What I am asking, is exactly the opposite of the yearly rain. It is well measured and it has century long databases.

Consider a given, generally dry area. What I am trying to know: if this area wouldn't be dry, but it would be an open water surface (for example, a sea), but without any water resupply (i.e. rain), what would be the yearly level loss by evaporation?

I think, this information would be useful to calculate the irrigation need of not arable territories because of the lack of rain.

Additional difficulty: in case of larger areas, this evaporation rate would be probably smaller, because the winds in the central regions get moisture on the borders.

I suspect, calculating this would require a relative complex weather simulation, which is not available on the enthusiast level. But maybe some fist rule, estimation or already existing databases can exist.