Moisture content in the air is an important environmental variable for plant production systems. There are different variables to express the moisture content in air, including relative humidity (RH) and dewpoint temperature. In addition, we use “VPD“ to evaluate the moist air when the plant response is the concern. However, there are some confusions about the definitions and calculation methods of VPD.
VPD is often spelled out as (water) vapor pressure deficit, which represents the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated. For this reason, it is also referred to as the water vapor pressure saturation deficit. Levels of moisture in the air are often measured using partial pressure of water vapor (i.e., water vapor pressure). VPD is recommended for use in climatology and ecology because it has a linear relationship with evapotranspiration under otherwise identical conditions (e.g., Anderson, 1936). Anderson (1936) recommended reporting VPD instead of relative humidity in ecology studies. In these literatures, VPD is clearly defined as the physical absolute quantity of moist air, independent from the status of the plant (for example its leaf temperature).
Using the leaf temperature, instead of air temperature, to determine the saturation water vapor pressure has been also used widely as another method to calculate the VPD (e.g., Faust, 2017). I do not know who first defined VPD with leaf temperature. But it is an extended definition of VPD to solely consider the transpiration from plant leaves. This approach makes sense for CEA (controlled environment agriculture), but when the VPD can be calculated in two different ways, it can be confusing. Leaf temperature can be different by several degrees from air temperature, and consequently, VPD values can be largely different depending on temperatures used for computing VPD. For example, when the air temperature is 20 oC (68 oF) with a relative humidity of 60%, and the leaf temperature is 18 oC (64 oF), VPD is 0.94 kPa using the air temperature for the calculation, and 0.66 kPa using the leaf temperature.
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