Not as easy as just plugging in temperature, but most of the parameters necessary should be available for your area of interest. Here's an intro on the topic with some formulas.
link didn't work on the prior post: http://ccc.atmos.colostate.edu/ET_Workshop/ET_Jensen/ET_water_surf.pdf
What you want is “lake surface evaporation.” Simplest way is to look up data from a local evaporation pan. In the US, NOAA and others have these. Get a monthly average rate, and multiply by the surface area. There are complications of course, based on wind, depth, etc. If you need a ballpark, this is OK. If you need defensible numbers, best advice is to hire a hydrologist! Check the Handbook of Hydrology or this doc:http://ccc.atmos.colostate.edu/ET_Workshop/.../ET_water_surf.pdf
Although it doesn’t appear to answer your question, I thought this discussion of evaporation control [pdf] might still be of interest to you.
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