Figuring out an exact measurement is almost impossible with the amount of factors at play. We can get close to a number you can use to plan out your ponds evaporation. First we will need to figure out how many gallons of water per day you are pumping and then the surface area of your pond. With these numbers we can easily determine about how many inches of water you will be losing to evaporation.
The best way to explain this is get a glass of 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
Natural swim ponds are a lot larger than the average pond which is a 12 x 7. Our average swim pond has a water surface of 30 x 60 not including rivers, waterfalls or bogs which will add to surface area and evaporation factors. Here are the up to numbers.
Example: If my pump pushes 13000 gallons per hour, and my pond is completely in the shade my math would look like this:
(0.005) X (13000) = 65 gallons of evaporation per day
Example: If my pump pushes 13000 gallons per hour, and my pond is completely in the sun, it would look like this.
(0.01) X (13000) = 130 gallons of evaporation per day.
Example, a 30x60 foot pond:
Rectangular shape: 30 x 60 == 1800 square feet of surface area
Irregular shape: 30 x 60 x 0.85 = 1530 square feet of surface area
Take square foot surface area of your pond and multiply it by 0.62 to find the gallons per one inch of water. So to use the rectangle shaped pond above as an example.
(1800 sq. feet) X 0.62 = 1116 gallons per inch of water.
Inches Of Evaporation Per Week
Now that you have your gallons evaporated per day, and your gallons per inch of water, we can figure out how many inches per day you can expect to lose due to evaporation. First, take gallons evaporated per day and divide that number by gallons per inch of water.
To use the same example above, I was losing 130 gallons of evaporation per day and had 930 gallons per inch of water.
130 / 1116 = 0.85 inches of evaporation per day
Multiply that number by 7 to get the inches of evaporation per week. 0.85 X 7 = up to 5.95 inches of evaporation per week