How Much Water Evaporates from a Pool

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How Much Water Evaporates from a Pool

Water conservation is crucial for every pool owner, and while many swimming pools will reuse the same water repeatedly, you will lose some of it through evaporation. Hence you often have to refill the pool to compensate for what evaporates.

Besides forcing you to add more water, evaporation can also lead to other issues, such as knocking your pool chemistry off balance.

As a pool owner, you need to understand why your water evaporation matters, how to calculate how much water you lose, and the best ways to minimize the rate of evaporation.

Why Does Pool Evaporation Matter?

The main reason you have to keep track of your evaporation rate is that it allows you to know when to refill your pool and how much water you will need to plan in advance. This becomes even more crucial for those that live in water-scarce areas.

Regardless of where you live, the more often you have to refill your pool, the pricier it will be to maintain in the long run. Remember that when you add fresh water to the pool, you still need to spend on other things like pool chemicals to balance the chemistry again.

The high water evaporation can also affect the integrity pool's filtration system, which ends up costing you even more money.

Factors that Affect Pool Water Evaporation Rate

Several factors will affect the water evaporation rate in a swimming pool, just like with all other water bodies. However, the following 4 are the main ones.

1. Average Temperature: The average temperature in your area is one of the key determinants of the evaporation rate you get. If you live in hot and dry areas with high average temperatures during the day and night, you will have a higher evaporation rate. On the other hand, you can expect reduced evaporation if you live in a cold climate.

2. Humidity Level: The higher the humidity levels in your area, the slower the evaporation rate because air can only hold a certain amount of moisture. High humidity means the air has a lot of water and hence cannot store more, which reduces the evaporation rate.

3. Pool Surface Area: Large swimming pools have more surface areas exposed to elements like sun and wind, meaning they will have increased evaporation.

4. Wind Intensity: Water will evaporate faster when you have more intense winds with high speeds. High wind speeds blow away humid air and replace it with less moist air, which can absorb more water and hence increasing the evaporation rate.

How to Measure How Much Water Evaporates from a Pool

There are three main methods of measuring the evaporation rate in your swimming pool: the direct measure, using a mathematical equation and estimating with evaporation rate maps.

1. Direct Evaporation Measure

The easiest way to determine how much water you are losing to evaporation is by measuring it directly. You can do this by measuring the distance between the pool deck and water surfaces at different times.

With this method, the most important thing is to record the specific date that you take the first and last measurements and make sure you do it at the same spot in the pool.

The difference in the two measurements you take is your evaporation rate, and you can easily convert it to gallons, whether it is in inches or feet.

2. Use Mathematical Equation

Although evaporation depends on several factors, from temperature to humidity, it is still possible to estimate it accurately using mathematical equations.

There are several equations you can use to estimate the evaporation rate for your swimming pool. However, the most common ones are the U.S. EPA, Stiver and Mackay, and the John W. Lund evaporation equations.

Each equation requires you to start by knowing factors like the pool surface area, temperature, and wind speed. The specific formulas provide all the parameters you need to know to calculate evaporation.

3. Estimate with Evaporation Rate Maps

You can use the evaporation rate maps for your geographical location to estimate your pool's evaporation rate.

For those in the USA, the National Weather Service provides some good evaporation maps and data that you can use.

With a simple conversion of the units that the map uses to something that you find more intuitive, you can use the data to estimate your pool's evaporation rate using its surface area.

Easy Ways to Reduce Evaporation

Once you know why water evaporates from your pool and how to determine the rate, the other crucial thing is understanding how to minimize the rate. While it is almost impossible to eliminate evaporation, here are some ways to slow it down significantly.

1. Cover Your Swimming Pool

Covering your swimming pool is one of the best solutions for dealing with evaporation. By covering your pool, you minimize contact between your water and the direct sun or wind, which are the leading evaporation causes.

Some studies estimate that a pool cover can reduce the water evaporation rate by up to 95%. Moreover, pool covers also help retain heat to keep the water warm and help conserve pool chemicals to reduce pool maintenance costs.

Read More: How to Build a Pool Cover from PVC Pipe

2. Add a Liquid Solar Blanket

If you prefer not to have a solid cover over your pool, you can try a liquid solar blanket. With a good liquid solar blanket like the Sunheater Liquid Heat Shield, you can create a protective barrier over the water that slows down evaporation.

Liquid solar blankets work round the clock, unlike solid covers that you can only use when the pool is not in use. Also, most are biodegradable, invisible, and super easy to use as there is nothing more to do besides pour them into the water.

3. Lower the Water Temperature

Warm water is always more comfortable and enjoyable, especially when swimming on colder days. However, if you are having issues with excessively high evaporation rates, you should try lowering your water temperature.

When your water temperature is higher than the air around it, water molecules will turn into mist and evaporate at a higher rate. On the other hand, it will be harder for cooler air to disperse into the atmosphere around it.

Read More: What’s Ideal Pool Temperature?

4. Switch off Water Feature

Water features add to the fun of the swimming pool and also help cool down the water. However, they can increase the water evaporation rate.

As water moves through water features like waterfalls and fountains, there will be more surface of the pool water exposed to the sun and air, increasing the overall evaporation rate.

The biggest culprits are the water features that often spray water into the air, such as fountains and sprinklers.

Other Ways Swimming Pools Lose Water

Evaporation might be the biggest cause of water loss for your pool, but there are still many others that you need to know and deal with, like the following three.

1. Leaks: Sometimes, the main reason your water levels are dropping fast is that you have a leakage somewhere in the pool. The leaks often occur around the pipe connections for inground pools, while tears and small holes are the primary sources of leaks for above ground pools.

2. Splashes: Swimmers can splash significant amounts of water out of the pool. Divers and aggressive swimming styles can splash out gallons of water from the pool if you do not have something like a gutter around the pool's edge for re-capturing the water.

3. Filter Backwashing: You have to backwash your filter often enough to clear out debris so that it continues working correctly. This backwashing usually entails taking out hundreds of liters from the swimming pool.

Bottom Line

Evaporation is something quite normal that you will encounter as a pool owner and so there is no need to stress over dropping water levels.

Instead, you should know how to determine the evaporation rate to plan well ahead by knowing how much water you need to add and the amount of pool chemicals you need.

Also, you can make sure it never gets out of hand or becomes a significant problem for you by taking preventative measures to minimize evaporation, such as covering your pool.