Evaporation is the natural process where liquid water transforms into water vapor and escapes into the atmosphere. This transition occurs continuously at the water’s surface. Some water loss from a swimming pool is normal and unavoidable. Understanding these mechanisms helps pool owners manage the water they lose.
Environmental Factors Accelerating Water Loss
Four primary environmental factors determine the speed of pool water evaporation. Warmer water molecules possess greater kinetic energy, making it easier for them to break free from the liquid surface tension and transition into vapor. A 10-degree Fahrenheit increase in water temperature can elevate the evaporation rate by approximately 25 to 30 percent.
The relationship between air temperature and humidity also influences the rate of loss. Hot, dry air readily absorbs moisture from the pool surface. Conversely, high humidity reduces the air’s capacity to accept new water molecules.
Air movement, particularly wind, is a powerful driver of evaporation. Wind constantly sweeps away the thin, saturated layer of air (the vapor boundary layer) hovering above the water surface. This continuously exposes the pool to drier air, accelerating the rate at which water molecules escape.
The total surface area of the pool directly correlates with the amount of water lost. Pools with water features, such as fountains or waterfalls, increase the effective surface area by creating spray and mist, further increasing evaporation.
Typical Evaporation Rates and Expected Ranges
For an uncovered, non-heated pool, the typical evaporation range is between one-eighth and one-quarter of an inch of water loss per day. This accumulates to approximately one to two inches per week under normal conditions. The rate is highly variable and depends heavily on local climate, particularly wind and humidity levels.
In arid or windy climates, or during periods of high temperature and low humidity, the daily loss can reach up to half an inch or more. Heated pools experience significantly increased evaporation because the water temperature is maintained higher than the surrounding air. Heating systems can add an additional one-quarter inch of water loss per day.
Pools with decorative water features, such as sheer descents or infinity edges, also experience substantially higher loss. These features maximize the water’s exposure to the air, easily adding another one-quarter inch or more to the daily total. When multiple accelerating factors are present, water loss can sometimes exceed three inches per week.
Testing Water Loss: Evaporation Versus Leaks
Pool owners often confuse high evaporation with a structural leak. The most reliable method to differentiate between the two is the bucket test, which compares water loss in the pool to water loss in a contained vessel where only evaporation can occur.
Performing the Bucket Test
To perform the test, place a small, weighted bucket on a lower step of the pool. Fill the bucket so the water level inside matches the pool level outside. Use a permanent marker to clearly mark the water level inside the bucket and the corresponding level on the outside wall.
Turn off the pool’s circulation system, including the pump and any automatic water fillers, and leave the setup undisturbed for 24 hours. The water inside the bucket drops only due to evaporation. The pool water level outside the bucket drops due to both evaporation and any potential leaks.
Interpreting Results
After the testing period, measure and compare the two water level marks. If the levels inside and outside the bucket have dropped by roughly the same amount, the loss is attributable to normal evaporation. If the pool level outside has dropped significantly more than the level inside, the difference indicates a likely leak.
Strategies for Minimizing Pool Evaporation
The most effective method for reducing water loss is physically covering the pool surface. Solid pool covers, such as solar blankets, block surface exposure to the air and wind, significantly reducing evaporation. These covers also trap heat, lowering the energy required to maintain the desired water temperature.
Another approach involves using liquid solar covers, which are chemical evaporation inhibitors. These products create an invisible, single-molecule layer (mono-layer) on the water surface that suppresses the escape of water molecules into the air.
Since wind is a major factor, installing windbreaks is a beneficial strategy. Landscaping, fencing, or structural barriers around the pool deck effectively block air movement. Reducing the wind speed prevents the rapid removal of the saturated air layer, slowing evaporation.
Pool owners can also manage evaporation by controlling the water temperature in heated pools. Lowering the thermostat setting reduces the temperature differential between the water and the air, decreasing vapor pressure at the surface. Limiting the use of water features to only when the pool is occupied will also conserve water.