How this calculator works
This estimate uses mass-transfer coefficient (25 + 19v) kg/(m² h), multiplied by area and the difference between surface-saturated and ambient humidity ratios. Separate water and air temperatures determine their vapour pressures using ASHRAE saturation equations. The air-speed correlation is empirical and does not close the surface energy balance: solar heating, waves, covers and changing water temperature can alter the actual rate. Use it as an approximate open-surface estimate.
Formula
mdot = (25 + 19v) A (ws - wa)/3600 in kg/s, with v in m/s; w = 0.621945 pv/(p - pv).
Worked example
At v = 1 m/s, A = 2 m², ws = 0.02 and wa = 0.01 kg/kg dry air, evaporation is 0.88 kg/h, or 0.0002444444 kg/s. At zero air speed, the same surface gives 0.5 kg/h.
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Frequently asked questions
Why can the rate be negative?
Ambient air wetter than the surface saturation state drives condensation rather than evaporation.
Does the model include solar energy?
No. Supply a water surface temperature that reflects the actual energy balance.
Is the coefficient a universal physical constant?
No. It is an empirical estimate for exposed water surfaces and may differ from field measurements.