How much water does a pond lose to evaporation?

Is your pond losing water?  Is it evaporation or do you have a leak?  The process below will help you answer the question: How much water should my pond lose to evaporation?

Average Evaporation

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 1000 gallons per hour:

For sheltered / shaded ponds use ½% (.005) X Gallons pumped per hour (1000) = 5 Gallons of Evaporation per day

For full sun / exposed ponds use 1% (.01) X Gallons pumped per hour (1000) = 10 Gallons of Evaporation per day

Head Pressure

To determine how many gallons are being pumped per hour, you will need to determine what is called “Head Pressure.”  Head Pressure is measured in feet.

1 Foot of vertical lift = 1 Foot of Head Pressure

10 Feet of horizontal distance = 1 Foot of Head Pressure

Example: If you are pumping water 4 feet vertically (from where the pump sits to where the water comes out) and 10 feet away, you would have 5 Feet of Head Pressure (4 Feet Up + 1 Foot for the 10 feet horizontally)

Most pumps come with a chart that lets you know how much they are pumping at a certain head pressure.  Find the documentation for your pump and use the head pressure you calculated to find out what the flow rate should be.

Use the gallons per hour flow rate you calculated to figure out what your evaporation per day should be on average by using the ½% – 1% formula described above.

Once you’ve figured out about how much water you should be losing to evaporation, you can take that number and figure out what difference that should make as far as the water level in your pond.

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.

Contributing Factors

In addition to normal weather conditions there are many factors that could influence evaporation.

  • Low Relative Humidity – Low amounts of water in the air can increase evaporation.
  • High Winds – Increases evaporation
  • Very Sunny Days – Successive very sunny days can also increase evaporation
  • No Rain – Without the occasional rain to help keep levels up, it will make your water loss seem greater.
  • Long Streams – The temperature in the shallow areas will be higher, and the more exposed surface area will both increase evaporation
  • Excessive Splashing – If there is a lot of splashing from waterfalls or drops, there will be increased evaporation.
  • What is around the pond such as mulch and how easy water can drain into the pond.

So there can be some fluctuations in evaporation water loss due to the factors above.  Every water feature is different, but hopefully these formulas will help give you an idea of what you should be losing per week from evaporation.

Recap of Calculations:

  1. Actual Flow Rate X .005 (sheltered) or .01 (exposed) = Average evaporation loss in gallons per day
  2. Pond Length X Pond Width X 1 (Rectangular) or .85 (Irregular) = Square Feet of Surface Area
  3. Square Feet of Surface Area X .62 = Gallons of water in 1” of your pond.
  4. Average Evaporation Loss per day X 7 = Average Evaporation Loss per week in gallons
  5. Average Evaporation Loss per week in Gallons Divided By The Number of Gallons In 1” of your pond will tell you about how many inches of water you should be losing to evaporation in a week.

If you’re concerned with the amount of water you’re losing on a regular basis to evaporation, you can always Install An Automatic Fill Valve to keep your water level constant.