We see it every day: pool owners calling frantically because their pool is losing more than they ever remember.

Hold tight! It may not be a true leak but simple evaporation. Our experience shows that a pool with no heater and no waterfalls or water features can lose as much as a quarter inch per day, averaged over several days. That’s more than one and a half inches in a week. Of course, this is the high side of evaporation and most pools will lose less than this, but it’s still a very reasonable and useful guideline.

What causes evaporation?

There are three main factors for evaporation: Exposure, temperature and humidity levels.

• Exposure. The more your pool is exposed to the sun and wind, the more evaporation you’ll see. A screened pool will evaporate less than an unscreened pool. A pool surrounded by a fence, trees or a structure will evaporate less than a pool that’s facing an open sky. A pool that isn’t on a lake, golf course or other large piece of land will evaporate less than a pool that is.

• Temperature. Throughout our cooler months in Charleston, we see a significant contrast between the high and low temperatures, sometimes in excess of 25 degrees. This allows your pool to heat up during the day and evaporate all night. In fact, if the sun is down and you turn on your pool light, you’ll likely see a mist above the pool. This is the water evaporating and it does this all night, every night.

• Humidity. High humidity equals very low evaporation, whereas low humidity equals high evaporation. The air is like a towel or a sponge: The more dry it is, the more water it can hold. In other words, ladies, if you know your perfectly styled locks will fall as soon as you walk outside, you can expect very low evaporation from your pool.

These factors vary greatly from area to area and from season to season. For example, Arizona summers are dry, whereas between daily showers and 100 percent humidity, South Carolina summers are wet. Therefore, evaporation is high during an Arizona summer but very low during a South Carolina summer.

Test your water loss

To assess your water loss, here’s an overview of how to take reliable measurements:

  1. You’ll need a pencil and a ruler.
  2. Before you begin, turn off all waterfalls and water features for the entirety of this test. The exception is a simple spillover from a spa into the pool. Please allow this to run as normal.
  3. Turn off solar, propane, electric or other heater types and allow the pool to cool down for two days prior to beginning this test.
  4. “Day 0” is the starting day. Fill pool to normal level, being careful not to overfill.
  5. Turn your pump off for five minutes, allowing your water level to settle from the turbulence of the pump.
  6. Use a pencil on the wall of your skimmer canister to mark the current water level.
  7. Turn your pump back on and allow it to run as normal for four days.
  8. On the fourth day, turn your pump off for five minutes, allowing your water level to settle from the turbulence of the pump.
  9. Use a ruler to measure the distance from the pencil mark made on “Day 0” to the current water level. Now you have a true, reliable measurement of your actual water loss over a four-day period.

Keep in mind that daily pool use tends to result in higher water loss, and use by kids and pets tends to result in much higher water loss.

Evaluate the results

A quarter inch per day or less (or 1 inch in the Day 0-4 test) would be considered reasonable evaporation in Charleston for the dry season (October through May, depending on current rain levels). Losing more than a quarter inch per day in a pool running with no waterfalls, features or heaters is considered a leak.

Did you lose more than a quarter inch per day averaged over the four days? If so, you very likely have a leak and should consider contacting a leak detection company.  Share your test results with the company and their professionals will be able to give you a more specific expectation of your area’s evaporation rates. Did you lose less than a quarter inch per day? Rest easy knowing that most likely your pool is simply experiencing normal evaporation.