Evaporation and water loss in the pool: what is normal

What increases evaporation

Evaporation happens at the water surface and depends mainly on four factors:

• Wind: the most important. With the meltemi or on an exposed plot, the level drops noticeably faster.
• Dry air: the lower the humidity, the more water the air "pulls" out.
• Temperature difference: warm water with cool nights evaporates strongly, which is why heated pools lose more water.
• Disturbed surface: waterfalls, jets, diving and play increase the surface that comes into contact with the air.

A useful calculation: every millimetre the level drops equals as many litres as the pool has square metres of surface. In a 4 × 8 m pool, a drop of 5 mm a day means about 160 litres. In a Greek summer a few millimetres a day is normal, and even more with strong wind or warm water.

Losses that are not a leak

Before you worry, also account for water that leaves in normal ways:

• backwashing the filter and rinsing, which send pool water to waste every time;
• splashing from play and diving, especially when children are swimming;
• in an overflow pool, fluctuations in the balance tank, where the level goes up and down with use.

The bucket test step by step

The test compares the pool with a bucket kept in exactly the same conditions. The bucket loses water only to evaporation; if the pool loses clearly more, there is a leak somewhere.

1. Bring the pool to its normal water level and switch off any automatic top-up.
2. Place a plastic bucket on the first or second step so its rim stays above the water. If there are no steps, hold it firmly in place with a weight inside.
3. Fill it with pool water to the same height as the pool level.
4. Mark the level inside the bucket and the pool level at the skimmer with tape or a marker.
5. Leave the pump on its normal schedule for 24 hours, with no swimming and no filter backwash. If it rains, start again.
6. Measure and compare the two drops.

If the drops are about the same, it is evaporation. If the pool loses clearly more, repeat the test with the pump off: comparing the two results shows whether the problem is in the pipework or in the pool shell. The next steps are in the guide to pool leaks.

How to reduce evaporation

• Cover: the most effective measure. A summer or solar pool cover at night and on windy days keeps in water, heat and chemicals.
• Windbreaks: a fence, glass screen or dense planting on the side of the prevailing wind in your area.
• Water features in moderation: switch off waterfalls and jets when you are not enjoying them, especially at night.
• Sensible heating: the warmer the water compared with the air, the greater the evaporation. See which water temperature makes sense.

Top up so the level stays at about the middle of the skimmer mouth; any lower and the pump risks drawing in air. Remember too that evaporation removes only pure water and the minerals stay behind. In areas with hard water, constant top-ups gradually raise the hardness, so in high summer it is worth testing it more often.

Questions

See also