To use the Penman-Monteith equation for finding water loss from plants, what two important things about the air and sun do you need to know?

To use the Penman-Monteith equation for finding water loss from plants, you need to know two important things: the net radiation from the sun and the vapor pressure deficit of the air. First, you need net radiation. Net radiation is the total amount of energy available at the plant surface to evaporate water. It represents the balance between all the solar energy (shortwave radiation) hitting the surface and the energy reflected or emitted back into the atmosphere (longwave radiation) by the plant and surrounding ground. This energy, primarily from the sun, is crucial because it provides the heat needed to change liquid water in the plant into water vapor, a process called latent heat of vaporization. Without this energy, water cannot evaporate. Second, you need the vapor pressure deficit (VPD) of the air. Vapor pressure deficit measures the "drying power" of the air. It is the difference between the amount of water vapor the air *couldhold if it were completely saturated at its current temperature, and the amount of water vapor it *actuallyholds. When the air has a high vapor pressure deficit, it means the air is relatively dry and has a strong capacity to take up more moisture, thereby pulling water vapor away from the plant leaves. This 'pull' is a key driving force for transpiration, the process of water moving through a plant and evaporating from its aerial parts, such as leaves, stems and flowers.