Evaporation Rate Calculator — Open-Water Aerodynamic Method
Estimate how fast water evaporates from an open surface using the aerodynamic method — enter surface and air temperatures, relative humidity and wind speed.
°C
°C
%
m/s
Vapor-pressure driven component — radiation adds more in sunlit conditions
- 1
Sat. vapor pressure at water surface (Magnus)
0.6108 × exp(17.27 × 25 ÷ (25 + 237.3)) = 3.1678Saturation VP at the water surface temperature in kPa. - 2
Actual vapor pressure of air
(50 ÷ 100) × 4.2431 = 2.1215 - 3
Vapor pressure deficit (VPD)
3.1678 − 2.1215 = 1.0462 - 4
Wind evaporation factor
6.43 × (1 + 0.536 × 2) = 13.323 - 5
Evaporation rate
13.323 × 1.0462 = 13.94
How does this calculator work?
Aerodynamic evaporation Ea = 6.43 × (1 + 0.536 u₂) × VPD [mm/day], where VPD = esW − ea (saturation VP at water surface minus actual VP of air, both in kPa) and u₂ is wind speed at 2 m in m/s. This captures the humidity-driven component only; radiation adds 60–75% more in sunny conditions.
Formula
How this is calculated
Evaporation from a free water surface is driven by two forces: the vapor pressure gradient between the saturated air just above the water and the drier air above it (the aerodynamic component), and the energy input from solar and thermal radiation (the radiation component). This calculator implements the aerodynamic component using the formula developed by Doorenbos and Pruitt (1977) and documented by the FAO: Ea = 6.43 × (1 + 0.536 u₂) × VPD, where u₂ is wind speed at 2 m height in m/s and VPD = esW − ea is the vapor pressure deficit in kPa.
Saturation vapor pressure at any temperature is computed using the Magnus formula: es(T) = 0.6108 × exp(17.27 T / (T + 237.3)) kPa, accurate to ±0.1% from −40 to 60 °C. The actual vapor pressure of the overlying air is ea = (RH/100) × esA, where esA is the saturation pressure at the air temperature. The VPD is the difference between the water surface saturation pressure and the actual air vapor pressure — a higher VPD means drier air relative to the water and faster evaporation.
Important limitation: the aerodynamic term alone underestimates total evaporation in sunny conditions because it excludes the radiation term. In the full Penman (1948) equation, radiation typically contributes 60–75% of the total in summer at mid-latitudes. The aerodynamic-only result here is most applicable at night or in shaded/overcast conditions. For a full estimate, the Penman-Monteith method (FAO-56) is recommended when net radiation data are available.
Frequently asked questions
Two main factors: (1) the vapor pressure deficit (VPD) — how much drier the overlying air is relative to the saturated air just above the water; and (2) energy input from solar and thermal radiation, which supplies the latent heat needed to convert liquid water to vapor. Wind enhances both by continuously replacing moist air with drier air.
This calculator only computes the aerodynamic (vapor-pressure) component, not the radiation component. On a clear sunny day the radiation term can contribute 60–75% of total evaporation, so the actual rate may be 2–3× the number shown here. For a full estimate, use the Penman-Monteith equation with net radiation data.
VPD is the difference between the saturation vapor pressure at the water surface temperature and the actual vapor pressure of the overlying air. It quantifies how much "room" the air has to absorb more water vapor. A higher VPD (hot, dry air) drives faster evaporation; VPD = 0 (saturated air) means no net evaporation.
TG we-Calculate Editorial Team. (2026). Evaporation Rate Calculator — Open-Water Aerodynamic Method [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/evaporation-rate-calculator
TG we-Calculate Editorial Team. "Evaporation Rate Calculator — Open-Water Aerodynamic Method." TG we-Calculate. 2026. https://we-calculate.com/calculator/evaporation-rate-calculator.
TG we-Calculate Editorial Team, "Evaporation Rate Calculator — Open-Water Aerodynamic Method," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/evaporation-rate-calculator
@misc{wecalculate_evaporation_rate_calculator, title = {Evaporation Rate Calculator — Open-Water Aerodynamic Method}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/evaporation-rate-calculator}}, year = {2026}, note = {TG we-Calculate} }
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