How Fast Does Water Evaporate From a Swimming Pool

Having a swimming pool throws you at the need to study and observe more than usual.

In your pursuit of the best experience in its usages and prevention of forthcoming issues, you might get lost in determining the accurate measures of different characteristics like pH, water level, evaporation rate, filtration sufficiency, etc.

Those who don’t find it necessary to calculate this find themselves at the risk of chemical disproportion. It is normal to ask questions like this as a new swimming pool owner or maybe after you have a different type of swimming pool installed.

How Fast Does Water Evaporate From a Swimming Pool

How Fast Does Water Evaporate From a Swimming Pool

To start with, the evaporation rate differs according to the climatic condition of your region, the mechanical mechanism you installed in your pool, and the chemical composition.

While general answers are given directly in the form of “quarter of an inch gets absorbed by evaporation,” put to heart, the level of evaporation on a body depends on the surface area and volume.

Therefore, you can only get an accurate measure of loss in your swimming pool water due to evaporation by considering the major criteria involved – evaporation rate per region, wind speed, the volume of the swimming pool, and human contributions – to solve the precise measure yourself.


Getting The Requirements

Statistical analysis from the weather station, on the morning news, or in the local newspaper can equip you with the figures of the relative humidity, temperature, wind speed, air pressure, and other requirements that might be needed later. Still, more cases of people not knowing the gallons of water their swimming pool contains.

If you are aware of the amount present, you can skip to the next section, but if you aren’t, kindly read through how to measure your pool water volume, it will help in other things.

The pool volume is calculated using the length × width × average depth × multiplier formula. If the swimming pool is rectangular, free form, or square, the multiplier is 7.5, but if oval, circle, or ellipse, it is 5.9.

You can measure the length and the width easily, but in the case of the average depth, it is just (shallow depth + deep depth)/2. With a width and length of 12 ft by 24 ft average depth of 4ft, your rectangular swimming pool will have a volume of 12 × 24 × 4 × 7.5 = 8640 gallons. Take your measurements and do the calculation now.


Different Methodology to Calculate Water Loss Due To Evaporation In Your Pool

There are different approaches to finding how fast evaporation drains the water content in a swimming pool. While you can figure out the evaporation rate in your swimming by manual methodology, there is a professional and scientific approach to doing so too, but this comes with a little bit of mathematical calculation and preciseness.

The manual method:

All that you require is a long meter rule or a tape rule with a recording paper. Make a table with column titles of “Time started,” “Initial length of water,” “Time ended,” and “Final length of water.” Move towards your pool, insert the meter rule/tape rule and record the level of water present.

Write down the time too. Limit human influence, and exactly after a day, go back to measure the water level in the same spot you did the other day. The difference in the water level records (which will be in inches or whatever the unit of measurement of the meter rule/tape rule) is the level of water evaporated in your swimming pool for the day. If it is in inches, divide by 12 to get your feet per day rate. Then, multiply that with your length, width, and 7.5 to get the gallons per day.


You can also fill a container with the known measurement with water and put it beside the pool. Find the evaporation rate with the formula, meter/time. Then multiply your result by the gallons of water in your pool. It will give you the evaporation rate of your pool.


Usage of evaporation maps:

There is usually a broadcast of weather conditions like pressure, evaporation rate, humidity, and others in every country. Check your local climate agency’s data news to know the rate of evaporation to expect. A simple conversion method is explained earlier in this article; follow it to get the volume of water lost in gallons.


Rules of thumb:

A general rule states that your pool drains by 0.25 inches per day due to evaporation in summer. Convert this to gallons, it is:

0.25/12 = 0.021

  • × 12 × 24 × 7.5 = 45.36 gallons.

Therefore, your pool loses 45.36 gallons of water every day if your pool is 12 feet by 24 feet and rectangular/square.


Stiver and Mackay Evaporation Equation:

Where E is evaporation rate,

P = Water’s Vapor Pressure (mmHg) at Ambient Temperature

A = Pool Surface Area (ft2)

W = Wind Speed Above Pool (mph)

T = Temperature (°F)


You can get all these figures from your local weather broadcast. The U.S. EPA evaporation law and John W. Lund Evaporation Equation. While these are scientific approaches and might be uneasy, they are provided to give readers a general overview of calculating how fast water can leave your swimming pool from all angles of knowledge.


Which One Is The Most Accurate?

If you make use of the four approaches, you will get different answers, but, in the end, the manual method is advised. This allows you to have a direct measurement with your pool without complication.

The others are proved ineffective sometimes due to differing conditions in the swimming pool’s location.

Still, in commercial places where there is less time to take the manual method, most swimming pool coordinators rely on the formula or thumb rule estimation.


Other Means a Swimming Pool Can Lose Water

A faulty skimmer, main drain, or leaf strainer can withhold more water than required in the system’s filtration cycle.

There might be structural damages resulting in a leaking process, a series of human activities like splashing during swimming, and water loss can occur during backwash. At this moment, professional swimming pool maintenance personnel should be invited for rectification.


Should I Panic When My Swimming Pool Loses Water?

Depends on the level. Invite a specialist if you notice an incredible fall in the water level. Still, natural factors like sunshine, wind speed, human activities, and others can contribute to the loss of water, and they are considered normal.


Final Thoughts

Why are you focused on determining how a normal swimming pool will lose its water content to evaporation? Have you asked yourself how often you should refill the water in your swimming pool?

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