It is a closed system for ease of description .So what does go in the exact same amount that goes out. Actually that is just a distraction from the problem.


There just are too many variables plus unknowns to use other than actual measurement. From what information has been presented. The air velocity above the water also makes it difficult. A calibrated laser fired in might eliminate the air velocity factor. In high wind velocity water is blown flat for a period. So you might get a reading of the level. Before the surface reaction starts to establish itself.