VPD (Vapor Pressure Deficit)

Detailed Explanation

VPD (Vapor Pressure Deficit) is the difference between the maximum water vapor pressure air can hold at the same temperature (Psat) and the actual water vapor pressure (Pv). Its unit is kPa or Pa. Unlike RH, which is a percentage, VPD is an absolute pressure difference; it therefore accurately reflects the true thermodynamic driving force of plant physiology.

Plants release water vapor through their stomata (leaf pores) — a process called transpiration. This process is directly proportional to VPD: high VPD = rapid vapor transfer from leaves to air = excessive water loss, closed stomata, and loss of photosynthesis. Low VPD = insufficient transpiration = poor nutrient uptake, increased risk of mold and disease.

Controlled agriculture (greenhouses, vertical farms, vegetative rooms, drying cabinets) uses VPD as the control parameter instead of temperature and humidity separately. Optimum VPD varies by plant species and growth stage; the general target in controlled environment agriculture is 0.8–1.2 kPa, with lower values preferred for seedlings.

Calculation

VPD = Psat × (1 − RH/100)

Saturated vapor pressure (Tetens equation): Psat = 0.6108 × exp[17.27 × T / (T + 237.3)]

VPD: vapor pressure deficit (kPa) Psat: saturated vapor pressure (kPa) T: air temperature (°C) RH: relative humidity (%)

Leaf surface VPD (LVPD) — more accurate: LVPD = Psat(Tleaf) − Pv(Tair)

Tleaf: leaf surface temperature (typically 1 to 3°C below Tair due to transpiration cooling) Pv: actual water vapor partial pressure in air

Practical Example

Compare two vertical farming rooms. Same plant, same LED, different climate:

Room A: 24°C, 65% RH Psat = 2.98 kPa → VPD = 2.98 × (1 − 0.65) = 1.04 kPa ✓ vegetative optimum

Room B: 28°C, 75% RH Psat = 3.78 kPa → VPD = 3.78 × (1 − 0.75) = 0.95 kPa ✓ vegetative optimum

The two rooms have different temperatures and RH values, yet the same VPD — physiologically similar environments for the plant. This shows why VPD is so valuable as a single control parameter.

Real-world error scenario: If the same operator designed a 30°C environment with a 65% RH setpoint: Psat(30) = 4.24 kPa → VPD = 1.49 kPa (well above the 0.8–1.2 kPa range!) During the vegetative stage, the plant would over-transpire, leaves curl, and growth slows. When temperature and RH are controlled separately, this error goes unnoticed; with VPD, it is visible at a glance.

Engineering Note

Considerations in VPD-based climate control design:

• HVAC controller selection — modern climate computers (e.g. Priva, Hortimax) operate directly with a VPD setpoint; temperature and humidity become auxiliary parameters. • Leaf temperature vs air temperature — LVPD, measured with IR thermometers or leaf temperature sensors, differs from air VPD. Under LEDs a transpiring leaf is usually 1–3 °C cooler than the air; under intense light or weak transpiration it can exceed air temperature. • Local microclimate — a single sensor does not represent the entire growing area; the upper zone near LEDs can have different VPD from the shaded lower zone. Multi-point monitoring and circulation fans are critical. • CO₂ interaction — at high CO₂ levels, stomata partially close and transpiration decreases; for this reason, VPD can be raised slightly in CO₂-enriched facilities.

In vertical farming and greenhouse applications the dew point is typically +14…+18 °C, so base dehumidification is done with condensing units; they can be run on a relative humidity setpoint derived from the VPD target in the BMS.