Vapor Pressure Deficit at the Leaf
Computes the deficit at leaf temperature and beside it the air-based figure a room sensor reports.
Example
You enter
- Air temperature (deg F) 75
- Relative humidity (%) 65
- Leaf minus air temperature (deg F) -2
- Alternative leaf offset (deg F) 3
- Alternative relative humidity (%) 80
You get
- Saturation pressures 2.9641 kPa at air, 2.7719 kPa at leaf
- Leaf saturation kpa 2.7719
- Actual vapour pressure 1.9266 kPa
- Leaf vapour pressure deficit 0.8452 kPa
- What a room sensor would report 1.03743
- Air over leaf ratio 1.22736
- At the alternative leaf offset 1.34748
- Alternative change (%) 59.4184
- At the alternative humidity 0.400637
Details, formula, and sources
A leaf 2 degF below air makes the room reading 23% high, and a leaf 3 degF above air raises the deficit 59% with the air and humidity unchanged. The humidity lever is the weaker one.
VPD = es(leaf temperature) - RH x es(air temperature), with es = 0.6108 exp(17.27 T / (T + 237.3)) in kPa for T in degrees Celsius.
The Tetens saturation relation over water. The crop's own published VPD band governs.
Public relation; crop VPD bands are published by extension services.
Estimate. AHJ and licensed professional govern.
Field names used by the API: air_temp_f, relative_humidity_pct, leaf_offset_f, alternative_leaf_offset_f, alternative_humidity_pct, air_saturation_kpa, leaf_saturation_kpa, actual_vapor_pressure_kpa, leaf_vpd_kpa, air_vpd_kpa, air_over_leaf_ratio, alternative_leaf_vpd_kpa, alternative_change_pct, humid_leaf_vpd_kpa
- Leaf temperature entered as an offset from air; an infrared reading is the cheap instrument for itIR thermometer
- Humidity lever weaker than it feels -- a large RH change moves the leaf deficit less than a few degrees of leaf temperaturespec-v1754 finding