How Much Water Your Pond Will Lose To Evaporation


The pond in your backyard will most definitely lose water over time. But many people wonder if the water loss is due to a leak somewhere or evaporation.

How much water will your pond lose to evaporation? On average a pond will lose around 1 inch of water per week to evaporation. In larger ponds up to 3 inches per week of evaporation is normal. Dry and hot weather can increase evaporation another inch or two per week.

There are many contributing factors that play into how much water will evaporate from your pond. Surface area, volume, and shade all play a role in determining how much water should be evaporating from your pond.

Average Water A Pond Will Lose To Evaporation

Nailing down an exact measurement is nearly impossible with the amount of factors at play. But we can get close to 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.

If you already know how many gallons are pumped and how big your pond is, you can use this chart to get a round about idea. If you need to figure out how many gallons are pumped, surface area, or just want a more accurate measurement, keep reading below.

Gallons Pumped Per HourSurface AreaEvaporation Per Week
5005×100.84 inches
10005×101.68 inches
100010×100.84 inches
15005×102.5 inches
150010×101.26 inches
20005×103.36 inches
200010×101.68 inches

How Many Inches Of Water Per Week Should Evaporate

Knowing how many gallons are pumped per hour, will help give you the amount of gallons evaporated per day and week. We then need find the gallons per 1 inch of water by using your surface area. From there you will know how many inches of water per week you should see evaporating from your pond.

How Many Gallons Are Pumped Per Hour

Most pumps come with a chart telling you how much water they are pumping, but they typically require you to figure out the head pressure first. The head pressure is basically the amount of work a pump must do to pump water out the end of a pipe.

To find your head pressure measure the pipe your pump is pumping water through. For every 1 foot of vertical pipe add 1 foot of head pressure. For every 10 feet of horizontal pipe add 1 foot of head pressure. For example, a pipe running 5 feet horizontally and 2 feet vertically will have 2.5 foot head pressure. 2 feet for the vertical portion and .5 foot for the horizontal.

With your head pressure measurement, use the chart that came with your pump to determine the flow rate, usually in gallons per hour. Every pump is different so you need to turn to your pumps user manual or documentation for this.

Gallons Evaporated Per Day

Once you’ve found your head pressure and know your gallons pumped per hour, you can easily find out how many gallons will evaporate per day. Typically a pond in the shade will lose 0.5% of the gallons pumped per hour over the course of a day. A pond in direct sunlight will lose around 1% of the gallons pumped per hour over the course of the day.

Find a number between 0.5% and 1% depending on how much of your pond is in direct sunlight throughout the day. If half your pond is in the shade and half in the sun, 0.75% is what you would go with. Take the percentage you chose and multiply it by the gallons pumped per hour to determine how many gallons of evaporation per day you will have.

Example: If my pump pumps out 1500 gallons per hour, and my pond is completely in the shade my math would look like this:

(0.005) X (1500) = 7.5 gallons of evaporation per day

Example: If my pump pumps out 1000 gallons per hour, and my pond is completely in the sun, it would look like this.

(0.01) X (1000) = 10 gallons of evaporation per day

Keep in mind, you need to move the decimal place over two places for a percentage.

Surface Area And Volume Of Pond

We need to know the surface area so that we can find the volume of the top one inch of water in your pond. Surface are is simply the length times the width of your pond. If your pond isn’t a perfect square or rectangle, you will want to multiply length times width times 0.85 for a more accurate surface area.

Example, a 5×10 foot pond:

Rectangular shape: 5 X 10 == 50 square feet of surface area

Irregular shape: 5 X 10 X 0.85 = 42.5 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.

(50 sq. feet) X 0.62 = 31 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 10 gallons of evaporation per day and had 31 gallons per inch of water.

10 / 31 = 0.32 inches of evaporation per day

Multiply that number by 7 to get the inches of evaporation per week. 0.32 X 7 = 2.24 inches of evaporation per week.

Causes Of Water Evaporation

There are a few factors that come into play that could affect the numbers above. The changes shouldn’t be drastic but I want to mention that these conditions could speed up or slow down your water loss to evaporation.

Plants

Certain plants can help decrease the amount of evaporation you pond will have. For example, water lilies or other plants that rest on the water actually reduce your surface area. As we saw with the equations above surface area is big part of water evaporation. Most plants will suck up some of the water to survive, but that isn’t an evaporation factor.

Shade

Having your pond in the shade can make a huge difference in the amount of evaporation. If your seeing 10 gallons of evaporation per day in the sun, that number might only be 5 gallons per day in the shade with the same pond.

If you’re concerned about water evaporation, try planting small trees or bushes that will protect your pond from too much sun exposure. Also consider installing a partial canopy of some kind over the pond.

Heat And Humidity

In very humid areas your pond evaporation may decrease. But in hot dry areas like Arizona or on the plains in Colorado you might only see 8% humidity some days. This will cause major evaporation, adding an extra 1-3 inches of evaporation per week in some cases.

Wind

On very windy days you will notice more water will evaporate than normal. Your surface area remains the same obviously, but the wind causes your waters surface to come into contact with more air. Having your pond near the side of your house or having something to block the wind will help minimize water loss due to wind.

Surface Area

The bigger your pond the larger its surface area. With a large surface area, more water will evaporate into the air. You won’t necessarily see a difference as far as inches evaporating because the pond will be larger, but you will be losing more gallons per hour to evaporation.

Splashing

Additional water features such as water falls or water spouts can increase water evaporation. These types of features could also be splashing water out of your pond. I wouldn’t say the extra evaporation is bad enough to remove them from the pond, but do make sure they aren’t splashing water out of the pond itself.

Automatic Fill Kit

If you don’t want to worry about filling your pond every week due to evaporation, you should consider getting an automatic water fill kit. Some of these kits will hook to your garden hose and fill your pond when it drops to a certain level.

You will need to do some installation work if you want to connect it to a 1/2″ line for instance. But it’s a great option for keeping your pond topped off without you needing to remember to do it.

If you feel your pond is losing more water than it should from normal evaporation, you may have a leak somewhere. Check out my Pond Water Loss Troubleshooting Guide if you want help finding and fixing the problem.

Robert Sampson

I'm Robert Sampson and I live in Colorado where I spend a lot of time in the backyard with my family either grilling, playing games and sports, or working on a project to make our backyard a better place to be.

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