Vapour Pressure Deficit (VPD)

Definition

Vapour Pressure Deficit (VPD) is the difference, in kilopascals, between the saturation vapour pressure of air at its current temperature and the actual vapour pressure present. It expresses how much more water the air could hold — its evaporative demand on the canopy. VPD is a derived quantity, computed by CHLORA from the co-located air-temperature (VC-MET-100) and relative-humidity (VC-MET-101) signals of each zone. A leaf-temperature correction is applied where a valid canopy-surface reading is available, yielding leaf-to-air VPD, the form the plant actually responds to.

Why it matters

VPD is the leading environmental driver in the Canopy Vitality Index (VC-MET-001) model — when CVI softens, CHLORA inspects VPD first. Low VPD suppresses transpiration, slows nutrient uptake, and raises disease pressure through leaf wetness; high VPD forces stomatal closure, stalls photosynthesis, and drives water stress that shows in turgor and conductance before it shows in greenness. Because temperature and humidity are controlled in order to hold VPD, it is the true climate set-point variable on the Operations Dashboard, not a passive readout.

Formula

Air VPD is the saturation deficit; leaf-to-air VPD substitutes leaf-surface temperature in the saturation term:

e_s(T) = 0.6108 · exp( 17.27·T / (T + 237.3) )    [kPa]   (Tetens)

Air VPD:
  VPD_air = e_s(T_air) · ( 1 − RH/100 )

Leaf-to-air VPD (preferred when T_leaf valid):
  VPD_leaf = e_s(T_leaf) − (RH/100) · e_s(T_air)

  T_air   zone air temperature  (VC-MET-100, °C)
  RH      relative humidity     (VC-MET-101, %)
  T_leaf  canopy-surface temp   (Spectral Canopy Array, °C)

Reported value = VPD_leaf where T_leaf confidence ≥ 0.8,
                 else VPD_air with a LEAF-EST flag.
Inputs older than 3 min are excluded; VPD then withheld.

Inputs

Units & Scale

Reported in kilopascals to two decimal places. VPD is intensive and not summed; facility roll-up is the canopy-area-weighted mean. The value carries a method flag (LEAF or LEAF-EST) and a confidence flag that degrades to DEGRADED when either input is stale or when the leaf-temperature channel is unavailable and VPD falls back to the air form, which can read 0.1–0.3 kPa low under high radiant load.

Sampling & Source

• Recomputed every 1 minute by CHLORA from the latest valid temperature and humidity.
• Inputs are co-located per zone (single air parcel) to avoid spatial error in the deficit.
• Leaf-temperature correction applied from the Spectral Canopy Array when confidence ≥ 0.8.
• Stale handling: any input older than 3 min withholds the VPD value and sets DEGRADED.

Thresholds

Recent Trend

Zone-A leaf-to-air VPD, last 14 sampling roll-ups (kPa):

Interpretation Guidance

Related Metrics

Related SOPs

• SOP Library — VPD-targeted climate-control procedures.
• Canopy water-stress triage on high-VPD WARN/CRIT — see Environment SOP set in the SOP Library.
• Alarm escalation & humidification interlocks — Monitoring Systems.