient. The incoming PAR and NIR are in W m⁻² of ground area.

using PlantBiophysics, PlantSimEngine, PlantMeteo, Dates

meteo = Atmosphere(
    T=20.0, Wind=1.0, P=101.3, Rh=0.65,
    Ri_PAR_f=300.0, Ri_NIR_f=350.0, duration=Hour(1),
)

scene = CompositeModel(
    BeerShortwave(0.6);
    status=Status(LAI=2.0),
    environment=meteo,
)

run!(scene)
canopy = only(model_objects(scene))
(aPPFD=canopy.status.aPPFD, Ra_SW_f=canopy.status.Ra_SW_f)

CompositeModel applies the light model to one object, representing the canopy. Its status supplies the leaf area index and stores the calculated radiation.

The output aPPFD is in µmol photons m⁻² ground s⁻¹, and Ra_SW_f is in W m⁻² ground. BeerShortwave also provides the absorbed PAR and NIR separately as Ra_PAR_f and Ra_NIR_f:

(Ra_PAR_f=canopy.status.Ra_PAR_f, Ra_NIR_f=canopy.status.Ra_NIR_f)

These are rates at the simulated conditions. Use outputs=:all when running several timesteps to keep a history, as in the several-timestep tutorial.

Use the light in a leaf simulation​#

A canopy output is per unit ground area, while leaf photosynthesis and energy balance need radiation per unit leaf area. To obtain the canopy mean per leaf area, use GroundToMeanLeafPPFD for photons and GroundToMeanLeafShortwave for shortwave radiation. These conversion models use the canopy's LAI; they do not describe differences between sunlit and shaded leaves.

Coupling light and leaf physiology shows how to connect these models, how to use 3D radiation from ArchimedLight, and how to record daily radiation totals. If you already have measured or prescribed radiation per leaf area, you can provide it directly, as in the TL;DR leaf example.