Pottsy - Your greenish water indicates you have a good phytoplankton (high #'s microalgae) bloom in the pond. All these microalgae absorb CO2 (carbon dioxide) during the day and reducing the carbnonic acid in the water thus raising the pH. At night they release CO2 and it forms carbonic acid which lowers your pH. Since you have naturally hard and well buffered pond water your pH will on the low end, usu be high 7's low 8's and upward from there.
No need to worry (in your case) even with evaporation and water loss.
