Is my pond leaking?

A pond that loses water is not always leaking. In a hot, windy spell, water leaves through the surface and through the plants. A pump, a waterfall or a fountain can throw water out of the pond without any hole at all. A puncture in the liner, a split pipe or a sunken edge is a different problem with a different fix. The job is to work out which one you have before you drain anything.

Evaporation and plants

Open water loses water to the air, and more so in sun and wind. Marginal plants and bog plants draw water out of the pond and lose it through their leaves. In a long dry summer, the level of a perfectly sound pond can drop noticeably.

The RHS care page notes that evaporation in hot or windy weather can reduce oxygenation and cause problems for fish. It also says topping up is not usually necessary if there are no fish, because most wildlife copes with a changing water level. Topping up a pond covers that, and why rainwater is the better choice when you do.

Signs it is probably evaporation:

• The level drops faster in hot, sunny, windy weather and slows when it is cool or overcast.
• It drops roughly evenly, without stopping at a particular height.
• Rain brings it back up and it stays there for a while.

This page does not give a typical evaporation rate. It varies with weather, shade, surface area and planting.

Water leaving without a leak

Before blaming the liner, check the things that move water about:

• Fountains blow spray onto the lawn on a windy day.
• Waterfalls and streams can run over a low bank or under a stone and soak away. The waterfall guide explains how a stream liner should overlap the pond.
• Pipes and fittings on a pump or filter can drip or split, especially after a frost.
• Filters can overflow if the pump pushes more than the filter passes. The pump size and filter pages cover that balance.
• Turf and soil touching the water can wick it out through the edge.

The Freshwater Habitats Trust FAQ says the same thing in a sentence: check pumps, filters and fountains first, because overspray can cause water loss.

A simple test is to turn the pump off, isolating it safely as the electrics page describes, and watch the level for a few days. If the loss stops, the leak is in the pump circuit, the waterfall or the stream, not the pond.

Finding a leak in the pond itself

The Freshwater Habitats Trust says that if water levels are falling, you may have a leak, and that to locate it you wait until the falling water level stabilises. The logic is simple: water escapes until it drops below the hole, then stops. The line where the level settles is near the height of the leak.

Once it has settled:

• Walk round the edge at that height and look for a tear, a puncture, a fold where a stone has worn through, or a place where the liner has slipped below the water line.
• Check the edge as well. A liner that has sagged, or soil that has slumped under the edging, can let water spill out at one low point. That is a level problem, not a hole.
• Look for wet ground, unusually lush grass or a soggy patch outside the pond. Water goes somewhere.

The RHS care page says that if the level drops in winter, it may indicate a leak. Evaporation is low in cold weather, so a winter drop is harder to explain away. If the pond holds fish, winter care covers keeping it topped up until a repair is possible.

Repairing it

What you can do depends on what the liner is made of and where the damage is.

• A patch. Liner makers and pond shops sell repair kits for particular liner materials. A patch is only as good as the maker's instructions and how well they are followed: the right kit for that material, a clean and dry surface, and the conditions the instructions ask for. If you do not know what the liner is made of, find out before you buy a kit. This site does not give a universal patching method or claim any patch is permanent.
• Re-seating the edge. If the leak is at the rim, lifting the edging, building up the soil and relaying the liner can be enough.
• A new liner. A liner that is brittle, has several holes, or was never protected underneath may be past patching. Size a replacement from the dug hole using the liner size page, and lay it over fresh underlay as described in fitting a pond liner.

Preformed shells crack rather than puncture. A cracked shell needs a repair product made for that material, or replacing.

Draining for a repair

If you have to lower the level to reach the damage, take only as much water out as you need. Keep the water you remove, if you can, and keep animals and plants in it. Avoid doing it in the amphibian breeding season. The spring clean page covers how to drain a pond with the least harm if you must.

Prevention

Most liner leaks start with something under the liner, something dropped on it, or a fork pushed through it while planting. Underlay, a careful edge and soft shoes while working in the pond prevent most of them.

How do I know if it is evaporation or a leak?

Evaporation speeds up with heat and wind and slows when it is cool. A leak keeps going until the level falls to the hole, then stops. A drop in cold winter weather points to a leak.

Can I patch a liner while the pond is full?

Some products claim they can be used underwater. Follow the maker's instructions for the product and the liner material. If the instructions say dry, drain to below the hole first.

The level stops dropping when I turn the waterfall off. Where is the leak?

In the waterfall, the stream or the pipework. Check for water running over a low bank or under a stone, and for split hoses and loose fittings.