What is the typical evaporation rate for a garden pond in the UK

In the UK during the milder weather you can expect to loose approximately 25mm (1″) of water per week from your garden pond due to evaporation, this is quite normal and nothing to worry about, as long as you top your pond up with water and keep an eye on the level you should be fine.

So when it’s sunny you may notice the level of the water dropping in your pond, as long as it’s not more than 25cm over a period of a week it’s usually down to evaporation, any more than that and your pond could have sprung a leak.

Obviously evaporation is down to weather conditions, if it’s really hot and sunny the more evaporation is going to take place, if its cool and cloudy then there is going to be less evaporation.

In between these hot and sunny days you could have a day of constant rain (happens a lot in the UK) so it tops up your pond and you don’t even notice a drop in water.

There is a complicated formula for working out evaporation in your pond which is calculated on flow rate, pond size (width x length) and surface area but as far as we are concerned as long as your pond doesn’t lose a lot of water and stays around the average of 25cm water loss per week in the summer months then that can be put down to water evaporation.

How water evaporates from your pond.

The reason your pond water evaporates is that the surface of the water is exposed to warm air allowing molecules to escape which forms water vapor, and it’s this vapor that rises up into the air.

Low humidity has low amounts of water in the air and this can increase evaporation, high winds can also increase evaporation.

If you have a large surface area of exposed water the more evaporation will occur, if you can cover some of that area with floating plants then the less evaporation will happen.

Long waterfalls and streams add to the water surface and will also increase water evaporation.

If you suffer water evaporation or any type of water loss you should top up your water on a regular basic especially if you are using tap water.

Tap water contains chlorine which is harmful to fish in large doses, but just topping up an inch or so of water will not harm them as the chlorinated tap water will be diluted with the pond water.

If you want to stay safe you can fit an automatic water top up system, it has a water level sensor that detects a drop in water and sends a signal to a control box which turns on the water feed to your pond and turns off when the water level is reached. It tops up small amounts of water frequently so any chlorinated water is diluted to safe levels and you can keep your water levels constant.

Other reasons why your pond might be loosing water.

Leaking ponds.

As we touched on above, your pond might have sprung a leak so the first thing to do is turn off your pond pump.

If the water levels in your pond continue to drop you know it’s not being pumped out and the leak is in the pond itself. If the level of the water stays the same then it must be the pump actually pushing the water out through the hoses or fittings within the system and not returning to the pond.

If you think it’s in the pond itself, let the water drop, when the water stops dropping you know the leak will be somewhere in the perimeter at the level of the water.

We have written an article on finding a pond leak that you can read here

Water features.

If you have a waterfall or a fountain in your pond these can lose water through spillage and splashing.

If you have a fountain that is close to the edge of your pond or it’s a windy day the water that should go back into the pond is being spilled outside of the pond.

If you have a waterfall you might find it has sprung a leak or has got blocked causing the water to spill over the edge.