Leaf Transpiration Rate Calculator

Why plants transpire

A plant uses about 100-1000 water molecules for every 1 CO₂ molecule it fixes through photosynthesis. The water loss happens through the same stomata that let CO₂ in — there’s no way to keep one open while closing the other. For most plants, 95-98% of water uptake from soil ends up evaporating through leaves, never appearing in any biomass.

Transpiration isn’t waste, though — it serves three real functions:

1. Cools the leaf (each gram of water evaporated removes 2,260 J of heat)
2. Drives nutrient uptake by creating water mass flow from roots upward
3. Maintains turgor through the steady water column

The trade-off between CO₂ gain and water loss is fundamental to plant ecology and is what the Water Use Efficiency (WUE) metric quantifies.

The Fick’s Law transpiration equation

Transpiration follows Fick’s law for diffusion:

E = gs × VPD ÷ P

Where:

• E: transpiration rate (mol H₂O/m²/s)
• gs: stomatal conductance (mol H₂O/m²/s)
• VPD: vapor pressure deficit between leaf and air (kPa)
• P: atmospheric pressure (kPa, typically 101.3 at sea level)

The math says: transpiration = (how open stomata are) × (how much drier air is than leaf interior) ÷ (atmospheric pressure).

Vapor Pressure Deficit (VPD) — the driving force

The leaf interior is essentially 100% relative humidity (saturated at leaf temperature). The air outside is typically drier. The vapor pressure deficit is the difference:

VPD = es(T_leaf) − ea VPD = es(T_leaf) × (1 − RH ÷ 100) [if leaf and air same temperature]

Where es is the saturation vapor pressure at temperature T:

es = 0.6108 × exp(17.27 × T ÷ (T + 237.3)) kPa [Tetens equation]

VPD is the single biggest driver of transpiration. Doubling VPD roughly doubles water loss.

Notice how VPD climbs exponentially with temperature, even at the same RH. A 10°C rise at 50% RH triples the VPD from 0.61 to ~2 kPa. This is why plants struggle in hot, dry weather: not just heat itself, but the dramatically higher water demand.

Optimal VPD for plant growth

Commercial greenhouse VPD control is now standard — temperature alone doesn’t tell you whether plants are stressed; VPD does.

Stomatal conductance (gs) — what the plant controls

While VPD is set by environment, gs is set by the plant. Plants can close stomata in minutes via guard cell osmoregulation. Typical values:

Plant species also differ in their default open state:

• C₃ crops (wheat, rice): max gs ≈ 0.3
• Tomato: 0.4-0.5
• Sunflower: up to 0.6
• Trees: 0.15-0.25 (typically lower)
• Cacti and CAM plants: stomata open only at night (when VPD is low)

Converting to mm/day

For agriculture and irrigation, transpiration is typically expressed as mm of water per day:

E_mm/day = E (mol/m²/s) × 18 (g/mol) × 86,400 (s/day) ÷ 1000 (g/L) ÷ 1 (density) ≈ E × 1.555

Typical transpiration rates:

• Well-watered tomato crop: 4-6 mm/day
• Well-watered corn: 5-8 mm/day
• Mature forest: 2-4 mm/day
• Desert shrub: 0.2-1 mm/day
• Drought-stressed crop: 0.5-2 mm/day
• Hot dry day at high VPD: up to 10-15 mm/day

For perspective: a 1 mm/day transpiration rate is 1 liter of water per square meter per day — that’s substantial agricultural water demand.

Water Use Efficiency (WUE) — the agricultural goal

WUE is the ratio of carbon fixed to water lost:

WUE = A ÷ E

Higher WUE = more carbon per unit water. This is what drought-tolerant crops are bred for. C₄ plants (corn, sorghum, sugarcane) have inherently higher WUE than C₃ plants (wheat, rice, soybean) — typically 2x.

CAM plants (cacti, succulents, pineapple) have the highest WUE — they open stomata only at night, store CO₂ as malate, and “release” it during the day for photosynthesis. WUE can be 5-10x higher than C₃ plants.

Stomatal closure feedback

Plants don’t just respond to environment — they respond to their own water status. When leaf water potential drops, the hormone ABA (abscisic acid) signals stomata to close. The cascade:

1. Soil dries
2. Root sensors detect dryness
3. ABA produced in roots
4. ABA transported to leaves
5. Guard cells lose turgor
6. Stomata close
7. Transpiration drops; CO₂ uptake also drops

This is why drought-stressed plants stop growing even before they wilt — they’ve shut down stomata to conserve water, but this also stops photosynthesis.

The leaf-air temperature gap

The Fick’s law formula assumes T_leaf = T_air. In reality, transpiring leaves are 2-5°C cooler than surrounding air (evaporative cooling). On a hot day with closed stomata (drought stress), leaves can be 5-10°C warmer than ambient — because the cooling has stopped.

Thermal imaging of crops can detect water stress before visible wilting: stressed leaves show up hotter on IR cameras. Agricultural drones routinely use this for precision irrigation.

Worked example

A well-watered tomato leaf at 25°C in 60% RH air, gs = 0.2 mol/m²/s:

• es(25°C) = 0.6108 × exp(17.27 × 25 ÷ 262.3) = 0.6108 × 5.20 = 3.17 kPa
• VPD = 3.17 × (1 − 0.6) = 1.27 kPa
• E = 0.2 × 1.27 ÷ 101.3 = 2.51 × 10⁻³ mol/m²/s
• E_mm/day = 2.51 × 10⁻³ × 1.555 × 86,400 / 1000 ≈ 3.9 mm/day

That’s a typical, healthy transpiration rate.

Bottom line

Transpiration is the cost of doing photosynthesis. VPD is the dominant environmental driver; gs is what the plant controls. For commercial growers, maintaining VPD in the 0.8-1.4 kPa range optimizes photosynthesis while minimizing stress. For drought-resistance breeding, the goal is higher WUE — more carbon per liter of water lost.