How this calculator works
The FAO saturation-vapour-pressure equation estimates vapour pressure over liquid water. Actual vapour pressure comes from relative humidity, dew point, or the FAO coefficient 0.000662 per C for an aspirated psychrometer. Air VPD is saturation pressure at air temperature minus actual pressure. Leaf VPD instead uses the leaf temperature; a negative leaf VPD indicates possible condensation. Below freezing, this calculation remains referenced to liquid water rather than ice.
Formula
es(T) = 0.6108 × exp[17.27T/(T + 237.3)] kPa, with T in C. ea = RH × es(Tair), or es(Tdew). Wet bulb: ea = es(Twet) - 0.000662 × P × (Tair - Twet). VPD = es - ea.
Worked example
At 0 C, es = 0.6108 kPa. With 50% relative humidity, air VPD is 0.3054 kPa. If dew point, air and leaf are all 0 C, both VPD values are zero.
Advertisement
Frequently asked questions
Can leaf VPD be negative?
Yes. A leaf colder than the dew point has saturation pressure below the actual vapour pressure of the air.
Does any wet-bulb thermometer work?
The implemented FAO coefficient assumes a ventilated Asmann-type psychrometer. Natural or unventilated instruments require a different coefficient.
Which VPD should I target?
Crop stage, species and temperature affect a suitable target. This tool calculates the physical quantity and does not prescribe a universal range.