Vapour pressure-deficit, orVPD, is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated.

In equation form:^[2]

${\displaystyle VPD=e_s(T_a)-e_a}$

${\displaystyle e_a}$ = actualvapor pressure

${\displaystyle e_s(T_a)}$ =saturation vapor pressure at temperature T_a

Once air becomes saturated, water will condense to form clouds, dew or films of water over leaves. It is this last instance that makes VPD important forgreenhouse regulation. If a film of water forms on a plant leaf, it becomes far more susceptible to rot. On the other hand, as the VPD increases, the plant needs to draw more water from its roots. In the case ofcuttings, the plant maydry out and die. For this reason the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 and 0.95 kPa.^{[citation needed]}

Inecology, it is the difference between thewater vapour pressure and the saturation water vapour pressure at a particulartemperature. Unlikerelative humidity, vapour-pressure deficit has a simple nearly straight-line relationship to the rate ofevapotranspiration and other measures of evaporation. Also, vapor pressure deficit is a more concrete measurement of the difference of the moisture content in the air, while relative humidity is a ratio of the actual vapor pressure to the saturation vapor pressure at the given temperature.^[2]

To compute the VPD,^[3] we need the ambient (greenhouse) air temperature, therelative humidity and, if possible, the canopy air temperature. We must then compute the saturation pressure. Saturation pressure can be looked up in apsychrometric chart or derived from theArrhenius equation; a way to compute it directly from temperature is

${\displaystyle v{p}_{\text{sat}}=e^{A/T+B+CT+D{T}^2+E{T}^3+F\ln T},}$

where

${\displaystyle v{p}_{\text{sat}}}$ is the saturation vapor pressure in PSI,

${\displaystyle A=-1.0440397\times {10}^4}$,

${\displaystyle B=-11.29465}$,

${\displaystyle C=-2.7022355\times {10}^{-2}}$,

${\displaystyle D=1.289036\times {10}^{-5}}$,

${\displaystyle E=-2.4780681\times {10}^{-9}}$,

${\displaystyle F=6.5459673}$,

${\displaystyle T}$ is temperature of the air in theRankine scale.

To convert between Rankine and degrees Fahrenheit:${\displaystyle T[\text{R}]=T{[}^{\circ }\text{F}]+459.67}$

We compute this pressure for both the ambient and canopy temperatures.

We then can compute thepartial pressure of the water vapour in the air by multiplying by the relative humidity [%]:

${\displaystyle v{p}_{\text{air}}=v{p}_{\text{sat}}\times (\text{relative humidity})/100}$,

and finally VPD using${\displaystyle v{p}_{\text{sat}}-v{p}_{\text{air}}}$ or${\displaystyle v{p}_{\text{canopy sat}}-v{p}_{\text{air}}}$when the canopy temperature is known, or simply

${\displaystyle VPD=v{p}_{\text{sat}}\times (1-\text{relative humidity}/100)}$.

It can easily be seen from this formula that if${\displaystyle T}$ rises (which raises${\displaystyle v{p}_{\text{sat}}}$), but relative humidity remains constant,${\displaystyle VPD}$ will increase.

VPD can be a limiting factor in plant growth.Climate change is predicted to increase the importance of VPD in plant growth, and will further limit growth rates across ecosystems.^[4][5]

VPD is also a strong influencer ofpotential evapotranspiration, which has important effects in the warming climate.

In controlled environments, such as greenhouses, vapor pressure deficit can be managed to maximize production. Excessive VPD can be reduced with misting or irrigation, or low VPD can be boosted by dehumidification or ventilation.^[6]

As the vapor pressure deficit increases, the amount of moisture in the vegetation and soil decreases, leading to drier conditions. VPD correlates strongly with the area that has been burned by wildfires in the southwest United States.^[2] Warming temperatures and drops in actual vapor pressure have contributed to an increase of VPD in the southwest U.S. Conversely, in the northern Plains and midwest, an increase in actual vapor pressure has caused a decrease in VPD.^[2]

The vapour pressure deficit can be utilized when predicting behaviour of a wildfire. Such predictions are an essential tool ofwildfire suppression.^[7]

• Water vapour

• Psychrometrics

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