Is It Evaporation or a Leak? Part 1: How to Tell the Difference
Normal evaporation rates in New England, and the simple tests that tell you whether your falling water level is a real problem.
Every summer, we get the same panicked phone call. The water level is dropping. They are sure they have a leak. They are picturing a torn liner, a flooded yard, a multi-thousand-dollar repair, and an empty pond by Labor Day.
About half the time, they do not have a leak. They have evaporation.
The other half of the time, they do have a leak. Knowing which is which saves a lot of stress, and getting it right matters because the response is completely different. In this post I want to walk you through what normal evaporation looks like in New England, what counts as a leak, and the simple diagnostic tests you can run before you call us.
What Counts as Normal Evaporation in New England
Every pond loses water to evaporation. Always. The question is how much, and at what rate. In our climate, here are the rough numbers we expect to see:
- Spring (April to May): Cool temperatures, lower wind, moderate sun. Expect roughly half an inch to one inch of loss per week.
- Summer (June to August): Peak heat, more wind, longest sun exposure. Expect one to two inches of loss per week, sometimes more during a heat wave.
- Fall (September to October): Cooler nights, less sun. Loss slows back down to half an inch to one inch per week.
- Winter: Loss is minimal once the pond is shut down or partially frozen, but sublimation and any active features still pull some water.
These numbers are baselines. Your pond may run a little higher or lower depending on a handful of factors that bump the rate up or down.
What Drives Evaporation Up
Some ponds evaporate more than others, and it has nothing to do with a leak. The drivers we see most often:
Active waterfalls and streams
Every time water tumbles over rocks, it picks up surface area and splashes off the system. A pond with a tall, wide waterfall can easily evaporate twice as fast as a still pond of the same size.
Direct sun exposure
A pond in full sun all day evaporates significantly faster than one shaded by trees or a structure for part of the afternoon.
Wind exposure
Wind is one of the most underrated evaporation drivers. Open, breezy yards lose more water than sheltered ones. A windy week alone can double your normal weekly loss.
Heat waves
Three or four consecutive days above 90 degrees will dramatically increase evaporation. We see customers panic during the first major heat wave every summer.
Low humidity
Dry air pulls water out of the pond faster than humid air. New England summers are mostly humid, but the dry stretches can sneak up.
Splash loss from a busy waterfall
If your waterfall sends water onto adjacent rocks and soil instead of back into the basin, that water is technically "lost" the same way evaporation is. We often correct this with rock adjustments.
What Counts as a Leak
A leak is when water leaves the system through a route it should not be taking: a hole in the liner, a failed seam, a cracked plumbing connection, or a settled rock edge that water is now flowing over and out of the system.
The clearest sign of a leak is loss that exceeds normal evaporation. If you are losing more than two to three inches per week consistently and the weather is not extreme, something is wrong.
Some other signs worth watching:
- The pond keeps dropping even when the waterfall is shut off.
- Wet, soggy, or unusually green grass somewhere along the pond perimeter.
- Water staining or wet rocks along one specific section of the stream or waterfall.
- The pump runs dry or starts sucking air before you expect it to.
- You can hear the pump struggling because the water level has dropped below the skimmer intake.
- You are topping the pond off more than once a week with the hose.
Any one of those signs is not a guarantee that there is a leak. Together, they build a strong case.
Every spring I get a few calls about new leaks that turn out to be mouse damage from the winter. Mice will burrow into the rockwork around a pond looking for a warm spot to nest, and they sometimes chew through the liner in the process. The holes are usually on a vertical surface, often hidden behind rocks at the stream or waterfall edge.
If you fired your pond up this spring and noticed a sudden, fast loss, do not assume the worst. It might be a small hole that is easy to patch. We see this pattern every year, and most of these repairs are simple if you catch them early.
The Five-Minute Test Anyone Can Do
Before you call us, run this quick diagnostic. It takes a few minutes of attention spread over a day or two, and it answers the question most of the time.
- The Bucket Test Place a clean five-gallon bucket on the top step of the pond or on a level surface right next to the pond. Fill it with pond water to within an inch of the rim. Mark the water level inside the bucket with a piece of tape. At the same time, mark the water level on the inside of the pond with another piece of tape. Wait 24 to 48 hours. Compare the loss in the bucket to the loss in the pond. If they dropped about the same amount, it is evaporation. If the pond dropped significantly more, you likely have a leak.
- The Pump Off Test Shut your waterfall pump off completely for 24 hours. Mark the water level. If the pond stays steady when the pump is off but drops noticeably when it is running, the leak is somewhere in your stream or waterfall, not in the main basin. This is the most common scenario we see.
- The Stop Point Test With the pump off and the pond still leaking, watch where the water level stabilizes. If it drops to a specific point and then stops, that level usually tells you where the hole or low spot is. Walk the perimeter at that level and look for wet rocks, soft soil, or visible damage.
- The Refill Calendar Even if you do not run the bucket test, just keep track of how often you are topping off the pond. Once a week during a heat wave is normal. Every day or two, regardless of weather, is not normal.
When to Stop Diagnosing and Call Us
You can usually figure out whether you have a leak or evaporation on your own. Finding and fixing the actual leak is harder, and that is where we come in. Part 2 of this series covers that in detail. The short version: call us if
- The bucket test confirms loss faster than evaporation, and you cannot see an obvious cause.
- The waterfall keeps losing water even after you have checked the visible rockwork.
- You suspect mouse damage but cannot locate the hole.
- Your pond is more than a few years old and the loss is gradually getting worse year over year, which often points to liner aging or settling.
- You just want a professional eye on it before the loss becomes a fish problem.
We service ponds throughout Middlesex, Norfolk, Suffolk, Worcester, Essex, and Plymouth counties in Massachusetts, plus Rhode Island, Connecticut, and southern New Hampshire. We work on ponds we built and ponds we did not build. If you would rather have us run the diagnostic, we can do that too.
Frequently Asked Questions
How much evaporation is normal in summer?
In New England, one to two inches of loss per week during summer is normal, sometimes more during a heat wave. Ponds with active waterfalls and direct sun lose more. Ponds without features or with significant shade lose less.
Why does my pond lose more water when the waterfall is running?
Active waterfalls increase evaporation significantly because they expose more surface area to air and wind. If the loss only happens when the pump runs, the cause may also be a leak somewhere in the stream or waterfall path rather than evaporation alone.
Can a small leak fix itself?
No. Small leaks tend to get bigger over time as water continues to wash away soil behind the liner or erodes the area around the hole. Catching a leak early is the cheapest repair.
Is it ever safe to ignore a slow leak?
If the loss is minimal and stable, you can monitor it through the season. But we recommend identifying the cause before the next winter, because freeze-thaw cycles often make small leaks worse.
Does adding fresh water hurt the fish?
Topped-off water from a hose contains chlorine and chloramines that can stress fish. Use a dechlorinator when you add more than a small amount, or top off slowly over time. We can recommend products that work well for New England water supplies.
Not Sure If It Is Evaporation or a Leak?
We can run the diagnostic for you and give you a clear answer. If you do have a leak, we will tell you what it will take to fix it.
Request a Pond Assessment
