Is My Pond Leaking or Is It Evaporation? A Practical Water-Loss Test

A falling water level does not automatically mean a pond liner has failed. Warm, dry or windy weather can remove a surprising amount of water, while waterfalls, streams and dense marginal planting increase the exposed, moving surface. The useful question is not simply whether the level has dropped, but how, when and where it stops.

Start with a consistent water-level record

Mark the current level at a fixed point with tape, chalk or a photograph against a stone or liner fold. Check it at roughly the same time each day and note rainfall, strong wind and any further top-ups.

A universal “normal loss” figure is not dependable. Pond size, shade, weather, planting and moving water all change evaporation. The RHS notes that hot or windy conditions increase evaporation, so compare the pond’s behaviour across several days rather than relying on one reading.

Separate the pond from the circulation system

Water often escapes from a waterfall, stream, filter, skimmer or pipework rather than through the pond itself. If it is safe for the pond’s fish, oxygen levels and equipment, switch the circulation system off for a controlled observation period and continue recording the level.

• The loss slows or stops: inspect the waterfall, stream edges, filter chambers, hoses, unions and return pipes.
• The loss continues: evaporation is still possible. If loss exceeds the comparison test, investigate the pond, edges and lining penetrations.

Do not leave essential aeration or filtration off in warm weather or where fish depend on it. If you are unsure, get advice before isolating equipment.

Check the edge before assuming there is a hole

Low edges are a common cause of persistent loss. Soil can settle, stones can press the liner down and marginal plants can wick water over the side. Walk the full perimeter and look for wet ground, displaced folds, overflowing planting pockets and roots or vegetation bridging the waterline.

If the pond repeatedly settles at the same level, inspect the entire perimeter at that height. This can help narrow the search, but it is not proof of a puncture: a leaking pipe fitting or watercourse may sit at a similar level.

Use a simple evaporation comparison

Where a stable shelf is available, place a watertight container in the pond, partly fill it and mark both the water inside the container and the pond level outside it. Both are exposed to similar weather, but only the pond is connected to its liner, edges and pipework. If the pond level falls noticeably faster than the water in the container, a leak is more likely.

Compare the vertical drop over the same period, not the volume lost. Different water temperatures, shade, wind, rainfall or splashing can distort the result. Keep the container stable without damaging the liner, and prevent wildlife becoming trapped. This is a rough comparison, not proof of a leak.

The pond type changes the investigation

With a flexible liner, the likely causes include a low edge, damaged fold, puncture or leaking penetration. Concrete and fibreglass ponds need checks around cracks, joints, fittings and previous repairs. A clay pond may be responding to seasonal inflow, drying or a failure in the compacted layer; adding bentonite without understanding the ground and water source is not a dependable first step. Read our guide to bentonite for pond sealing before treating it as a simple cure.

When to stop testing and investigate properly

Seasonal drawdown can benefit wildlife in unlined ponds; a falling level is not automatically a defect. Do not drain a pond or disturb banks or protected wildlife to run these checks. Repeated topping-up can hide the pattern and delay the right repair. Arrange an assessment where loss is rapid, the pond contains fish or protected wildlife, the liner is old or brittle, or the repair would require draining and disturbing established habitat.

Our guide to common pond leak causes explains the main failure points. For a persistent problem, see our leaking pond investigation and repair service and pond refurbishment options.