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Q & A: Garden pond level lowering

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Most recent answer: 10/22/2007
Q:
hi there i have a small garden pond, 9ftx12ftx3ft, can you tell me if its normal for the water level to drop over night by at least 1 inch the best info i can give you is yesterday and last night the daytime temp was 29c in the shade and about 35c in the sun the pond sits in sunllight for about 4 hours from midafternoon the night temp was about 20c there was no wind, i have changed my pond liner several times thinking it was a leak hope you are are able to answer my question
- ian (age 43)
suffolk uk
A:
Your water appears to be about 2.7% lower in the morning than in the night. Evaporation may contribute quite a lot to this, although I wouldn't rule out a leak. The evaporation rate depends on lots of variables, like the humidity, tempeture, and wind (all through the night). If you lived in Arizona, for example, I wouldn't hesitate to suspect that your water would evaporate very quickly (it is very hot and dry there). At least your location in Suffolk is sufficiently hot, although it probably is humid as well.

Another effect is going on which can help explain things, but is about a factor of 10 too small. Water expands when it heats up and shrinks when it cools off. The thermal expansion coefficient for water is 0.0003 per degree C. This number is the fractional change in volume per degree C temperature change. So for a 10 degree change in temperature, you can expect your water to shrink by 0.3%, accounting for a tenth of what you see.

There may also be local wildlife which are hot and thirsty, and may take splashy baths in your pond at night. Is the ground near the pond wet in the morning?

One experiment you can do is to cover your pond with plastic to see if that stops the water loss. If it's evaporation, then the concentration of impurities in the water should go up over time, but I wouldn't suggest adding anything to the water to check this out. I still wouldn't rule out the leak hypothesis.

Tom

(published on 10/22/2007)

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