Pool evaporation: how much water do you really lose?
Many owners believe they have a leak when what they are seeing is normal evaporation. The phenomenon is particularly marked in a hot, dry climate like that of Marrakech: it is worth knowing about, because it changes the way a pool is designed.
What we are talking about
A pool loses water every day, without any valve being opened: that is evaporation. In temperate conditions it stays discreet; in high heat and dry air, it changes scale and becomes the pool’s biggest consumption item, far ahead of draining.
The figures depend too much on the pool, its exposure and the weather at the time for us to give one here without misleading. What matters is knowing that the loss is daily, that it is easily measured on your own pool, and that topping up is part of routine maintenance.
Heat is not the main factor
Air temperature plays a part, obviously. But two other factors weigh as much, or more.
Humidity. The drier the air, the more vapour it absorbs. That is what makes evaporation so strong in arid regions: at equal temperature, dry air dries out a pool far faster than humid air.
Wind. It constantly renews the saturated layer of air just above the water, and restarts evaporation. A pool exposed to wind loses markedly more than a sheltered one, in identical conditions.
Telling evaporation from a leak
The bucket test, simple and reliable: place a bucket filled with pool water on a submerged step, so that the water is at the same level inside and outside. Mark both levels. After 24 to 48 hours, compare the drops. If they are identical, it is evaporation. If the pool has dropped more than the bucket, look for a leak.
What works to limit losses
A cover. A bubble cover or a slatted cover cut off most of the evaporation; even a simple sheet put on in the evening and taken off in the morning removes a good part of it. It is by far the most effective measure. Note: the housing for a slatted cover must be allowed for in the quote.
Break the wind. A hedge, a low wall or planting at the edge of the pool reduce evaporation and improve bathing comfort. This is thought about when the garden is designed.
Do not overheat. Every degree gained by heating is a degree that speeds up evaporation. At our latitudes, the sun is more than enough for a good part of the year.
Go easy on water features. A waterfall is magnificent, but it atomises the water and increases its exchange surface with the air. Running it in sequences rather than continuously limits the loss.
Recover the filter backwash water. It can go to irrigation rather than to the drain, provided this was planned when the plant room was installed.
What this implies at the design stage
Almost all these measures are decided before digging. The housing for a slatted cover, the pool’s orientation relative to the prevailing wind, shade cast over part of the shallow area, the backwash recovery circuit: added afterwards, they are expensive or become impossible. In a region where water is a counted resource, it is a conversation to have from the first meeting.