Pond Evaporation in a Heatwave: Water Loss and Compensation
In a full heatwave, a pond can lose 1 to 3cm of water a day to evaporation — a natural process, not a leak. On a 10m² surface, that is up to 300 litres a day. Top it up with small amounts of untreated water, and learn to tell evaporation apart from a genuine leak.
The Chatelain Method for a falling water level
You observe how fast the level drops; you diagnose evaporation or a leak; you correct with a controlled top-up; you prevent excess loss through shade and fewer dripping surfaces.
How much water really evaporates?
Evaporation depends on heat, wind, sun and exposed surface area. Evaporation in hot or windy weather can reduce oxygenation as well as water level, so it is worth checking regularly during a prolonged heatwave.
| Conditions | Estimated daily loss |
|---|---|
| Mild, overcast | A few mm/day |
| Dry and sunny | ~1cm/day |
| Heatwave, dry wind | Up to 3cm/day |
A waterfall, sheet of water or fountain speeds up evaporation by multiplying the contact surface between air and water.
The Chatelain note. Do the maths: 1cm of water over 1m² is 10 litres. A 10m² pond losing 2cm a day is therefore giving up 200 litres to the air every day of the heatwave. Nothing abnormal, but it needs replacing.
Evaporation or a leak: how to tell
- Mark the water level precisely (on the liner, a stone, or a stake).
- Switch off the waterfall and any jets for 24-48 hours (keep an air pump running for oxygenation).
- Observe: a slow, even drop across the whole pond means evaporation. A fast drop that stops at a specific level points to a leak at that spot.
If the drop stops as soon as the waterfall is switched off, the problem lies in the waterfall circuit, not the pond itself.
Topping up without shocking the fish
- Use rainwater where possible — ideal, and free.
- Otherwise, let tap water stand for 24-48 hours to let chlorine dissipate.
- Add small, regular amounts rather than one large dose.
- Top up in the morning when the pond water is at its coolest.
The hidden effect: concentration
Water evaporates, but dissolved minerals and nitrates stay behind. As the level drops without topping up, their concentration rises, which can favour algae and add stress to fish.
FAQ
Is it normal for my pond to drop several centimetres a day in summer?
Yes. In hot, dry, windy weather, evaporation can reach 2-3cm a day, especially with a waterfall or fountain running. It’s natural — just top it up.
How do I know if my pond is leaking or just evaporating?
Mark the level, switch off the waterfall for 24-48 hours and watch. A slow, even drop is evaporation; a fast drop that then stabilises at a set level points to a leak.
What water should I use to top up during a heatwave?
Rainwater if you can, otherwise tap water left to stand for 24-48 hours to remove chlorine. Add it in small amounts, ideally in the morning.