If you’re noticing your pond is losing water while your water bill is increasing, it’s probably not evaporation. Evaporation rates change due to many factors such as location, temperature, humidity, wind, and some localized variables. This can make for a lot of different numbers if you search for pond leak or evaporation on the internet. Seeing as how it’s different for every pond, the first thing to do is figure out how many gallons of water per inch your pond has by using this formula:
Length of Pond x Width of Pond x .75 x .0833 x 7.48
Those last three numbers didn’t magically appear. .75 is a typical percentage of the pond area, .0833 is an inch (which is how much water will be lost from your pond every X days), and 7.48 is how many gallons of water are in a cubic foot. Afterward, follow this formula to see how much water your feature should be losing per day:
.5% of your pump flow per hour per day.
Here are the formulas in action! Let’s say you have a 15′ x 20′ pond. 15 x 20 x .75 x .0833 x 7.48 = about 140 gallons per inch of pond surface. This pond is moving 5000 gallons total per hour through your pump(s) every day. 5000 x .5% = 25 gallons. So, with this pump, you can expect to lose about 25 gallons per day to evaporation.
Now you put those numbers together by dividing the gallons per inch by the number of gallons you should be losing: 140/25 = 5.6. This means your pond can lose 1 inch of water every 5.6 days because of evaporation. If it’s more than that then you have the answer to if it’s a pond leak or evaporation.
Let’s say you have a disappearing pondless waterfall or fountain basin with a 5000 gallons per hour pump. It’s a 200-gallon basin and is 2′ (24 inches) deep. 200/24 inches = 8.33 gallons per inch of the basin. In the previous example, this pump was figured to lose 25 gallons per day to evaporation. 25/8.33 = 3. You should expect to lose 3 inches of water per day to evaporation based on the pump size.