There are many factors on this subject and research has come up with is. Here in Texas depending on size you could lose up to 5 gallons per 1000 gallons in one day.
The best way to explain this is get a glass off water and dump it on your driveway and watch how fast it evaporates. Rocks in a natural swimming pond also do the same thing. The hotter and dryer it gets the more water you will lose.
These factors apply to pool and natural swimming ponds.
You will notice in the picture above the rock beach line is absorbing water and the sun is heating the rock. This happens all day and contributes to the evaporation. You can also notice this on larger stones where you can see the water line about an inch above the water.
Humidity in Texas is always fluctuating, we may get rain once a week or we could go months without it. Austin average humidity is 67 but with no rain this number goes down and evaporation goes up. Austin Texas morning are the most humid part of the day and dropping by almost half by the end of the day. This can increase the evaporation rate of a pond.
When the heat rises you may see an additional inch or two of pond evaporation
Windy days are also a big culprit in the evaporation scheme. You might even see a daily loss in water. Some may think that the pond plants are sucking up all the water. However, plants drink a relatively small amount. In fact, if your pond has water lilies, they are reducing the amount of surface area and are actually reducing the amount or evaporation occurring. On the natural swimming pond we really strive to achieve this on the bog sides.
Pools and pond both evaporate around the same rate. The difference between the two are natural swimming ponds tend to be larger and are set and built more out in the open to give it the natural feel. We build natural swimming ponds starting around 22000 gallons to our latest coming in close to 36000 gallons.
On average a water feature will lose ½% to 1% of the gallons pumped per hour in a day. Remember to use the actual gallons pumped per hour, not just the size of the pump. See below to figure out the actual flow rate.
Example: In a pond that is pumping 16000 gallons per hour:
For sheltered / shaded ponds use ½% (.005) X Gallons pumped per hour (1000) = 80 gallons of Evaporation per day
Surface Area / Volume
You need to figure the square feet of surface area in your pond. Take the Length X Width of the pond if square or rectangular, or Length X Width X .85 if irregular shaped.
Example: A 7’ X 11’ pond
Rectangular Pond: 7 X 11 = 77 sq. ft. of surface area
Irregular Shaped Pond: 7 X 11 X .85 = 65 sq. ft. of surface area
You then take the sq. ft. and multiply it by .62 this will give you the number of gallons you have in 1 Inch of your pond.
So an irregular shaped 7 X 11 Pond would have 7 X 11 X .85 = 65 Sq. Ft. X .62 = 40 Gallons per Inch of water.
So if we take the numbers in the examples above and assume we have a sheltered pond and are pumping 1000 gallons per hour we should be losing around 5 Gallons per day to evaporation. If you multiply that by 7 (5 X 7) you get 35 Gallons lost to evaporation per week. That compared to how many gallons are in an inch of water 40 Gallons (from the example) you can see you would be losing about 1 Inch of water per week to evaporation.
To be exact, divide Gallons Lost Per Week by Gallons In 1 Inch of Water: 35 / 40 = .875 Inches per week of evaporation
For full sun / exposed ponds use 1% (.01) X Gallons pumped per hour (1000) = 160 Gallons of Evaporation per day