Using the Evaporation Rate Calculator
The calculator requires five key inputs. First, enter the surface area of your water body in square metres—a 20 m² pond, for example. Next, specify the wind speed in metres per second; local weather stations or anemometers provide this value. Air temperature and relative humidity follow, both measured in standard units. The tool then computes evaporation using humidity ratios derived from psychrometric equations.
Two calculation paths are available. You can supply temperature and relative humidity directly, allowing the tool to derive the maximum humidity ratio internally. Alternatively, if you already know the current and maximum humidity ratios (common in engineering practice), enter those values instead. The result appears as kilograms per hour, which you can scale to daily or seasonal totals.
Evaporation Rate Formula
Evaporation from a water surface scales with three primary factors: the capacity of air to absorb moisture (controlled by wind and saturation), the available surface area, and the difference between saturated and actual humidity levels.
Evaporation rate (kg/h) = (25 + 19 × v) × A × (Xs − X)
where saturation humidity at temperature T:
Xs = 0.003733 + 0.00032T + 0.000003T² + 4×10⁻⁷T³
and actual humidity ratio:
X = RH × Xs
v— Wind velocity in metres per second; higher speeds remove moisture-laden air fasterA— Water surface area in square metres; larger surfaces expose more liquid to evaporationXs— Maximum humidity ratio of saturated air at the water surface temperature, in kg water vapour per kg dry airX— Current humidity ratio of ambient air, calculated as relative humidity multiplied by saturation ratioRH— Relative humidity as a decimal (0 to 1); lower values increase evaporation potential
Daily Evaporation Calculations
For planning purposes, multiply the hourly rate by 24 to obtain total daily evaporation, using average wind speed and humidity across the entire day:
Daily evaporation (kg) = 24 × (25 + 19 × v_avg) × A × (Xs_avg − X_avg)
v_avg— Average wind speed over 24 hours, in metres per secondXs_avg— Average saturation humidity ratio based on mean daily temperatureX_avg— Average actual humidity ratio across day and night
Physical Basis of Evaporation
Evaporation occurs because water molecules possess random thermal motion. Those with sufficient kinetic energy—typically at the liquid surface—escape into the gas phase despite intermolecular attraction. The rate accelerates when the surrounding air is unsaturated, because escaped molecules do not immediately recondense.
Wind removes the thin moist boundary layer above the water, replacing it with drier air from the environment. Without air movement, humidity near the surface climbs, reducing the driving force for further evaporation. Temperature amplifies molecular motion, increasing escape rates and the air's moisture-holding capacity simultaneously. Relative humidity directly opposes evaporation: saturated air (100% RH) permits zero net evaporation, whilst dry air (20% RH) sustains vigorous evaporation.
Practical Considerations and Pitfalls
Real-world evaporation varies with factors beyond calculator inputs; account for these when planning water management.
- Stratification and temperature gradients — Still water develops cooler layers at depth. The formula assumes uniform temperature at the surface, but in practice, afternoon heating creates a warm upper layer that evaporates faster than the bulk temperature suggests. Measure or estimate surface temperature for greater accuracy.
- Wind speed variability — Weather reports often give peak gusts rather than sustained averages. Use 10-minute average wind speed, not instantaneous values. Sheltered sites (surrounded by vegetation or structures) experience lower effective wind speeds than open weather stations nearby.
- Seasonal humidity swings — Morning dewfall and overnight condensation can exceed hourly evaporation rates in temperate climates. The calculator assumes steady-state conditions; over multi-day periods, net loss depends on daily cycling and rainfall contributions.
- Salinity and mineral content — Salt water and irrigation runoff evaporate slightly slower than pure water because dissolved solids lower water vapour pressure. This effect is small (1–3%) for typical concentrations but matters for precision agricultural or industrial water balances.