To compute the VPD
[2] we need the ambient (greenhouse) air temperature, the
relative humidity and if possible, the canopy air temperature. We must then compute the saturation pressure. Saturation pressure can be looked up in a
psychrometric chart or derived from the
Arrhenius equation, a way to compute it directly from temperature is
where:
is the saturation vapor pressure in kPa
Temperature of the air in Kelvin
To convert between Kelvin and Celsius:
We compute this pressure for both the ambient and canopy temperatures.
We then can compute the actual
partial pressure of the water vapour in the air by multiplying by the relative humidity [%]:
relative humidity/100
and finally VPD using
or
canopy sat
above my pay grade