Search around on the NOAA website. You should be able to find "pan evaporation rates" for your area. That will tell how much you will lose per day/week/month. Time of year does matter. Sun is the most direct now, so the rate should be highest now.

www.hoosierpondpros.com

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).