Vapour pressure deficit
Temperature & humidity VPD saturation deficit
Vapour pressure deficit, or VPD, describes the gap between current water vapour and saturation. It helps indicate how strongly the air drives evaporation.
The gap in vapour pressure
Water vapour contributes a small part of the total air pressure. Vapour pressure deficit is saturation vapour pressure at the current temperature minus the vapour pressure actually present. It is commonly measured in kilopascals, or kPa.
A numerical example
At 20 °C, saturation vapour pressure is about 2.34 kPa. With 50 per cent relative humidity, half that pressure is present. The deficit is therefore about 1.17 kPa. At the same relative humidity, VPD increases as temperature rises.
- Near zero: The air is almost saturated, so the driving force for evaporation is small.
- A larger value: The air can drive more drying. Actual evaporation also depends on wind, available energy and water supply.
Why plants respond
Leaves release water through tiny pores called stomata. If water loss becomes excessive, a plant may close them, which also reduces carbon dioxide uptake. A weather value uses air temperature; a sun-warmed leaf may experience different conditions.
VPD alone cannot decide when to water. Soil moisture, plant species, light and growth stage also matter.