Forage Crops & Agro-Forestry

In Monsi and Saeki's formulation of Beer-Lambert's Law for crop canopies, I = I0 e^(-K L), K represents the canopy light extinction coefficient.

• Albedo: Fraction of incident solar radiation reflected by a surface.
• Photosynthetically active radiation: Denoted by PAR (spectral band 400-700 nm), represented by I and I0.
• Extinction coefficient: K is the light extinction coefficient depending on canopy architecture, leaf angle, and orientation.
• Pan coefficient: Denoted by Kp, used in pan evaporation equations to estimate reference evapotranspiration.

The solar constant is the rate at which solar radiation is received on a surface perpendicular to the solar rays at the outer boundary of Earth's atmosphere, standardly measured as 1.94 cal/cm²/min (1367 W/m²).

• 0 cal/cm²/min: Approximate rounded figure, but 1.94 cal/cm²/min is the precise standard accepted value.
• 0 cal/min: Lacks area unit (per cm²).
• 94 cal/cm²/min: The standard precisely defined value of the solar constant.
• 94 cal/min: Incomplete unit lacking the area component.

Photosynthetically Active Radiation (PAR, 400-700 nm) is measured in terms of photosynthetic photon flux density (PPFD) using a specialized quantum sensor.

• Pyranometer: Measures total global solar radiation across the entire solar spectrum (300-3000 nm).
• Quantum sensor: Measures photosynthetically active radiation (PAR, 400-700 nm) in micromoles of photons per square metre per second.
• Precision pyranometer: Calibrated instrument for total broadband solar irradiance, not selective PAR.
• Piezometer: Hydrological instrument measuring water table hydraulic head.

Albedo is the fraction or percentage of incident solar radiation reflected diffusely by a surface (such as vegetation, water, bare soil, or snow) back into space.

• Lux: Unit of illuminance or luminous flux per unit area.
• Albedo: The ratio of reflected shortwave radiation to incident shortwave radiation from a surface.
• Net radiation: The balance between incoming and outgoing shortwave and longwave radiation components.
• PAR: Photosynthetically active radiation (400-700 nm spectral band).