How Much Water Do You Actually Lose in Your Pool From Evaporation?

Before you became a pool owner, you likely knew about basic maintenance items, such as needing to shock the water and keeping it free of debris. So, if you notice your pool’s waterline starting to lower, it might have you scratching your head about what the problem could be.

If you already know that kids splashing around in the water aren’t to blame for your decreasing water volume, could it be natural water evaporation that is the culprit?

Don’t worry—in the grand scheme of swimming pool ownership, evaporation isn’t a significant issue. Nevertheless, it’s important to rule out other possible causes of your declining pool water, and I’ll share tips on how you can prevent evaporation from happening.

How Much Pool Water Do You Lose from Evaporation?

Your pool may lose anywhere from two millimeters to two inches of water every week as a result of evaporation.

Many factors cause evaporation, and I’ll cover them shortly. But for now, know that while evaporation is a natural process, there are strategies you can use to prevent Mother Nature from taking away your pool water.

Why Does Pool Water Evaporate?

From a scientific standpoint, evaporation happens when water molecules begin moving around so quickly that they bump into each other, releasing water vapor molecules into the atmosphere.

If that’s too technical for you to follow before your first cup of coffee, here’s the layman’s version of it: Evaporation is when a liquid turns into gas. If you’re a visual learner, imagine a boiling pot of water; the steam that comes out of it is evaporation at its finest.

Luckily, you don’t have to worry about your swimming pool’s water evaporating as quickly as boiling water. Nevertheless, evaporation is a natural part of the water cycle powered by the sun, and where you live has a significant impact on how quickly or slowly your pool water will evaporate.

I’ll cover more environmental factors that determine how quickly pool water evaporates shortly. However, there’s one other important factor worth mentioning here—air pressure.

The higher the air pressure on the surface of your pool, the less likely your pool water will evaporate quickly. As its name suggests, the air pressure literally presses down on the water’s surface, making it difficult for water molecules to escape.

Thunderstorms are typically a high-pressure system, so if you live in an area that rains a lot, you receive the double benefit of less evaporation and rainwater refilling the small amount of water your pool loses.

How Quickly Does Pool Water Evaporate?

There’s a large range regarding how quickly pool water evaporates—it can be as little as two millimeters to as much as two inches per week.

Climate is the biggest factor that determines how quickly pool water evaporates. However, there are other factors that you have some control over.

For example, if you’re building a pool, consider the location of where you place it. A pool that sits in the sun all day will evaporate more quickly than a pool that spends a portion of the day in full or partial shade.

Of course, few people want to swim in a shaded pool, so enjoyment is also a factor when determining where to put your pool.

On the other hand, if your pool sits in a hot, sunny climate all day but you keep it covered when it’s not in use, then you’ll significantly reduce the water evaporation rate.

Similarly, running a water fountain or other water features speeds up evaporation since you’re exposing a larger amount of water surface area to the air.

Therefore, if you have water features and you’re concerned about how quickly your pool loses water, consider only turning them on at night or when you’re using your pool.

Long story short, even if you live in a hot climate with high evaporation rates, you can keep your pool evaporation levels closer to the two millimeters per week mark by using a solar pool cover and being mindful of how you use water features.

Factors That Go Into Water Evaporation Rate

As I mentioned earlier, numerous factors determine a pool’s evaporation rate, and climate is the most important. Furthermore, your pool will likely change its evaporation rate throughout the year, depending on the environment.

Examples of climatic situations that impact water evaporation in pools include:

  • Rainfall
  • Wind velocity
  • Solar radiation
  • Temperature
  • Difference in vapor pressure between the water and air

From an environmental standpoint, the highest water evaporation in pools often happens in areas where water is already scarce. For example, swimming pools in hot, dry climates like Arizona typically have higher rates of evaporation than those in cooler, humid climates like New York.

As I alluded to, human factors also influence pool evaporation rates. Below are some of the most common scenarios that impact them:

  • Pool location
  • Traffic (the more people use the pool, the faster evaporation occurs)
  • Amount of splashing
  • Use of water fountains and other features

Furthermore, sometimes the pool itself is to blame for water loss. Towards the end of this article, I’ll talk about how to know whether your pool is losing water because of evaporation or a leak.

How to Calculate Your Pool Water Evaporation Rate

If you’ve already spent some time researching pool evaporation rates, you’ve likely stumbled upon water loss calculators.

Such calculators are great if you have the data to feed them, but they require you to measure the water loss per day, which can get tricky if your pool only loses a couple of millimeters of water per week.

Furthermore, rainfall and pool use can all make a pool’s water level fluctuate throughout the day.

Therefore, I recommend filling up a bucket of pool water and placing it on your pool steps. With a permanent marker or piece of duct tape, mark where the water level comes to.

The next step can get a little tricky, but you’ll want to situate the bucket on the step so that the rim is slightly higher than the water level. That way, it prevents the sides of the bucket from heating up, which would lead to higher rates of evaporation than your pool.

After 24 hours, mark the new water line in your bucket and measure its distance from the mark you made the day prior. That’ll give you your evaporation rate per day.

As my final piece of advice, pick a time to do this when rain isn’t in the forecast. Otherwise, the rain will undo your hard work!

How to Tell if the Problem is Actually a Leak vs. Evaporation

If you notice that your pool is suddenly losing water at a quicker rate than usual and the weather isn’t in an abnormally long hot and sunny stretch, there’s a good chance you have a leak.

Using the bucket method I described above is a great way to help you determine a leak, provided that you couple it with marking the side of your pool; if the bucket loses less water than your pool, then a leak is likely the culprit.

Now that you know you have a leak in your pool, the next part is determining where the leak is. To do so, repeat the bucket test with the pump off, and determine which of the following scenarios is the case for your pool:

  1. You lose more water. In that case, the leak is in the return lines.
  2. You lose less water. In that case, the leak is in the skimmer or drain.
  3. You lose the same amount of water. In that case, the leak is structural—in the shell, pool liner, or fittings.

By knowing the general area of your leak, it’ll make the next step easier, for you’ll need to apply a colored dye in that region; the dye will point you to the exact location of your leak.

Once you identify the leak, you’ll need to make a trip to your local pool store to purchase material to patch it up. You can read my full guide on pool leak detection for more details on how to find the leak and fix everything up.

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How to Slow Down Pool Water Evaporation

Let me start with this—it’s impossible to prevent your pool water from evaporating entirely. However, there are numerous strategies you can use to reduce water from leaving your pool.

  1. Keep your pool covered. Any solid pool cover will significantly reduce evaporation since it lowers the surface area where water particles can escape. Even better, use a solar cover if you can. That way, your water will heat up so that it’s closer to the air temperature, which will reduce evaporation rates.
  2. Build some shade. If your pool isn’t already in a shady spot, consider planting a few tall trees near it or install a shaded enclosure.
  3. Shut off water features with aeration. Fountains and misters are fun to look at, but they significantly increase evaporation rates.

By implementing these strategies, you’ll spend less time filling your pool with more water, you’ll save money on your water bill, and you can feel good knowing you’re implementing these environmentally friendly strategies.

Questions about evaporation or other issues with your pool water levels? Drop me a line.

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