Why pools lose water
Evaporation is usually the main cause of water loss in a swimming pool, ahead of splash-out and backwashing. It also carries away a lot of heat: evaporating each pound of water removes about 1,000 BTU from the pool, which is why heated pools lose most of their heat through the surface.
How the calculator estimates it
The rate depends on the difference between the vapor pressure at the water surface (set by water temperature) and the vapor pressure in the air (set by air temperature and humidity), and on the wind speed over the water. The calculator uses the ASHRAE evaporation equation:
W = A × (pw − pa) × (0.089 + 0.0782 × V) ÷ Y
where A is the surface area, pw and pa the vapor pressures, V the air velocity and Y the latent heat of water. A cover is taken to cut evaporation by about 90%.
Example
A 16 × 32 ft pool (512 ft²) at 80 °F, air at 77 °F, 60% humidity and a light breeze of 1 mph:
- water lost: about 88 gallons a day;
- level drop: about 0.28 in a day, almost 2 in a week;
- heat carried away: about 32,000 BTU/h;
- top-up cost at $8 per 1,000 gallons: about $21 a month.
What makes it worse
- Warm water and cool, dry air: evaporation climbs fast when the water is much warmer than the air, such as on cool nights with a heated pool.
- Wind: even a few miles per hour over the surface can double the rate. Fences, hedges and windbreaks help.
- Low humidity: desert climates lose far more than humid ones.
- Sun heats the surface, which this simple model does not capture directly.
How to cut it
- A pool cover is the most effective measure; solar covers also hold in heat.
- Lower the water temperature by a few degrees when the pool is not in use.
- Turn off fountains and waterfalls, which increase the evaporating surface.
- Shelter the pool from prevailing winds.
Evaporation or leak?
Use the bucket test: set a bucket of pool water on a step, mark the water levels inside the bucket and on the pool wall, and compare after 24 hours with the pump running as usual. If the pool drops noticeably more than the bucket, look for a leak.