How do I CALCULATE water EVAPORATION from RELATIVE HUMIDITY ?

Question

I'd like to calculate water loss at 30%rh, 50%rh, 70%rh & 90%rh from the surface of water at a constant temperature of 30degC.
I can assume a constant breeze at a lower temperature than the water surface.

Answer 1

I do not believe this problem can be answered until a net heat flow into the water is first determined. And I do not believe you are going to have much evaporation from a water surface into a convection media at a lower temperature regardless of its RH..

Answer 2

Well, I'll tell you. Water loss(evaporation) decreases with increasing rh. Because more the rh, more the water in the atmosphere and lesser the the amount of water required to saturate that volume of air.
Relative humidity=(Absolute humidity/Saturation point * 100)%
eg. Absolute humidity is 10g/cc and the saturation point is 20g/cc. Then:
rh=(10/20*100)%=50%.