I have been adding 10 to 20 gallons a day here lately. I was even concerned that I might have a leak or something somewhere that I wasn't finding. The question of evaporation came up and I did a search on "Evaporation rate for Phoenix Arizona" and came to a report from the Western Regional Climate Center for several western states.
Click on a State: Arizona, California, Colorado, Hawaii & Pacific Islands, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, Wyoming
As it turns out the evaporation rate for my area is around 11 to 14 inches a month for June. That is about a half an inch a day.
To find out how many gallons that is for my system I first needed to find out how much area I have that is subject to evaporation. That is my sump, all of my fish tanks and all of my grow beds. Any area that is moist and open to air circulation. For me that is around 8750 square inches (+/- 60 ft^2). Then multiply the area times the average daily evaporation rate (0.5in) to get the volume and then convert that to gallons (+/- 18 gal).
So that is what is happening to all of my water!
Below is a spreadsheet that does all of these calculations for you. You can overwrite my numbers and fill in your own. You can use either or both rectangular or round beds and tanks. Be sure to find the evaporation rate for your own area and enter it as inches per month. The sheet will find the average gallons per day that you should be losing and even tells you how many inches that should be in your sump.
These calculations are pretty much the worst case for evaporation. There is nothing to allow for shaded areas and doesn't account for the water that the plants use or transpire. But it does give you a good idea of what you could be losing each day.
