Leaf VPD (leaf-to-air vapour pressure deficit) is the difference between the saturation vapour pressure inside the leaf, assumed saturated at leaf temperature, and the actual vapour pressure of the surrounding air. Because water moves from the wet leaf interior to the air, leaf VPD is the true driving force for transpiration. Air VPD only equals leaf VPD when the leaf is at air temperature.
In practice
Leaf temperature differs from air temperature by light source and transpiration. Under LED, leaves commonly run about 1–3 °C below air; under HPS they can be near or above air temperature (Nelson and Bugbee 2015). Using air VPD alone can mislead you by several tenths of a kPa.
Example: a grower at 26 °C air and 60% RH reads an air VPD of 1.35 kPa and thinks the room is dry. An infrared thermometer shows the leaf at 24 °C. Leaf VPD is about 1.0 kPa, comfortably in the flower range. Raising humidity here would have been the wrong call.
Measure leaf temperature on top-canopy leaves facing the light, several leaves, with an IR thermometer or thermal camera.
Units and formula
Leaf VPD (kPa) = es(T_leaf) − es(T_air) × RH ÷ 100, with es = 0.6108 × exp(17.27 × T ÷ (T + 237.3)).
Don't confuse with
- Air VPD: uses air temperature for both terms.
- Leaf temperature offset: the input; leaf VPD is the result.
