Rincon -"Use it or lose it!", page-28

Could you check for long term rainfall and evapotransporation data for the actual locality (ie for a representative weather station). This should show the net loss/plus for each month. The maths then is pretty simple. E.g. if you have 1m worth of water in the ponds that you need to evaporate simply add up the monthly net losses/gains until it totals 1m.

Note that even long term weather, esp in that region is variable and changes rapidly due to climate change. Disregard data older than 15 years. Past performance is no guarantee for future.